For whatever reason, I was thinking about the most profound idea or concept I've ever heard and learned, and one does stick in my mind: the most famous equation ever written down, E = mc^2.
It certainly does not look like much. Most people you ask to name an equation will state this one, and almost none of them will understand what it means or implies. It is just sort of there, thanks to Einstein. But what does it really mean?
I look at it this way  and it has helped me make sense of a good chunk of modern physics, which includes quantum mechanics. Keep in mind this is coming from special relativity, from 1905.
This states in no uncertain terms that Energy = Matter. It is equivalence. This means that whatever matter can do, so can energy; and whatever energy can do, so can matter. The properties of one must be the properties of the other. The one exception, I suppose, is that energy (i.e. photons) move at the speed of light, whereas matter cannot.
To me, this alone helps me understand why there is particlewave duality and the uncertainty principle.
For instance, energy has wave properties, in particular diffraction and interference. If this is true for energy, then necessarily it must be true for matter. Particles like electrons have wavelengths and can diffract, and we know this because people use electron microscopes all the time. Another example is matter has momentum, which we have always been taught since Newton is p = mv. This must mean energy has momentum...but wait, energy has no mass! Einstein figured out that p = E/c for photons, and this has been confirmed by something called the Compton effect; NASA also wants to make solar sails, and use the momentum of sunlight to help propel a spacecraft. DeBroglie dervied his famous equation from relativity and this principle for matter waves, wavelength = h/p.
E = mc^s changed geopolitics and the course of human history forever, as nuclear reactions, power plants and bombs literally changed the world in just a few short years in the 1930s and 1940s.
This equation effectively outlines a phase transition, where matter can change to pure energy, or energy can change to matter. This reminds me of ice and steam being able to change back and forth into each other through phase transitions due to temperature. Ice and steam certainly don't look like each other, but upon closer inspection they are two forms of the same stuff, H_2O. Matter and energy are two forms of the same stuff.
But this notion of a phase transition goes even deeper. Our universe exists because of it! The Big Bang was a burst of pure energy. Because of this phase transition and equivalence, as the temperature cooled, energy transformed into matter and the forces of Nature. My own background in particle physics at Fermilab relied on miniBig Bangs, where protons and antiprotons annihilated into pure energy, and then back into matter. We produced top quarks and hundreds of other particles from this phase transition process on a daily basis!
So let's summarize. E = mc^2 holds the equivalence of matter and energy, explains waveparticle duality and the uncertainty principle, changed our world politically forever through an understanding of nuclear processes, and explains how a phase transition produced our universe. Not bad for one little algebraic formula anyone can quote.
For some derivations as to begin to see where this comes from, check out one of my videos. There is another video on where the notion of antimatter comes from, momentum of photons, and matter waves.
Monday, December 29, 2014
Saturday, December 13, 2014
Top Physics Discoveries of 2014
Check out a topten list put out by Physics World. I personally like the sonic tractor beam! Pretty cool.
Tuesday, December 9, 2014
For Classes
For the periods 12, 89 classes, please show the video on inelastic collisions with a ballistic pendulum. Click here for the video.
For periods 34, please show the video on rolling AND slipping by clicking here. Also, please show the video for real pulleys that do spin, by clicking here.
For periods 34, please show the video on rolling AND slipping by clicking here. Also, please show the video for real pulleys that do spin, by clicking here.
Monday, December 8, 2014
New Astrophysics Experiment trying to Catch Direct Evidence for a Black Hole
Thanks to Lucy S. for the link.
A new group of scientists and astrophysicists from 13 institutions are putting together the Event Horizon Telescope, which is the combination of a number of other, individual telescopes and detectors, to catch any glimpses of gas clouds being 'sucked' into the supermassive black hole at the center of the Milky Way galaxy. It is thought that this particular massive critter is some 4 million times more massive that the sun. If this can be accomplished, a ring of light should appear around a perfectly black center, as that gas moves inward towards its final demise of passing through the event horizon. Check out the article! Very cool idea!
A new group of scientists and astrophysicists from 13 institutions are putting together the Event Horizon Telescope, which is the combination of a number of other, individual telescopes and detectors, to catch any glimpses of gas clouds being 'sucked' into the supermassive black hole at the center of the Milky Way galaxy. It is thought that this particular massive critter is some 4 million times more massive that the sun. If this can be accomplished, a ring of light should appear around a perfectly black center, as that gas moves inward towards its final demise of passing through the event horizon. Check out the article! Very cool idea!
Sunday, November 30, 2014
Rolling Object going up a FrictionLESS Hill
Rotations can be challenging, but over time hopefully things start to click a bit more and make a little more sense. Here is one of those strange examples of a 'what if' situation: a ball rolling without slipping on a flat surface, but then going up a frictionLESS incline. What would this look like? Are there any differences from what we are used to seeing, which is something rolling without slipping even up the incline.
In everyday life, we are used to seeing something roll up a hill and both stop moving linearly as well as stop spinning at its maximum height. Would this happen if there is no friction on the hill?
The answer is NO...at the maximum height, the linear motion stops, that much we know. But here is the kicker: without friction between the ball and the surface it sits on, there is no torque. Without torque, there is no change in the rotational motion of the object. In other words, the ball will still be spinning with the same angular velocity as it had at the bottom of the hill! It would look weird.
What's more, the ball would then start to move back down the hill, since a component of gravity produces a net force. As it accelerates linearly down the hill, the spin is opposite what we would normally see since that original spin has not at all changed! Very odd! Check out this in a bit more detail.
In everyday life, we are used to seeing something roll up a hill and both stop moving linearly as well as stop spinning at its maximum height. Would this happen if there is no friction on the hill?
The answer is NO...at the maximum height, the linear motion stops, that much we know. But here is the kicker: without friction between the ball and the surface it sits on, there is no torque. Without torque, there is no change in the rotational motion of the object. In other words, the ball will still be spinning with the same angular velocity as it had at the bottom of the hill! It would look weird.
What's more, the ball would then start to move back down the hill, since a component of gravity produces a net force. As it accelerates linearly down the hill, the spin is opposite what we would normally see since that original spin has not at all changed! Very odd! Check out this in a bit more detail.
Moment of Inertia when there are Holes in the Object  Yikes!
Moments of inertia are essential to rotating objects. This quantity replaces mass in our equations that we are used to for linear motions, and it represents the distribution of mass about the axis of rotation. For a point mass, for instance, I = mr^2, where r is the distance from the axis to the mass. I have videos for sticks and disks, using the integral version for I.
Then there is the parallelaxis theorem, which is for finding the moment of inertia for an object that rotates about an axis that is not its center of mass. Imagine spinning a disk around a point on its edge! This theorem says I = I_cm + md^2, where I_cm is the usual moment of inertia for its axis at the center of mass, and d is the distance from the center of mass to the new axis of rotation.
