Ohm's Law - Adding Resistors in Series and Parallel



Lab #5: Ohm's Law - Adding Resistors in Series and Parallel

Copyright R. Mahoney 2013

Name: _______________

The purpose of this lab is to verify that resistors add when in series, and combine using the "one over" or "product over sum" rule when in parallel. We will also test if currents split according to the inverse ratio of the two resistors. We will also check to see if there is a difference between VL and VTPD.

1. Set up the following circuit:

[pic]

Use the following resistors: R2 = 1000 ohms, R3 = 330 ohms, R1 = 220 ohms. Use a power supply, not a battery, and dial in a voltage of 12 VDC.

1. With the switch open, measure the power supply voltage, terminal potential difference, VTPD = _______. With the switch closed, measure the voltage under load, VL = _______. Are they the same or different?

2. With the resistors out of circuit (one at a time), and using an ohmmeter, measure the resistors' resistances.

R1 = _______ R2 = _______ R3 = _______

3. With the switch open, and using a voltmeter, measure the voltage drops across R1, R2 and R3.

V1O = _______ V2O = _______ V3O = _______

4. Anything odd to report about the three voltages just measured?

5. With the switch closed, and using a voltmeter, measure the voltage drops across R1, R2 and R3.

V1M = _______ V2M = _______ V3M = _______

6. With the switch closed, and using an ammeter, measure the currents at points A, B, C and D. Be sure to hook your ammeter in series, and be sure to choose the right meter input jacks so as to not blow the meter's low current internal fuse.

IA = _______ IB = _______ IC = _______ ID = _______

7. Using the measured resistances (step 2), calculate the theoretical (or true) equivalent resistance of the circuit R EQ1 = _______.

8. Divide VL (step 1) by IA (step 6). This is the measured equivalent resistance,

or R EQ2 = _______.

9. Calculate the % error between R EQ1 and R EQ2. % error = _______.

10. Using the measured resistances (step 2), calculate the ratio of R2 over R3.

Ratio = _______.

11. Using the measured currents (step 6), calculate the ratio of IC over IB.

Ratio = _______.

12. Calculate the % error between the two ratios (from the last two steps; the ratio in step 10 is the true value). % error = _______.

13. Using the measured resistances (step 2), and the measured currents going through each resistor (step 6), calculate the voltage drop across each resistor.

V1C = _______ V2C = _______ V3C = _______

14. How does each calculated voltage drops compare to its measured value from step 5 (for example, V1C versus V1M)?

15. Add IB and IC and compare the result to IA , and to ID. What do you notice?

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