## 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 (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:

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?