Here, we take it one step further. What if the object has a hole drilled out, so some of the mass is missing, and the object still rotates? Huh?!?! Check it out. These are nice examples of rotations, moments of inertia, and the parallelaxis theorem for an unusual situation.
Then there is the parallelaxis theorem, which is for finding the moment of inertia for an object that rotates about an axis that is not its center of mass. Imagine spinning a disk around a point on its edge! This theorem says I = I_cm + md^2, where I_cm is the usual moment of inertia for its axis at the center of mass, and d is the distance from the center of mass to the new axis of rotation.
Here, we take it one step further. What if the object has a hole drilled out, so some of the mass is missing, and the object still rotates? Huh?!?! Check it out. These are nice examples of rotations, moments of inertia, and the parallelaxis theorem for an unusual situation.
Saturday, November 29, 2014
History of the Universe in 18 Minutes!
Here's an interesting and well done TED talk by David Christian, as he takes us through the evolution of the universe in 18 minutes. Keep in mind that this has as a key idea that what started off as something very simple, an early universe of just energy and very little matter and no atoms (in the first few hundred thousand years of time), which then had huge complexity develop up to the present.
Saturday, November 22, 2014
Quantum Biology
As nanotechnology advances, and biological studies reduce down to molecular and even atomic levels, that fuzzy world of quantum mechanics may be more relevant for biological systems than previously thought. Quantum biology and biophysics are seemingly growing fields of study, and make for a neat area of research for those who are interested in both disciplines.
For example, think about animals (and even microorganisms) that can sense and navigate via magnetic fields. How does that work? Or what are the quantum features of photosynthesis and many other life processes that take place in the atomic and molecular realms? Or how does smell work, when certain molecules are released into the air? What are the physical processes at small size scales when electrochemical signals more through neural networks in the body? What is consciousness? A review of a new book on this topic can be found here.
For example, think about animals (and even microorganisms) that can sense and navigate via magnetic fields. How does that work? Or what are the quantum features of photosynthesis and many other life processes that take place in the atomic and molecular realms? Or how does smell work, when certain molecules are released into the air? What are the physical processes at small size scales when electrochemical signals more through neural networks in the body? What is consciousness? A review of a new book on this topic can be found here.
Sunday, November 16, 2014
Advanced Topics for Olympiad
Here are some quick links to videos about topics that are more likely to make appearances on the Olympiad exams than on the AP.
 air friction, horizontal
 air friction, with a second applied force (think sky diver)
 Gauss's law for gravity
 Gauss's law for gravity (inside earth)
 Nongauss cases for gravity
 Momentum, ballistic pendulum type cases
 Moment of inertia, sticks
 Moment of inertia, disks
 Moment of inertia, where axis of rotation is not center of mass (parallel axis theorem)
 Pendulum, finding tension as it swings
 Pendulum, smallangle approximation for SHM
 SHM for springs, general
 SHM for springs, initial conditions (i.e. phase angle)
 SHM, spring and stick system oscillating through small angle
 Springs in series and parallel
Go to video page of this blog for numerous videos on all aspects of rotations.
 air friction, horizontal
 air friction, with a second applied force (think sky diver)
 Gauss's law for gravity
 Gauss's law for gravity (inside earth)
 Nongauss cases for gravity
 Momentum, ballistic pendulum type cases
 Moment of inertia, sticks
 Moment of inertia, disks
 Moment of inertia, where axis of rotation is not center of mass (parallel axis theorem)
 Pendulum, finding tension as it swings
 Pendulum, smallangle approximation for SHM
 SHM for springs, general
 SHM for springs, initial conditions (i.e. phase angle)
 SHM, spring and stick system oscillating through small angle
 Springs in series and parallel
Go to video page of this blog for numerous videos on all aspects of rotations.
Tuesday, November 11, 2014
Capacitors in Series and Parallel
Similar to resistors, we can connect multiple capacitors in series and parallel. Also, we must commonly find the total capacitance, measured in Farads, for series and parallel. But what are these rules, and where do they come from?
It turns out, Kirchhoff's rules, just like we do for resistors in series and parallel.
In series, the voltage of the battery = sum of voltages of components.
In parallel, the total charge being stored by the circuit = sum of charges stored on branches.
Check out this video to see we end up with the two same rules for capacitors, only they are flipflopped compared to resistors: in parallel we just add capacitors, and in series we have the reciprocal rule.
It turns out, Kirchhoff's rules, just like we do for resistors in series and parallel.
In series, the voltage of the battery = sum of voltages of components.
In parallel, the total charge being stored by the circuit = sum of charges stored on branches.
Check out this video to see we end up with the two same rules for capacitors, only they are flipflopped compared to resistors: in parallel we just add capacitors, and in series we have the reciprocal rule.
Resistors in Series and Parallel
Resistor circuits are the starting point for learning about circuits. Resistance is a natural feature of all everyday circuits, and is purposely used to regulate how much current is in a circuit. Generally, there are two main ways of combining multiple resistors in a circuit, series and parallel. One feature of circuit analysis is to try and find the total, or effective, resistance, in order to find total currents.
Two fundamental rules we will use over and over in circuit analysis are Kirchhoff's rules. For series, the rule is whatever voltage is put into a circuit, it is all used up by the time you make it around. FOr parallel, whatever your total current is coming into a junction, the sum of the currents in individual branches must equal the total current that came in (i.e. you don't lose current). The resistor rules for series and parallel follow directly from Kirchhoff's rules.
Check it out!
Two fundamental rules we will use over and over in circuit analysis are Kirchhoff's rules. For series, the rule is whatever voltage is put into a circuit, it is all used up by the time you make it around. FOr parallel, whatever your total current is coming into a junction, the sum of the currents in individual branches must equal the total current that came in (i.e. you don't lose current). The resistor rules for series and parallel follow directly from Kirchhoff's rules.
Check it out!
Wednesday, November 5, 2014
"Interstellar" has a Stellar look at Black Holes!
Thanks to Noah S. for giving me the link to a Wired article and video about the new movie Interstellar. Kip Thorne, a world famous expert on black holes and general relativity, consulted with the movie makers and their special effects and graphics teams to produce the best simulations of what Einstein's field equations from general relativity produce for black holes and worm holes. The graphics are stunning, and Thorne expects to get several research publications out of this because of the details they learned by seeing what a black hole should really look like! Please check it out, and be ready to be amazed!
