Applying Ohm’s Law

Purpose

To understand the relationships of amps, volts and ohms in an electrical circuit.

Materials

two multimetimeters, a rheostat, resistor 1 (R1) a 10 ohm, 10 watt resistor, resistor 2 (R2) a 20 ohm, 10 watt resistor, three connecting leads (with alligator clips at each end), a switch, a voltage source. (The voltage source could be a dry cell, 9 V battery, a car battery charger (6V) or a power pack.)

Procedure

1. Construct the circuit according to the schematic diagram below.

2.R2 in the above schematic is the rheostat. Close the switch and adjust the rheostat. Record measurements of the voltage and current for six different rheostat settings. Make the first setting where the voltage and current are both zero. Record the readings below. Calculate the resistance in the right column by dividing the voltage by the current for each pair of readings.

Device                Setting                Voltage drop (V)        Current (A)        Resistance V/I =

R1                 1

                 2

                 3

                 4

                 5

                 6

3. Remove R1 from the circuit and replace it with R2. (Leave the rheostat as it was.) Close the switch and adjust the rheostat. Record measurements of voltage and current for six different rheostat settings. Again,include the first setting where the voltage and current are both zero. Record the readings below. Calculate the resistance in the right column by dividing the voltage by the current for each pair of readings.

Device                Setting                Voltage drop (V)        Current (A)        Resistance V/I =

R2                 1

                 2

                 3

                 4

                 5

                 6

4. Prepare a line graph plotting voltage vs current for the six pairs of readings for R1. (Voltage on the y-axis and current on the x-axis.) Draw a best fit line beginning at the origin.

5. On the same graph, plot the six data points for R2. Again, draw a best fit line.

Discussion questions:

1. What conclusions can you draw from the data?

2. Explain why the calculated V / I values for R1 are close to each other.

3. What was the average resistance calculated for R1? Remember that voltage / current for the first setting is undefined, so average the five remaining quotients.

4. What was the average resistance calculated for R2?

5. How does the calculated resistance for R2 compare to the calculated resistance for R1?

6. What do the slopes of the lines on your graph represent?

7. Calculate the slopes.

8. What is the function of the rheostat in this activity?

Photo: Potentiometer / CC-BY 2.5