# Series & Parallel Circuits

## Purpose:

• to practice constructing electric circuits
• to observe characteristics of series and parallel circuits

## Discussion:

In this lab activity, you will construct a sequence of simple series and parallel circuits using small light bulbs.

## 1.5/2 VDC power supply 3 light bulbs & sockets 7 connecting wires 1 knife switch Procedure:

IMPORTANT: In this lab activity 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.

### Preliminaries:

1. Set up the circuit depicted in the schematic diagram shown at right using the power supply, switch, one light bulb, and 3 wires. Since you are probably not comfortable reading an electrical schematic, here's how:
1. Clip one end of a connecting wire to the 1.5/2 VDC terminal of the power supply.
2. Connect the other end of the wire to either side of the knife switch.
3. Be sure that the switch is open.
4. Using another wire, connect the other side of the switch to either screw on the light socket
5. Connect the third wire from the other screw on the light socket to the COMMON terminal of the power supply.
2. Close the switch. If the light bulb doesn't light, check your connections. If you can't get it to work, call your teacher.

### Series Circuits:

1. Open the switch. If the light doesn't go out, check your connections. If you can't figure it out, call your teacher.
2. Connect another light bulb in series with the first. To do this:
1. Be sure the knife switch is open.
2. Disconnect the wire at the COMMON terminal (only) and connect this end to either screw on the new light socket.
3. Connect a new wire from the other screw on the light socket to the COMMON terminal on the power supply.
3. Close the switch. How does the brightness of the two light bulbs in series compare with the brightness of the single light bulb?

4. Unscrew one light bulb. What happens?

5. Add a third light bulb in series with the first two. (Follow instructions in step 4.)
6. Compare the brightness of the three bulbs in series to the brightness of the original light bulb.

7. Unscrew one of the bulbs. What happens? Does it make any difference which bulb you remove?

### Parallel Circuits:

1. Remove three light bulbs from your circuit so that you have the original circuit from step 1 and 2 (a switch plus one bulb).
2. Connect a second bulb in parallel with the first. To do this:
1. Be sure the knife switch is open.
2. Connect a new wire from a screw on the original light socket to either screw on the new light socket.
3. Connect a new wire from the other screw on the original light socket to the other screw on the new light socket.
3. Close the switch. How does the brightness of the two light bulbs in parallel compare with the brightness of the single light bulb? How does it compare with the brightness of the two bulbs in series?

4. Unscrew one light bulb. What happens?

5. Add a third light bulb in parallel with the first two. (Follow instructions in step 2.)
6. Compare the brightness of the three bulbs in parallel to the brightness of the original light bulb, and to the brightness of the three bulbs in series.

7. Unscrew one of the bulbs. What happens? Does it make any difference which bulb you remove?

8. "It says in the book" that two light bulbs in parallel are just as bright as a single light bulb, but that probably didn't happen, did it? What are some plausible hypotheses for this discrepancy?

last update March 23, 2001 by JL Stanbrough