Thursday, October 23, 2014
Nobel Prize in Physics goes to 2 Japanese and 1 American  Blue LED
It you have an LED TV or computer monitor, you have this year's group of Nobel Prize winners to thank. Having a blue LED along with the other two primary colors, red and green, allows one to have the pixels necessary to make white light and all other colors for pictures. The great thing about LED lighting of any kind is the large drop in electrical energy necessary for the system. While presently some 20% of the world's energy is used for lighting, LEDs may drop this to about 4%! This is the sort of practical application to better the world we need to see more of, and that is celebrated with the Nobel Prizes.
The Nobel for Medicine and Physiology goes to and American and two Norwegians, for their discovery of how the brain figures out our positioning  that is, how we know where we are! Pretty cool stuff! Oh, and one of the three is indeed a woman.
The Nobel for Chemistry goes to two Americans and one German, for developing superfast fluorescent optical microscopes that can attain high resolution at the nanoscale (i.e. can watch molecules!). Not bad!
The Nobel for Medicine and Physiology goes to and American and two Norwegians, for their discovery of how the brain figures out our positioning  that is, how we know where we are! Pretty cool stuff! Oh, and one of the three is indeed a woman.
The Nobel for Chemistry goes to two Americans and one German, for developing superfast fluorescent optical microscopes that can attain high resolution at the nanoscale (i.e. can watch molecules!). Not bad!
Check this out, seniors: Gravity for Sticks and Rings
When it comes to the math, what we are working on in EM as far as electric potential and electric fields for charge distributions should be the same as gravity. Check out the following videos if you want to see some fancier math and strange situations for gravity, as they just follow the analyses we have done for electric charges. It is neat to see what happens with a hollow earth, or digging a tunnel through the earth and jumping in. Then there is Gauss's law for gravity, and the NONGauss geometries. Keep in mind you can also check out the main concepts Einstein used to come up with general relativity.
Lessons for October 23
Periods 12 and 89, check out how to begin circuit analysis. In the Ohm's law lab, from data I've seen in some groups, you should see current, I, is linearly related to voltage. And also, current is related to resistance^(1), or I proportional to 1/R. Combined we'll get an empirical formula of I = V/R. This is Ohm's law. We already know the rules for series and parallel resistance.
Periods 34, check out why momentum is conserved, and then an example of perfectly inelastic collisions with a ballistic pendulum.
Periods 34, check out why momentum is conserved, and then an example of perfectly inelastic collisions with a ballistic pendulum.
Monday, October 20, 2014
Classes for Oct. 20
For periods 12 and 89, check out this video on finding potentials at various locations of the charged sticks. Focus on how to set up the integrals, and what the proper limits of integration are. You can try the 1980 (back page) and 2002 problems (second to last page) of the packet from Friday.
For periods 34, check out this video for air friction and terminal velocity. You can try the 1984 problem, and then try to complete the lab (don't worry about the last analysis question for the time being).
For COMAP teams, have at least one person from each team come to one of the organizational meetings on Tuesday, either before school at 8 am, period 5, or period 6. Note that there are some online resources that can be found here.
For periods 34, check out this video for air friction and terminal velocity. You can try the 1984 problem, and then try to complete the lab (don't worry about the last analysis question for the time being).
For COMAP teams, have at least one person from each team come to one of the organizational meetings on Tuesday, either before school at 8 am, period 5, or period 6. Note that there are some online resources that can be found here.
Sunday, October 12, 2014
Resources for COMAP 36Hour Problem Contest
Want to try your hand at developing a viable solution to a realworld, openended problem with different approaches that can be taken, and no single 'right' answer! Complex problems hound us in all fields in real life, so being able to attack such problems and develop reasonable solutions is a skill set that is necessary in the 21st century. You are going to have to collaborate, do research to find reliable resources of large data sets, use technology to assist the process, communicate your work, use more advanced thinking and problem solving skills (such as developing a mathematical model), make predictions that can be tested against a real world sample, develop creative solutions, and so on!
To assist you, here are various resources that will hopefully be helpful for you and your team:
Check out other openended, complex problems from the COMAP High School Mathematical Modeling Contest (HiMCM) and the Moody's Mega Math Challenge. The home pages are: HiMCM and Moody's.
Problems from past contests are: HiMCM and Moody's.
Exemplar papers from Moody's. This includes an ETHS team paper that took 5th place nationally!!! See what a good paper looks like. See how teams took a problem and broke it down into simpler pieces, and what assumptions they made. See how a mathematical model was developed, and what math techniques went into the model's development. See how they explained their work. See how they used the model to make predictions that could be tested against real data and other information. See how they determined and explained the weaknesses of the model. Might as well learn from some others who did a good job with this process!
Judge's perspectives from Moody's can be found on this page, and for COMAP on this page (pages 1735). These are especially useful because they point out what makes for an average solution paper compared to an outstanding solution paper! Are you doing these things for your solution paper? If not, why not? How can you readjust and make the solution better? If you had more time, what would you focus on to try and improve your solution proposal?
For optimization problems, check out this file that outlines an example of using Excel Solver. You can find lots of YouTube videos for how to use Solver, including one here. Keep in mind Solver can be used to maximize, minimize or find set values of functions for openended problems.
Teams might consider drawing out a flow chart early on in the process that outlines key components and assumptions you want to make. Get a visual picture of what this all looks like, and think about what is connected to what. Do you expect direct or inverse relationships? Linear or nonlinear? What factor or parameter, when changed, will have an effect on other parts of the model/solution?
We hope these all help you in this process. Good luck, and have fun with it!
To assist you, here are various resources that will hopefully be helpful for you and your team:
Check out other openended, complex problems from the COMAP High School Mathematical Modeling Contest (HiMCM) and the Moody's Mega Math Challenge. The home pages are: HiMCM and Moody's.
Problems from past contests are: HiMCM and Moody's.
Exemplar papers from Moody's. This includes an ETHS team paper that took 5th place nationally!!! See what a good paper looks like. See how teams took a problem and broke it down into simpler pieces, and what assumptions they made. See how a mathematical model was developed, and what math techniques went into the model's development. See how they explained their work. See how they used the model to make predictions that could be tested against real data and other information. See how they determined and explained the weaknesses of the model. Might as well learn from some others who did a good job with this process!
Judge's perspectives from Moody's can be found on this page, and for COMAP on this page (pages 1735). These are especially useful because they point out what makes for an average solution paper compared to an outstanding solution paper! Are you doing these things for your solution paper? If not, why not? How can you readjust and make the solution better? If you had more time, what would you focus on to try and improve your solution proposal?
For optimization problems, check out this file that outlines an example of using Excel Solver. You can find lots of YouTube videos for how to use Solver, including one here. Keep in mind Solver can be used to maximize, minimize or find set values of functions for openended problems.
Teams might consider drawing out a flow chart early on in the process that outlines key components and assumptions you want to make. Get a visual picture of what this all looks like, and think about what is connected to what. Do you expect direct or inverse relationships? Linear or nonlinear? What factor or parameter, when changed, will have an effect on other parts of the model/solution?
