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Name ________________
OHM'S LAW
Objective:
To verify Ohm's Law
Apparatus:
100 ohm and 220 ohm resistors, 1 ohmmeter, 1 ammeter (0-220 mA DC), 1 voltmeter (0-15
volts), 1 DC power supply, conducting wires, graph paper
Introduction:
Ohm's Law states that the voltage across a resistor is directly proportional to the current through
the resistor. This relationship is expressed by the equation:
V = IR
where V = potential difference across the resistor (volts)
I = current through the resistor (amperes)
R = resistance of the resistor (ohms)
Figure 1
Procedure:
1.
Measure the "100” ohm (brown-black-brown) resistor using the ohmmeter:
Ohms.
Measure the "220" ohm (red-red-brown) resistor using the ohmmeter:
ohms.
These are the actual and theoretical values that you will use in the lab.
2.
Connect the "100" ohm resistor in series with the ammeter and the power supply.
Connect the voltmeter across both the resistor and ammeter as shown in Figure 1.
3.
Do not turn on the power supply before the instructor has checked your
circuit!
4.
2
For safety, make sure that the voltage knob is turned to its lowest position. Then turn on
the power supply.
5.
Set the voltage to one volt. Read the ammeter. Record the voltage and current in
Table 1.
6.
Repeat step 5 for a voltage setting of 2 volts, 3 volts, etc. up to 8 volts. DO NOT
EXCEED 8 VOLTS!
7.
Record the voltage and corresponding current in Table 1.
8.
Convert the current readings from milliamps to amps.
Data Table 1 (“100” ohm resistor)
Voltage (volts)
Current (mA)
Current (A)
9.
Using Excel, plot a graph of Voltage (y-axis) vs. Current (x-axis).
10.
Find the best fit line and include it on the graph. From it, determine the slope of the line.
Slope of line, which is equal to the resistance R: __________ Ohms
11.
Calculate the percent error. A = Actual value, E = Experimental value.
% error = A - E X 100% =
A
3
12. Repeat steps 1-8 for the “220 Ohm” Resistor
Data Table 2 (“220” ohm resistor)
Voltage (volts)
Current (mA)
Current (A)
13.
Using Excel, plot a graph of Voltage (y-axis) vs. Current (x-axis).
14.
Find the best fit line and include it on the graph. From it, determine the slope of the line.
Slope of line, which is equal to the resistance R: __________ Ohms
15.
Calculate the percent error. A = Actual value, E = Experimental value.
% error = A - E X 100% =
A
4
Questions:
1. State in your words the exact meaning of Ohm's Law.
2. If we completed an experiment where the voltage was kept constant but we kept changing the
resistors, what changes if any would we see in the current? Explain your choices.