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DC Fundamentals
Ohm’s Law
Exercise 2: Ohm’s Law – Circuit Current
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to determine current by using Ohm’s law. You
will verify your results with a multimeter.
DISCUSSION
When voltage and resistance values are known, apply Ohm’s law to determine the circuit current.
I = E/R
CURRENT (IN AMPS) =
VOLTAGE (IN VOLTS)
RESISTANCE (IN VOLTS)
Based on the formula, current is directly related to voltage but inversely related to resistance.
In this circuit, the applied voltage is 10 Vdc, and the circuit resistance is 1000:. Based on Ohm’s law, the
circuit current is 10 mA.
In this circuit, the voltage is increased and the resistance is decreased. Based on Ohm’s law, what is the
effect on circuit current?
a. The circuit current must increase.
b. There is no change in circuit current.
c. The circuit current must decrease.
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Ohm’s Law
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PROCEDURE
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Locate the OHM’S LAW circuit block, and connect the circuit shown.
฀
Measure the total resistance of your circuit and enter your value in ohms.
RT =
:(Recall Value 1)
฀
Remove the meter from your circuit. Energize the OHM’S LAW circuit block with a two-post
connector.
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Adjust the positive variable supply to +10 Vdc.
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Ohm’s Law
Calculate the expected circuit current based on an applied voltage of 10 Vdc and a circuit
resistance of
:(Step 2, Recall Value 1). Enter your result in milliamps
(0.001A = 1 mA).
NOTE: I = E/R
IT =
mA (Recall Value 2)
฀
Set your multimeter to measure current.
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Remove the two-post connector and connect the multimeter between the positive side of the
voltage source and the top of resistor R1. Be sure to observe proper polarity.
฀
Enter the current reading of your circuit in milliamps.
IT =
฀
mA (Recall Value 3)
The calculated circuit current equals
mA (Step 5, Recall Value 2) and the
measured circuit current equals
mA (Step 8, Recall Value 3) . Based on a
comparison of the two values, is a calculation based on Ohm’s law a true indication of circuit
current?
a. No. Ohm’s law cannot be considered accurate.
b. Yes. Ohm’s law provides a true indication of circuit current.
c. Yes. Ohm’s law provides a true indication of circuit current but only when circuit resistances
and voltages are measured.
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Ohm’s Law
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DC Fundamentals
Using Ohm’s law, calculate the circuit current with a resistance of
Recall Value 1) and an applied voltage of 5 Vdc.
:(Step 2,
NOTE: IT = E/R
IT =
mA (Recall Value 4)
฀
Ensure that you change the applied voltage to 5 Vdc. Measure the current of your circuit.
Compare your meter indication to the calculated current of
mA (Step 10, Recall
Value 4) . Does Ohm’s law hold true when the circuit’s applied voltage is changed?
a. No.
b. Yes, provided the calculation uses the original circuit voltage.
c. Yes, provided the calculation uses the new circuit voltage.
฀
Place CM switch 20 in the ON position. Measure and enter in ohms the total resistance of your
circuit.
NOTE: Remove power from your circuit block when measuring resistance.
฀
฀
RT =
: (Recall Value 5)
IT =
mA (Recall Value 6)
Adjust the positive variable supply for 10 Vdc. Based on a circuit resistance of
:
(Step 12, Recall Value 5 ) and a circuit voltage of 10 Vdc, measure and enter the circuit current
(in milliamps).
Based on the tabulated results recalled below, which statement represents the relationship
between current, voltage, and resistance?
a. Current, voltage, and resistance are directly related.
b. Current, voltage, and resistance are inversely related.
c. Current is directly related to voltage but inversely related to resistance.
CURRENT
VOLTAGE
mA (Step 10, Recall Value 4)
5 Vdc
mA (Step 8, Recall Value 3)
10 Vdc
mA (Step 13, Recall Value 6)
10 Vdc
RESISTANCE
:(Step 2, Recall Value 1)
:(Step 2, Recall Value 1)
:(Step 12, Recall Value 5)
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Ohm’s Law
Make sure all CMs are cleared (turned off) before proceeding to the next section.
CONCLUSION
•
Ohm’s law in the form of voltage/resistance determines circuit current.
•
Circuit current is directly related to voltage. It increases if circuit voltage increases.
•
Circuit current is inversely related to resistance. It decreases if circuit resistance increases.
•
Voltage and resistance values are required in order to determine current by Ohm’s law.
REVIEW QUESTIONS
1. Voltage and current are
a. measured in the same unit.
b. inversely proportional.
c. directly proportional.
d. always larger than the resistance.
2. If the voltage in a circuit is doubled and the resistance remains the same, the current
a. remains the same.
b. is doubled.
c. is halved.
d. may increase or decrease.
3. In an electric circuit, if the voltage is 20 Vdc and the resistance is 4000:, the circuit current is
a. 5 mA.
b. 0.005A.
c. All of the above.
d. None of the above.
4. In an electrical circuit having a voltage of 8 Vdc and a resistance of 1.5 k:, the circuit current is
a. 5.3 mA.
b. 9.5 mA.
c. 12 mA.
d. 18.75 mA.
5. Place CM switch 19 in the ON position. Refer to the OHM’S LAW circuit block. Adjust the positive
supply to 7 Vdc. Determine the value of total circuit resistance and circuit current.
a. 510:, 13.7 mA
b. 1245:, 5.6 mA
c. 1510:, 4.6 mA
d. 3710:, 1.88 mA
NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section.
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