We hope these all help you in this process. Good luck, and have fun with it!
Thursday, September 25, 2014
How to do AntiDerivatives (i.e. Integrals)
We have been using derivatives since the first couple days of school. We should be getting comfortable with the notion that derivatives tell you slopes of tangent lines to curves, and in a practical sense allow us to find velocity and acceleration just by knowing a position function with respect to time. That is, v = ds/dt, and a = dv/dt. And hopefully this has made sense as we have worked with motion graphs and tried to see how these three motion graphs connect.
But a problem came up when we start with acceleration, and need to find velocity and position. We must figure out a way to 'undo' the derivatives, or in other words, find an antiderivative. This video introduces the concept, and graphically this is equivalent to finding the area below a curve. I hope this helps.
But a problem came up when we start with acceleration, and need to find velocity and position. We must figure out a way to 'undo' the derivatives, or in other words, find an antiderivative. This video introduces the concept, and graphically this is equivalent to finding the area below a curve. I hope this helps.
Friday, September 12, 2014
Lessons for classes
I am VERY sorry to be out again with a flulike illness, but here are some things we can still accomplish.
Periods 34: Tension problems are notorious for their detail, but with some practice and thinking about them systematically, are quite doable. Check out this video and take notes to get a technique down that we will use for just about any more complex, multiobject system we encounter, so we can find 'internal forces' of the system.
Periods 12, 89: We mentioned that the key quantity for everything in E&M is electric charge. All charges, whether a single particle or a charged balloon, produce TWO physical quantities: electric fields and electric potential (better known as voltage; this is not the same as potential energy!). Fields are vectors, and potential is a scalar. Check out this video for an introduction for finding total voltage and net electric fields from a system of point charges. Take notes, and see if you can work through the practice problems.
Everyone can also find a science article of interest and provide a summary  we do this every so often so you can have a chance to check out some current, cutting edge topics and get a sense of some very cool things going on out in the world! It can be from a Scientific American (please return at some point!) or online, and can be from any discipline.
Have a Happy Friday and weekend, and see you Monday. :)
Periods 34: Tension problems are notorious for their detail, but with some practice and thinking about them systematically, are quite doable. Check out this video and take notes to get a technique down that we will use for just about any more complex, multiobject system we encounter, so we can find 'internal forces' of the system.
Periods 12, 89: We mentioned that the key quantity for everything in E&M is electric charge. All charges, whether a single particle or a charged balloon, produce TWO physical quantities: electric fields and electric potential (better known as voltage; this is not the same as potential energy!). Fields are vectors, and potential is a scalar. Check out this video for an introduction for finding total voltage and net electric fields from a system of point charges. Take notes, and see if you can work through the practice problems.
Everyone can also find a science article of interest and provide a summary  we do this every so often so you can have a chance to check out some current, cutting edge topics and get a sense of some very cool things going on out in the world! It can be from a Scientific American (please return at some point!) or online, and can be from any discipline.
Have a Happy Friday and weekend, and see you Monday. :)
Monday, September 8, 2014
Quizzam Review
What is on the first Quizzam?
Vector addition  find components, add to get total x, y; create final right triangle
Vector multiplication: dot product, cross product
Examples of vectors vs. scalars
Constant speed problems
Constant acceleration problems
Basic projectile problems
Derivatives  v = dx/dt, a = dv/dt
Instantaneous vs. average velocity, acceleration
Antiderivatives (integration)
Motion graphs  sketch, interpret
Relative motion (e.g. airplane flying with a cross wind, boat across a river)
Standard deviation for multiple trials
Vector addition  find components, add to get total x, y; create final right triangle
Vector multiplication: dot product, cross product
Examples of vectors vs. scalars
Constant speed problems
Constant acceleration problems
Basic projectile problems
Derivatives  v = dx/dt, a = dv/dt
Instantaneous vs. average velocity, acceleration
Antiderivatives (integration)
Motion graphs  sketch, interpret
Relative motion (e.g. airplane flying with a cross wind, boat across a river)
Standard deviation for multiple trials
Monday, August 25, 2014
Welcome Back!!!
Here's to a wonderful 201415 school year!!
For my mechanics classes, if you need some vector addition review, check this out.
If you want a review of vector multiplication, check this one out.
A motto for us:
Learn a ton while having some fun!
For my mechanics classes, if you need some vector addition review, check this out.
If you want a review of vector multiplication, check this one out.
A motto for us:
Learn a ton while having some fun!
Wednesday, August 20, 2014
Lab Activity: Series RC Circuit Behavior
Purpose: Observe general behavior of RC circuits
before getting into the details in class. We'll look at both charging and discharging cases.
General Setup: You’ll have a power supply, some
resistors, a large capacitor, and some multimeters. Set up a circuit with the power supply, resistor and capacitor all in series with each other.
Case A – Small resistance
 Connect a 10ohm resistor in the circuit with the power supply set to zero. Make sure there is no initial potential difference across the capacitor.
 With one person measuring the potential difference of the capacitor and another measuring the potential difference of the resistor, quickly turn up the voltage of the power supply (to ~10 V or so). Write down observations below:
What happens to the voltage across the capacitor?
What happens to the voltage across the resistor?
Try to explain what you think is physically happening in the
circuit based on your observations:
Sketch graphs of V_{Cap} vs. time, V_{R} vs.
time, and I vs. time, as the capacitor charges. *Challenge: What function might produce each graph? Try to guess before we find the details!
 When the voltage across the capacitor is a maximum, how much charge is it storing? Use your max. voltage measurement.
 When the voltage across the capacitor is a maximum, how much energy is it storing? U = ½ QV = ½ CV^{2}.
 Calculate the product RC. This actually has units of time and is called the time constant of the circuit. Show that (ohms)(farads) = seconds.
Quickly turn off the power supply. What happens to the voltage across the
capacitor?
What happens to the voltage across the resistor? Explain
what you think is happening now, and sketch
graphs of V_{Cap} vs. time, V_{R} vs. time, and I vs. time,
as it discharges.
Case B – Large resistance
 Measure the resistance of the other resistor: __________ W. Connect it in the circuit with the power supply set to zero. Make sure there is no initial potential difference across the capacitor.
 With one person measuring the potential difference of the capacitor and another measuring the potential difference of the resistor, quickly turn up the voltage of the power supply (to ~10 V or so). Write down observations below:
What happens to the voltage across the capacitor?
What happens to the voltage across the resistor?
Is there anything different compared to your first
circuit? If so, what? Think rates.
Try to explain what you think is physically happening in the
circuit based on your observations:
 Calculate the time constant, RC. Based on the two circuits you have observed, interpret what the value of the time constant of the circuit refers to.
