# Ohm's Law - Genecontm Version

## Purpose:

• to perform an experimental check of Ohm's Law
• to practice constructing electric circuits
• to practice using an ammeter and a voltmeter

## Discussion:

In this lab, you will construct a simple circuit using a single known resistance, R. Then you will use an ammeter to measure the current (symbol = I) through the resistance and a voltmeter to measure the potential difference, V, across the resistance. With this data, you can check the validity of Ohm's Law (V = IR) in the circuit.

## Genecontm generator 3 connecting wires 1 - 3 , 5 Watt resistor 1 - 5 , 5 Watt resistor 1 - 10 , 5 Watt resistor 1 - 25 , 5 Watt resistor 0-1 A ammeter 0-3 V voltmeter assorted small objects

### About the Genecontm:

The Genecontm is a small hand-operated generator. When you turn the crank, the work that you do is converted into electric potential energy, which creates a potential difference (voltage) between the two alligator clips at the ends of the clear wires. The AC output voltage of the generator is converted to DC by a network of diodes inside the plastic housing, so the Genecontm produces a DC voltage.

## Construct the Circuit:

1. Construct and test the main circuit, consisting of the Genecontm, the ammeter, and the 3 resistor. Here's how:
1. Clip one end of the black wire to one of the terminals of the ammeter.
2. Clip the other end of the black wire to one of the wires on the 3 resistor.
3. Clip one of the Genecontm clips to the other end of the 3 resistor.
4. Clip the other Genecontm clip to the remaining terminal of the ammeter.
5. Testing:
1. Notice that if there is a voltage between the Genecontm clips, charges can flow through the resistor, through the black wire to the ammeter, through the ammeter, then back to the Genecontm. Since the ammeter measures current through the circuit, it must be placed in the circuit to do its job.
2. When you turn the crank on the Genecontm, you should get a reading (current in Amperes) on the ammeter. If the ammeter needle deflects to the left, turn the crank the other way.

2. Add the voltmeter to the circuit. Here's how: IMPORTANT! The picture at the right shows only the voltmeter connections that you should add to the circuit above. DO NOT remove the wiring from step 1! The completed circuit is shown below.
1. Connect a green wire from the one terminal of the voltmeter to the wire on one side of the resistor.
2. Connect a second green wire from the other terminal of the voltmeter to the other wire on the resistor.
3. Testing:
1. Notice that the voltmeter is not placed in the circuit like the ammeter. The voltmeter's job is to measure the potential difference across the resistance, so it just connects across the relevant points in the circuit.
2. When you turn the crank, you should observe both a current reading on the ammeter and a voltage reading on the voltmeter (voltage in Volts). If the voltmeter needle deflects to the left, switch the wires on the voltmeter terminal.

3. When both the ammeter and the voltmeter deflect correctly when you turn the crank, you are ready to make some measurements. The completed circuit is shown at right.

## Procedure:

Part I. In the first part of this experiment, you will measure the voltage and current across some known resistors placed in your circuit. You can then compare the measured current to the current predicted by Ohm/s Law (I = V/R) for the resistance and voltage in your circuit.

1. Construct a data table similar to the sample data table shown below.
2. Turn the crank on the Genecontm. It will take some practice and a steady hand to get relatively stable current and voltage readings!
3. Carefully read the voltage across the resistor and the current through the resistor, and record them in your data table.
4. Replace the 3W resistor with the 5 resistor, 10 resistor, and finally the 25 resistor and record the voltage and current in the circuit for each resistance.

Part II. In this part of the experiment, you will attempt to determine the resistance of (at least) four common objects by placing them in your circuit, measuring the voltage and current in the circuit, and applying Ohm's Law. A circuit to measure the resistance of an eraser is shown at right.

1. Construct a data table in which to record the name of your object, the voltage and current in your circuit, and your calculation of the object's resistance. If you have access to an ohmmeter, make a column in which to record the measured resistance of the object.
2. Put your object in the circuit in place of the resistor from part I.
3. Measure and record the voltage and current in your circuit.
4. Replace your object with something else and repeat.

## Results:

### Part I.

1. For each trial, calculate the expected current based on the resistance and measured voltage using Ohm's Law. Record your results in your data table, and be sure to show a sample calculation.

2. For each trial, calculate the percent of difference between the calculated current and measured current in the circuit. Record your results in the data table. Show a sample calculation.

### Part II.

1. For each trial, calculate the resistance of each item that you tested using Ohm's Law (V = IR) and record your results in your data table.

## Conclusions:

1. When the resistance in the circuit increased, what happened to the current in the circuit?

2. Do you think that your circuit follows Ohm's Law? Why or why not?

3. How well were you able to determine the resistance of your objects in part II?

last update May 7, 2007 by JL Stanbrough