Purpose
Measure the resistance value of series and parallel circuits using an ohmmeter.
Concept
An ohmmeter measures resistance in units of ohms (Ω). It measures both the
current flowing through a circuit and the voltage drop across the circuit
and calculates the resistance using Ohm's law.
A series circuit has one path for the electricity to travel through. The total
resistance in a series circuit is the sum of the individual resistances.
A parallel circuit has more than one path for the electricity to travel through.
The reciprocal of the total resistance in a parallel circuit is equal to the sum
of the reciprocals of the individual resistors.
Procedure A. Series
1. Select the proper range on the ohmmeter. If the proper ranges are not selected, the meter can be destroyed.
2. Select three 100 Ω resistors and hook them up in series, as in the top diagram.
3. Measure and record the resistance value of the series circuit.
B. Parallel
1. Select the proper range on the ohmmeter.
2. Select three 100 Ω resistors and hook them up in parallel as in the bottom diagram.
3. Measure and record the resistance value of the parallel circuit.
C. Series and Parallel Combination
1. Select the proper range on the ohmmeter.
2. Using the formulas for series and parallel circuits, predict what the theoretical total resistance would be for a circuit that had two parallel resistors and one in series. Record you prediction.
3. Select three resistors and hook them up so that two are parallel and one is in series.
4. Measure and record the resistance value of the combination circuit.
Analysis
1. What is the theoretical total resistance for the circuit in part A? How did this compare to the measured value? Explain.
2. What is the theoretical total resistance for the circuit in part B? How did this compare to the measured value? Explain.
3. How did your theoretical predicted values in part C compare to the measured value? Explain.
4. Were your calculated values exactly the same as the theoretical values? What factor(s) could explain why or why not?
5. If the two headlights on a car are wired in parallel, and each headlight was 20 Ω, what would the total resistance for headlight circuit be? If the battery supplied 12 V, what current would the headlights draw?