Wait a couple minutes to let the voltage build up on the
capacitor. Quickly turn off the power
supply. What happens to the voltage
across the capacitor?
What happens to the voltage across the resistor? Explain what you think is happening now.
Theory into Reality: (wait until we do this type of
calculation in class)
What is the equation we
derived for charge as a function of time for a charging capacitor?
For the first circuit (10ohm resistor), how much charge is
on the capacitor after 0.05 seconds?
After 0.5 seconds?
What is the equation we
derived for charge as a function of time for a discharging
capacitor?
When the capacitor discharges, how long does it take
to lose 25% of its original charge?
Tuesday, August 19, 2014
The Need for Cybersecurity
For those who have an interest in computer science, politics, economics, international relations, business, the military, espionage, and many other subjects and topics, cybersecurity is something you have hopefully heard of, but does not get an enormous amount of coverage in the popular press. In my opinion, this is perhaps the most important topic for personal and national security that exists in the modern era. Here is a discussion with Richard Clarke, who has served multiple presidents on national security issues over many years, giving examples of what will almost certainly happen at some point somewhere in the world, via attacks on computer networks.
Lab Activity: Determine Speed and Error Analysis of Data
Purpose: Given the essential
equipment, you and your partners will have to determine methods of measuring
speed as precisely and with as little uncertainty as possible. The focus is on data analysis, particularly
on methods to determine uncertainties and propagate them to results.
Materials: Marble Ramp Meter Stick Stopwatch Graph
paper
Procedure:
You are given the task of trying to
measure the speed (distance / time) of a marble that rolls off a ramp. You want to try and maximize accuracy and
precision while limiting uncertainties.
As far as procedure, write detailed steps you decide on for a method
that involves extracting a speed from
a graph.
Do not expect much help from me as
far as measurements. I suggest that you
take a few minutes before starting to write down possible uncertainties (experimental,
which you may be able to minimize) and then give this a try.
Data Table: Develop a neat, organized,
and appropriate data table for all measurements for your procedure. Your group will need to take some number of
trials for time measurements for each distance that you use. In your data table(s), include the standard deviation for each set of time measurements.
s_{time} = Ã¦ [S(t_{avg} – t_{i
})^{2}] Ã¶ ^{½}
Ã¨ N – 1 Ã¸
Questions/Analysis:
1. What potential experimental
errors did you and your partner(s) decide were the most significant? Explain why these caused the most worry. Do not just say something like “human error”
or “calculation error;” but be detailed and thorough.
2. Is it possible to “minimize”
errors in this experiment? Why or why
not? (Think about your equipment) What did you try in order to minimize
uncertainty on time measurements?
3. Why is it important to be
able to accurately determine
speeds? Think in terms of everyday
events or situations; try to come up with two examples.
4. Why is it important to be
able to precisely determine speeds
(i.e. measure down to more decimal places)?
Think in terms of everyday events or situations; try to come up with two
examples.
5. Make a graph of distance (y)
versus time (x) using Excel. Find the bestfit line and R^{2} value
for your data. Then, put on error bars
in the time dimension (this will likely have to be done by hand after printing
out your graph. Don’t forget to label
axes, include units on the axes, and so on, when making a graph). The error bars for time will make use of the
standard deviation for that particular set of time trials. It is OK for different points on a graph to
have different sized error bars; in fact, you should expect to have different
sized error bars for each data point.
6. If you started your marble
at the same height on the ramp each time you can assume it is going the same
speed when it rolls off on a flat surface.
When you made your graph of distance versus time, this is why you would
expect a straight line. Are all your
points on the line? Don’t feel bad if
they are not; rarely will data lie on the same line. Determine
and report the “best” speed of your marble as well as estimates of uncertainties on your speed result from your graph,
using max and min slope lines as determined by your error bars. In class, you will see an example of this.
7. Calculate the uncertainty for your speeds by
using the quadrature method for
independent measurements (it is safe to
assume that distance and time measurements
are independent of each other and random
since you’re using different measuring
devices for each). Do
this for each individual point of
distance and time. How do
these uncertainties compare to those from
your graph, using the max and min lines?
dv = v
[(dx/x)^{2}
+ (dt/t)^{2
}] ^{½} , where dt is
the standard deviation for that set
of times, and dx = 1 cm = 0.01 m.
Note that dx/x
and dt/t
are called the fractional uncertainties of the distance and time
measurements.
We are adding the
fractional uncertainties in quadrature (looks a lot like Pythagorean theorem!).
Grading:
Title (always have this) 1
point
Purpose (always have this) 1
point
Materials and Methods (Procedure) 3 points

procedures should be written such that someone who has not done this before can
recreate your lab
Data and Graph 5
points
Analysis Questions 10
points
Above and Beyond: If interested in learning the theory behind leastsquares fits to linear data, look here.
Lab Activity: Air Friction
Purpose: You will
investigate how air friction causes terminal velocity using coffee filters. Part of this will include Interactive Physics computer simulations
for multidimensional motion and air friction; this program is only on school
computers.
Materials: Meter stick Stop watch and/or video Coffee Filters
For your report:
You will need purpose; materials; data; and analysis sections for your
writeup. It is always a good idea to organize data in tables so they are clear
and neat, and include units on all measurements and results.
Keep in mind the BIG IDEA is that air friction (and friction in fluids in general) depends on how fast
you move, f_{air} = kv, where k is a positive constant.
Read through each analysis part below carefully, because it
will guide you through what we are looking for.
Write things up using complete sentences. I recommend Google Docs for your report (just
need a single report for the group).
Procedures:
Make some predictions prior to actually measuring the
terminal speeds of the falling coffee filters.
Question: Does
mass affect the terminal speed?
You can
control the mass by using different numbers of filters.
Predict: What
should happen to terminal speed as the mass increases?
Do it…make an appropriate data table with terminal velocity
as a function of mass. Do several time
trials and include standard deviations. Bonus: Determine the
uncertainties on the terminal velocity results. You will need to do this using propagation of
uncertainties as we have done in the past; see Above and Beyond below.
Do your best to estimate how long it takes for the filters
to reach terminal velocity upon release.
You’ll probably want to drop the filters from 23 meters high, so you
get terminal velocities.
Questions/Analysis:
1. Determine
the terminal speeds for at least five different masses of coffee filters. Estimate all measurement uncertainties and
record those with your data.
Above and Beyond: This includes using the quadrature method for
determining dv
values for each terminal speed. Remember
units are important on data and results.
dv = v
[(dt_{
}/ t_{avg})^{2} + (dd / d)^{2 }] ^{½}
You and your partners need to come up with a reasonable
estimate of uncertainty on the distance that the filters will fall; think of
how well you can read the metersticks.
