Lab #4: Ohm's Law - Adding Resistors in Series and Parallel
Copyright R. Mahoney 2013
Name: _______________
>> 101 students: do only steps 1 through 7. <<
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:
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 closed, and using a voltmeter, measure the voltage drops
across R1, R2 and R3.
V1M = _______
V2M = _______
V3M = _______
4. 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 to not blow the meter's internal fuse.
IA = _______
IB = _______
IC = _______
ID = _______
5. Using the measured resistances (step 2), calculate the theoretical equivalent
resistance of the circuit R EQ1 = _______.
6. Divide VL (step 1) by IA (step 4). This is the measured equivalent resistance,
or R EQ2 = _______.
7. Calculate the % error between R EQ1 and R EQ2. % error = _______.
8. Using the measured resistances (step 2), calculate the ratio of R2 over R3.
Ratio = _______.
9. Using the measured currents (from step 4), calculate the ratio of I C over IB.
Ratio = _______.
10. Calculate the % error between the two ratios (from the last two steps; the
ratio in step 8 is the true value). % error = _______.
11. Using the measured resistances (step 2), and the measured currents going
through each resistor (step 4), calculate the voltage drop across each resistor.
V1C = _______
V2C = _______
V3C = _______
12. How does each calculated voltage drops compare to its measured values
(for example, V1C versus V1M)?
13. Add IB and IC and compare the result to IA , and to ID. What do you notice?
14. State two non-trivial systematic errors for this experiment.