Investigation of the characteristics of series and parallel circuits

Components, eg bulbs, may be connected in series or parallel in a circuit. The specified practical investigates the differences between the two types of circuits by observing the brightness of the bulbs, the effect of 'blowing' or removing a bulb.

Aim of the experiment

To compare the differences between a series circuit and a parallel circuit.

Method 1 - Basic observation

A circuit showing two lamps in series with an open switch at the top left and a cell in top centre.
A circuit showing two lamps in parallel with an open switch at the top left and a cell in top centre.

1. Set up the circuits with 2 bulbs in series.

2. Switch on.

3. Observe the brightness of the bulbs.

4. Remove (by unscrewing) one of the bulbs. Observe the effect on the other bulb.

5. Set up the circuit with 2 bulbs in parallel.

6. Repeat steps 2 to 4 with this circuit.

Method 2 - Voltage measurements

A circuit diagram showing a voltmeter connected to a cell and two lamps in series which are connected to a voltmeter each.

1. Set up the series circuit with 2 bulbs. Switch on.

2. Connect a voltmeter across each of the bulbs in turn and also the battery.

3. Record all of your measurements.

A circuit diagram showing a voltmeter connected to a cell with two lamps in parallel to each other and connected to a voltmeter each.

4. Set up the parallel circuit with 2 bulbs.

5. Connect a voltmeter across each of the bulbs in turn and also across the battery.

6. Record all of your measurements.

Analysis

  1. What conclusions can you draw from your measurements of voltage?
  2. What is the link between the brightness of the bulb and the voltage across it?

Method 3 - Current measurements

1. Set up the series circuit with 2 bulbs.

2. Connect an ammeter in each of the 3 positions shown in turn.

3. Record all of your measurements.

A circuit diagram showing a cell at the top, two lamps on the bottom in series with an ammeter in between them, and two ammeters either side of the circuit.

4. Set up the parallel circuit with 2 bulbs.

5. Connect an ammeter in each of the positions shown in turn.

6. Record all of your measurements.

A van de graaff generator has a motor at the bottom whilst is connected to a rotating belt. The belt has a positive charge when the motor is switched on, as does the dome at the top.

Analysis

  1. What conclusions can you draw from your measurements of current?
  2. What is the link between the brightness of the bulb and the current in it?

Evaluation

Placing the bulbs in series causes the resistance of the pair to be double that of a single bulb because there is only one path for the electrons to follow - the supply must drive current through one bulb and then the other.

Suggest a reason why placing the bulbs in parallel causes the resistance to be half that of a single bulb.

Risk assessment

HazardRiskControl measures
Heating of wires and bulbs Minor burns Set up circuit before closing the switch and switch off before moving the voltmeter or ammeter in the circuit
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