2. Use
your measurements of terminal speed to determine values for k. Include these in a data table. What are the units of k?
3. Write
concise conclusions of what your data suggest about the effect of mass on
terminal speed. Make a graph in Excel of terminal speed as a function of mass (#
filters) from your data. Use as large a
range of mass as possible, up to a point where it does not have a measureable
terminal speed (where it continues to accelerate before hitting the floor).
4. Sketch graphs (i.e. do not need numbers
on the graph) of velocity as a function of time and acceleration as a function
of time. Put graphs for different masses
on the same set of axes so you can show a comparison of the effect of mass on
terminal velocity and acceleration. Use different colors, or soliddasheddotted lines, to distinguish the different graphs.
5. Do
a few trials for the other sized coffee filters, and draw any conclusions about
how the size of coffee filters affect the terminal velocity. Explain/support your conclusions in terms of
data and observations. Try to do this by
holding mass constant between the filters as best you can.
6. For
two of your small coffee filter examples, determine the percentage of kinetic
energy that is lost due to air friction. Hint:
think about how fast a filter should land if there is no air friction, and
compare to your terminal speed at which it lands.
7. Log
into your school account. Unfortunately, Interactive Physics is not online, only on school computers. Go to Programs, and go into the Science group of programs. You should find Interactive Physics. Go into IPFiles,
and then Physics Experiments. In that folder find the Air Resistance folder. There
should be 4 computer simulations, and run all four.
In each one, you can select
different values of k. In some you can change mass, and in some you can change
surface area. Run a series of controlled
computer experiments for each simulation, and write summaries of
observations/measurements and your conclusions about the effect of the
various parameters on the trajectories of projectiles when varying air
friction, terminal velocity, and so on.
Are these computer experiments consistent
with what you see with the coffee filters?
Explain.
The point of all this is to gain a good conceptual understanding of what air friction is all about,
and gain a better understanding of the complexity of reality, as opposed to the
‘physics land’ we tend to visit in most problems. Still, keep in mind that we
are using a highly simplified model for air friction, and reality is still quite
a bit more complex than we are treating air friction for things like cars,
planes and rockets moving through the atmosphere (aerodynamics). Aerospace engineers need to deal with the
complexities in a major way. J
Lab Activity  Simulate Halflife with M&Ms
Radioactivity  Mmmm & Mmmm, Good
Purpose:
Radioactivity was a phenomena that helped change the nature of science as the
twentieth century began (helped lead to fundamental changes in chemistry,
nuclear physics, biology, archeology and geology). And one of the most important ideas to come
from the studies of radioactivity is the concept that it is a statistical and probabilistic process. This means that one can apply statistical and
probabilistic methods to the study of radioactivity, and one of those
statistical measures is the halflife.
You will begin to gain understanding of halflives by using some pretty
sophisticated equipment.
Equipment: M & M’s (pretty sophisticated, huh?!)
Procedure:
Work
your way through the following questions with your M & M’s; choose one side
of your M & M’s to represent a ‘living’ radioactive atom, and the other
side to represent a decayed atom that will be removed from your sample. Dump your ‘living’ pieces on your table and
remove those that have just decayed. Continue this until all have decayed.
Questions and Analysis:
1.
Make a
data table to keep track of how many M & M’s decay each time you dump them
on the table. Make sure to count the
original number of pieces before you begin.
After all the pieces have decayed, make a graph of the number of pieces
left after each ‘halflife’ versus the number of the turn you dumped them. Is the shape of your graph linear? If not, how would you describe it?
2.
The
halflife for carbon14 (this is an isotope of normal carbon12; it just has
two extra neutrons that make it radioactive) is about 5700 years. What does this mean if you have a sample of
1000 carbon14 atoms? What does this
mean if you have a single carbon14 atom?
3.
Make a graph of the number of carbon14 atoms (suppose you have an
initial
sample of 1000 atoms) as a function
of time, knowing the halflife is 5700 years.
For an initial sample of 1000
carbon14 atoms, approximately how long would it
take to have only 100 carbon14
atoms remaining from the original sample?
Make your approximation from
your graph. Then, calculate it
from the decay law:
N = N_{o}e^{
t / }^{t}
where t = t _{½} / ln2; this is sort of like
an average time that is used for radioactivity
and is different for each radioactive
material. Work together to figure this
out, just
like a biologist or archeologist
would have to do. Some call Ï„ the decay constant or
time
constant for the material being considered.
We will see time constants again for
certain types of circuits in E&M.
4. Explain how scientists can
use carbon14 as a way of measuring the age of bones.
Would you trust carbon14
dating for objects that may be millions of years old?
Why or why not?
5.
Name
three other phenomena, events, activities, etc., that require probability to
describe them. Also, in your own words, define “probability.”
Above and Beyond: If curious, check out details about radiometric dating techniques and the math behind those techniques.
Lab  Video Game Physics Analysis
Lab: Video Game Physics Analysis
Purpose:
The research question is: does your video game follow
physics phact or phiction? You and a partner will need to play about 1020
seconds of an online video game that involves some type of collision, and then
analyze the motion to determine speeds, accelerations, and whether energy and
momentum are conserved.
Materials:
Students will need a computer with Internet access. Students also need an analysis program such
as Logger Pro or Tracker, which will be used to better analyze features of the
game being played. A screencast video of
the student playing the game will be made using the online program called Screencastomatic.
Background:
Using video technologies of all kinds have opened up new
ways of making connections between the physics we study and any phenomenon that
can be captured on video, including video games. Any game that has moving objects,
interactions between objects, projectiles, orbits, collisions, and any other
physical event, can be analyzed to see if our real laws of physics are being
used to create the video game environment, or if the game is simply, for lack
of a better word, being ruled by nonsense as far as the physics is concerned.
By measuring distances, times and paths, and estimating
masses, and scaling them by some appropriate scaling factors to better match
them to real object and events, you will be able to see if those measurements
and results are realistic, or if your game is being played in some other world
or parallel universe, with different laws of physics. You will be able to calculate speeds, accelerations,
forces, momentum, and so on.
PreLab: What do you
need?
You will need to make sure you have Internet access on your
computer. This will allow you to use
Screencastomatic (found at http://screencastomatic.com/),
as well as access an online video game.
It is possible to analyze a video game without additional
software, just by using the screencast video directly and playing it
framebyframe. You would need a small
ruler and possibly a protractor for 2D interactions to make measurements
directly on the computer monitor.
However, a more accurate analysis can be done with other
software, such as Vernier’s Logger Pro or Tracker, which is a free, open source
piece of software off the Internet. You
can download Tracker at http://www.cabrillo.edu/~dbrown/tracker/,
if you choose to use it for your lab.
Procedures:
I.
