# Ohm's Law

## 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, 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.

## 1.5/2 V power supply "knife" switch 3 , 5 Watt resistor 5 , 5 Watt resistor 10 , 5 Watt resistor 25 , 5 Watt resistor 0-1 A ammeter 0-3 V voltmeter 5 connecting wires Procedure:

IMPORTANT: In this lab you will use ONLY the "COMMON" and "1.5/2 VDC" terminals on the power supply. Connecting the circuit to any other terminals will certainly result in destruction of equipment and might well be hazardous to you and your lab partner. This will NOT be treated as a "harmless prank". Disciplinary action will be taken, and you will be responsible for damaged equipment.
1. Set up the circuit depicted in the schematic diagram shown at right using the 3 resistor. Since you are probably not comfortable reading an electrical schematic, here's how:
1. Screw one end of the resistor to the 1.5/2 VDC terminal of the power supply.
2. Using one of the connecting wires, connect the other end of the resistor to the red terminal of the ammeter (labeled D.C. Amperes on the meter face).
3. Be sure that the switch is open.
4. Using another wire, connect the black terminal of the ammeter to either side of the switch. Notice that when the switch is closed, current will flow through the resistor, the ammeter, and the switch in this circuit.
5. Connect the other switch terminal to the COMMON terminal on the power supply using a wire.
6. Connect a wire from the red terminal of the voltmeter to the 1.5/2 VDC terminal of the power supply.
7. Connect a wire from the black terminal of the voltmeter to the red terminal of the ammeter.

STOP!! Have your teacher check your circuit before you close the switch!

1. Construct a data table similar to the sample data table shown above.
2. Close the switch.
3. Carefully read the voltage across the resistor, R, and the current through the resistor, I, 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.

## Results:

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 and measured current in the circuit. Record your results in the data table. Show a sample calculation.

## Conclusions:

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

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

last update May 14, 2006 by JL Stanbrough