**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:**

**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**

*(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!***Change the resistance value. Select a voltage and record the current.**

*(B)*
After turning down the voltage,
select another resistance value. Set the
power supply back to the

__same__voltage (bus-to-bus) you just had, and record the new current. Do this for at least 4 more resistance values, and re-set to the same bus-to-bus 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 best-fit 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 best-fit functions to
your graphs.

3. Based
on your data and your graphs, combine the two best-fit 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:

__Post-Lab__
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/ohms-law

Basic
Circuit - http://phet.colorado.edu/en/simulation/battery-resistor-circuit

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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