Set up Screencastomatic:
Go to the Screencastomatic site, http://screencastomatic.com/, and
simply click on Start Recording. There
is nothing to download. When you get a
small setup window, the default is to record the whole screen, which will be
fine.
To start recording, all you will need to do is type ALTP (press these two buttons at the
same time), and you will get a countdown to recording. That’s it!
II.
Before typing ALTP, use a different tab/window in your Internet browser (this
should work well in either Chrome or Firefox) to go to a site of a favorite video game. Be sure the video game has motion and
interactions between multiple objects, because that makes for a more
interesting and useful analysis and discussion of how realistic the game is. For example, one of my favorites is a game I
grew up with, Asteroids. There is a free
online version at http://www.play.vg/games/4Asteroids.html,
if you are interested.
III.
Start Recording! All you need to do at this point is type ALTP, wait for the countdown, and then
when Screencastomatic tells you it is recording start playing the game. All you want to record is about 30 seconds –
that will be more than enough for an analysis sample. When you have what you want, type ALTP again to pause Screencastomatic.
IV.
To complete the screencast video, after
pausing Screencastomatic, you should get a window with an option to be Done with the screencast. Click on Done.
This will take you to a screen to
publish the video. It is best to choose Publish to Video File. This will then give you some options. First is the type of file you want. It is recommended to use the default option,
a QuickTime MP4 file. This will work
with other analysis software if you have it.
Keep the size Full Size (the default setting). Then click on Save Video.
You
will be asked where to save the file and to name the file. It might be easiest to save on your Desktop,
or start a folder for video files – that is all your choice. Choose an appropriate name for the file, and
then save it! Congratulations, you just
made a screencast video, and hopefully this entire process took no more than
about 5 minutes!
I.
Play and Analyze the video. Doubleclick the video file to make sure it
works and that you captured the action you wanted. You can always go back through this procedure
as many times as you want, with any and all video games you enjoy playing.
KEY GOAL: Determine with your partner if energy and momentum
are conserved? Is your game following
physics phact or phiction??
If you want to do a ‘quick
and dirty’ analysis and have no access to other analysis software, then
by pausing the video you can scroll it framebyframe. Each frame will be onetenth of a second of
time, so this is the time step we will use for your analysis. Use a small ruler and set a reasonable
distance scale for your video game.
For example, on my Asteroids video, I assumed the spaceship
was a oneperson ship that was 10 meters in length (about 30 feet seemed a
reasonable, realistic size – this is 1/5^{th} the length of a full
space shuttle, and I assumed a single, small fighter spacecraft for the
game. See Wikipedia.). Below is how one would analyze the Asteroids
video:
Measure the length of the ship
with your ruler. Let’s say the ship is 1
cm on your monitor. So the scale is 1 cm
of the video = 10 meters in reality; this is no different than a scale on a
map, where 1 inch = 10 miles, for example.
The large asteroids flying around might be 2.5 cm on the monitor, so
this would make them large rocks of about 25 meters in diameter. You’ll
need to set a scale for any video game, to find lengths, distances, masses,
speeds, etc.
I need the masses for each
object. Here we may need to do some
research. I looked up the mass of the
space shuttle on Wikipedia, and it is listed as about 2000 tonnes = 2 x 10^{6}
kg. Onefifth of this would be 4 x 10^{5}
kg, which is the mass of the spaceship I would use.
For a large asteroid in the game,
I wanted a good, realistic estimate for the mass of a 25 meter asteroid. I
looked up 'Asteroid' on Wikipedia and found that this is a reasonable size, as
the vast majority of asteroids are under a 100 meter diameter. The data for density of some asteroids
averaged around 3 g/cm^{3} = 3000 kg/m^{3}, so assuming a 25
meter diameter (volume of a sphere is (4/3)Ï€R^{3}) spherical asteroid,
this gives a mass of around 2.5 x 10^{7} kg.
We would need to do a similar
estimate of mass for smaller asteroids, when they are blasted apart in the
game. The diameters would need to be
measured, and then scaled in a similar way to get the masses.
Now, think about the physical
quantities that can be measured/calculated.
Speed = d/t, so suppose you run move the paused screencast video 10 time
steps. This would be one second of real
time. You measure one of the asteroids
moving 1 cm on the monitor in that time.
This means the asteroid moved 10 meters in one second, and its speed is
10 m/s. You can measure the speed of the
bullets the spaceship shoots in a similar manner, or the speed of the ship if
you have it drifting around. Or suppose
the ship is at rest on the screen, and you accelerate in a straight line. You can measure how many cm it moves in a
certain number of time steps, and then calculate the acceleration (assume a
constant acceleration) with d = ½ at^{2}. You could also calculate its final speed once
the acceleration ends.
When you blast an asteroid, if you
know its initial speed and direction (you can measure angles with a protractor
on the monitor or with the analysis software), you can measure the final speeds
and directions of the smaller chunks of rock – determine if momentum was indeed
conserved.
With the masses and speeds,
calculate the momenta of each object, and the kinetic energy of each
object. Is momentum conserved when you
shoot an asteroid? Is energy
conserved? If not, is the change in
kinetic energy negative or positive?
Calculate the gravitational forces
between different objects. Are those forces relevant? That is, are they strong enough to affect
each other’s motions?
How much would you weigh if you
stood on one of the asteroids?
Would you be able to jump off the
asteroid by exceeding the escape velocity with your jump?
What would the acceleration of
gravity be on one of the asteroids?
How does the mass and speed of one
of the alien spaceships compare to your spaceship?
Do all of these calculations and
see what the results are!
Summarize
your results: how realistic are they?
Does the video game seem to follow more
closely to phact or phiction?
For teachers, you may want to specify a game, or at least a type of game, and have students focus on figuring out if one or two physical quantities are realistic or not, such as is energy conserved, or are speeds and accelerations reasonable, rather than having students look at numerous quantities.
Have phun!! J
Wednesday, August 6, 2014
Lab Activity: Distance vs. Displacement Vector
Goal: To find
the total distance and displacement vector to your house from ETHS!
Problem:
Here is the
question for you – how can you determine the total distance you walk from ETHS
to your house, as well as the displacement vector from ETHS to your house? Think about what information you need to find
these, and how can you use Google Maps to help. ETHS is 1600 Dodge Avenue.
How:
On graph paper, make a map (to scale) from ETHS to your house and use data from Google
Maps to get the total distance and displacement vector. These will all be different for each person
in class. Be sure to define your scale,
such as 1 cm on the map is equal to 100 feet, or whatever you want it to be for
your map. Also orient your map with N,
S, E, W, so you can calculate the angle of the displacement vector from ETHS to
your home! Give your angle in terms of
how many degrees from 0degrees, which is the positive xaxis. Let ETHS be the origin of your graph.
