# Series & Parallel Resistances

## Purpose:

• to determine the equivalent resistance of two resistors in series
• to determine the equivalent resistance of two resistors in parallel
• to practice constructing electric circuits
• to practice using an ammeter and a voltmeter

## Discussion:

In this lab, you will construct simple circuits using combinations of known resistances in series and in parallel. Then you will use an ammeter to measure the current, I, through the resistor arrangement and a voltmeter to measure the potential difference, V, across it. With this data, you determine the equivalent resistance of the arrangement using Ohm's Law (V = IR) and compare this to the separate resistor values.

## 1.5/2 V power supply "knife" switch 2 - 3 , 5 Watt resistors 2 - 5 , 5 Watt resistors 2 10 , 5 Watt resistors 2 - 25 , 5 Watt resistors 0-1 A ammeter 0-3 V voltmeter 5 connecting wires Procedure:

### Resistors in Series:

1. Choose 2 resistors R1 and R2, and enter their values in the data table.
2. Predict the equivalent resistance of these two resistors in series, and enter your prediction in the data table.
3. Wire this series resistor combination into the circuit shown below.
4. Record the current through the resistor combination and the voltage across it in the data table.

### Resistors in Parallel:

1. Choose 2 resistors R1 and R2 of equal value, and enter their values in the data table.
2. Predict the equivalent resistance of these two resistors in parallel, and enter your prediction in the data table.
3. Wire this parallel resistor combination into the circuit shown below.
4. Record the current through the resistor combination and the voltage across it in the data table.

### The Circuit:

IMPORTANT: In this lab you will use ONLY the "COMMON" and "1.5/2 VDC" terminals on the power supply. Connecting the circuit to any other terminals will certainly result in destruction of equipment and might well be hazardous to you and your lab partner. This will NOT be treated as a "harmless prank". Disciplinary action will be taken, and you will be responsible for damaged equipment.

This lab uses the circuit depicted in the schematic diagram shown at right. This is essentially the same circuit you constructed for the Ohm's Law lab - the only difference is that you will have two resistors in the circuit instead of just one. Since you are probably not comfortable converting an electrical schematic into an actual circuit, here's how to set it up:

1. Screw one end of the resistor combination to the 1.5/2 VDC terminal of the power supply.
2. Connect the other end of the resistor combination to the red terminal of the ammeter (labeled D.C. Amperes on the meter face).
3. Be sure that the switch is open.
4. Using another wire, connect the black terminal of the ammeter to either side of the switch.
5. Connect the other switch terminal to the COMMON terminal on the power supply using a wire. Notice that when the switch is closed, current will flow through the resistors, the ammeter, and the switch in this circuit.
6. Connect a wire from the red terminal of the voltmeter to the 1.5/2 VDC terminal of the power supply.
7. Connect a wire from the black terminal of the voltmeter to the red terminal of the ammeter.

STOP!! Check your circuit before you close the switch!

## Results:

1. For each trial, calculate the equivalent resistance of the resistor combination based on the measured current and voltage using Ohm's Law. Record your results in your data table, and be sure to show a sample calculation.

## Conclusions:

1. How do you find the equivalent resistance for two resistors in series?

2. How do you find the equivalent resistance for two resistors in parallel?

last update May 14 , 2006 by JL Stanbrough