Analysis/Questions:
1. In
your own words, what is the distinction between ‘distance’ and ‘displacement?’
2. Write
down your results for distance and displacement vector measurements from your
map. You’ll need the total x component
(along EastWest axis) total ycomponent (along the NorthSouth axis) of your
displacement vector, as well as the direction (i.e. angle) of the displacement
vector from ETHS to your house. Express
the results in feet, meters and miles.
1 mile = 5280 feet = 1609.3 meters
3. Can
the magnitude of displacement ever have a larger magnitude than distance? Explain.
4. Explain
how displacement vectors would be used in the navigation of ships or planes.
5. What
about global travel – motion over a 3D surface – does anything change? If so,
what new variables do you need to consider? Explain any thoughts on this (talk
it through with others if you wish). Think of the Sphereland movie.
Bonus: How do space scientists and engineers navigate space probes
that are sent to other objects in the solar system? For instance, what
navigation issues and considerations come up with sending people to Mars?
Lab Activity: Use of Pendulum to Find g and Mass of Earth
Purpose: In this activity, you will determine the
acceleration of gravity and the mass of the Earth using a simple pendulum.
Background: A pendulum is a simple device that exhibits
periodic motion when the bob is lifted and given potential energy. Gravity does work on the bob and transforms
the potential energy into kinetic energy.
As potential energy and kinetic energy take turns transforming into each
other, the resulting motion is of a periodic nature. The period of a pendulum, which refers to the
time it takes to swing back and forth once (a full swing), is related to the
length of the pendulum and the acceleration of gravity (this assumes a ‘small
angle’):
Period = T = 2p(L / g)^{1/ 2}
Materials:
Stop
watch or video (phone, camera)
endulum
Meter
stick Your brain
Procedure:
You and
your lab partners need to determine an accurate way of determining the
acceleration of gravity, from which you will also calculate a value of the mass
of the Earth! You will need to do this
using a pendulum, which you know something about because of your last
experiment in determining a pendulum’s properties. Decide
what the best measurements and methods are, as well as how to minimize any
potential errors. Then, write down your
stepbystep procedure, and clearly and neatly present your data.
Analysis (answer in complete sentences):
1.
Based on your
measurements, determine the acceleration of gravity, g. Show your calculation.
2.
Using your result
in (1), calculate the percent error relative to the accepted g = 9.81
m/s^{2}. Recall that %error =
{[your value – accepted value] / [accepted value]} x 100.
3.
Explain, in your
own words, why the period of a pendulum is independent of the mass of the bob.
4.
What should the
period of your pendulum be if you performed this experiment on the moon
(gravity is about 1/6 that of the Earth)?
What about on Jupiter (gravity is about 20 times that on Earth)? Is this a direct or inverse relationship
between T and g?
5.
What are the main
sources of error in this particular activity?
How did your group try to minimize those errors?
6. Explain what a Foucault pendulum is
used for. You may need to look this one
up.
7. How long would you need to make a pendulum
in order for it to have a period of 1.0
second?
Show your calculation.
8. From your value of g, determine the mass of
the earth! Show your calculation. Other
useful information may include the radius
of the earth and G.
G = 6.67 x 10^{11} Nm^{2}/kg^{2}
and R_{E} = 6.4 x 10^{6} m.
*Note that by knowing the size (radius) of other
moons and planets, you could take a pendulum, measure the period, and determine
their masses, too. Sensitive
measurements of a pendulum’s period anywhere on earth also allows us to
determine small difference in g, which tells us something about how the radius
changes as well as densities of the earth at those locations.
Tuesday, August 5, 2014
Lab Activity: Develop Empirical Formula for Ohm's Law
Purpose: Using measurements taken in class, you will
develop an empirical formula for Ohm’s
law.
Background: You don’t get any this time since it might
“spoil the surprise.”
Materials:
Power supply Resistors
Multimeter/Ammeter Wires
Breadboard
Procedures:
(A) The
data you want consist of measurements of resistance, voltage and current, the
“Big 3” of electricity. Set up a data
table where you will have a single and constant resistance
value. Select a voltage value and then
measure the current. Do this for 5
different voltage values, and don’t go over 4 volts and do not max out the
ammeter. You will be making a graph of voltage vs current. Remember that resistors get hot
after a while!
(B) Change the
resistance value. Select a voltage and
record the current.
After turning down the voltage,
select another resistance value. Set the
power supply back to the same voltage (bustobus) you just had, and
record the new current. Do this for at
least 4 more resistance values, and reset to the same bustobus voltage value
each time. Try predicting what current
you will measure as you change resistance values.
Measure the
diameter and length of the resistor material (the cylindrical portion of the
resistor, and not the metal legs); use the digital micrometer for this. You will need these measurements for the last
analysis question.
Analysis:
1. Make
a graph of your data in part (A) above.
The graph should be one of Voltage (y) versus Current (x). Also make a graph of your data in part (B)
above. This graph should be one of
Current (y) versus Resistance (x). Make
your graphs using Excel, and find the bestfit functions for each graph.
2. What
shape are your graphs? What type of
relationship exists between voltage and current for a constant resistance? Between current and resistance for a constant
voltage? Find the bestfit functions to
your graphs.
3. Based
on your data and your graphs, combine the two bestfit functions from your
graphs and write down an empirical equation that shows how current depends on
resistance and voltage.
4. For
the following arrangements of resistors below, calculate the total resistance. (you’ll
get these in class)
a. b.
5. Why
do electronic devices get hot? Where
does the energy come from that you eventually feel as heat?
6. Based
on your observations, what role might resistors have in electronic
circuits? After all, resistors waste
energy and increase your power bill…why use them?
7. From
a chemistry perspective, why are conductors and insulators so vastly
different? What is it about these
various materials that make their electrical properties different? Also, from chemistry, what are
semiconductors, and why are they important to electronics? Discuss in terms of band theory.
8. In
your own words, why is the relationship E = dV/dr the key to understanding how
an electric circuit works? Be thorough,
and write in terms of potential and electric fields, and what they do to
delocalized electrons inside the wires/conductors of the circuit. Start with the
battery/power supply of the circuit.
9. Using
the diameter and length measurements of the resistor, along with the actual
resistance value, calculate the resistivity
of the material used to make the resistor.
Include appropriate units.
On your own:
PostLab
Run the following PhET computer simulations, in order to get
some good visuals related to what you directly observed and measured in this
experiment. The simulations are:
Ohm’s law  http://phet.colorado.edu/en/simulation/ohmslaw
Basic
Circuit  http://phet.colorado.edu/en/simulation/batteryresistorcircuit
Look at the Netlogo simulation for an electron moving
through material. Follow a single electron, and note the path. What defines ‘more
resistance’ versus ‘less resistance’ for an electron?
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