Suggested practical - Circuit components

There are different ways to investigate the relationship between current and potential difference for different circuit elements. In this practical activity, it is important to:

  • use appropriate apparatus and methods to measure current and potential difference
  • measure and record current and potential difference for a resistor, filament bulb and diode

Resistor and filament bulb

Aims

To investigate the relationship between current and potential difference for a resistor and a filament bulb.

Method

Circuit with a battery, variable resistor, resistor, ammeter and a voltmeter connected in parallel to the resistor.

  1. connect the circuit as shown in the diagram
  2. ensure that the power supply is set to zero at the start
  3. record the reading on the voltmeter and ammeter
  4. use the variable resistor to alter the potential difference
  5. record the new readings
  6. repeat steps 3 to 5, increasing the potential difference slightly each time
  7. repeat steps 1 to 6, but with the power supply reversed
  8. repeat the experiment, but replace the fixed resistor with a bulb

Results

Record the results in a suitable table. This one shows some example results for a 10 Ω fixed resistor.

Potential difference (V)Current (A)
0.50.05
1.00.10
1.50.15
2.00.20

Analysis

Plot a graph of current against potential difference for each component.

Graph plotting potential difference against current for a fixed resistor. Line is directly proportional.An I-V graph for a fixed resistor at a constant temperatureGraph plotting potential difference against current for a filament bulb. Line is an upward curve, that levels out and start to dip and potential difference increase.An I-V graph for a filament bulb

Evaluation

For a fixed resistor, the potential difference is directly proportional to the current, so doubling the amount of energy into the resistor results in the current in the resistor being doubled.

This relationship is called Ohm's Law and is true because the resistance of the resistor is fixed and does not change. A resistor is an ohmic conductor.

In a filament bulb, the current does not increase at the same rate as the potential difference. Doubling the amount of energy does not cause a doubling of current. The more energy that is put into the bulb, the harder it is for charge to flow – the resistance of the bulb increases. As the potential difference increases, so does the temperature of the thin wire inside the bulb, the filament. This increases the resistance of the filament.

Diode

Aims

To investigate the relationship between current and potential difference for a diode.

Method

This circuit shows a diode connected to a voltmeter, with a variable resistor, a battery and a milliamps symbol.

  1. Connect the circuit as shown in the diagram. The diode must be protected with a suitable protective resistor (between 100 Ω and 500 Ω).
  2. Set the variable resistor to give the lowest potential difference. Record the readings on the voltmeter and milliammeter (an ammeter capable of reading to ±1 mA).
  3. Alter the variable resistor to increase the potential difference by 0.2 V.
  4. Record the new readings.
  5. Repeat steps 3 and 4, increasing the current slightly each time.
  6. Reverse the power supply connections and repeat steps 2 to 6.

Results

Record the results in a suitable table. This shows some example results:

Potential difference (V)Current (mA)
0.20
0.40
0.60
0.80
1.00
1.20
1.41
1.63
1.88
2.020

When the power supply is reversed:

Potential difference (V)Current (mA)
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00

Analysis

Plot a graph of current against potential difference.

Graph plotting potential difference against current for a diode. Line is horizontal on the x-axis for a part, and then it curves upwards sharply.An I-V graph for a diode

Evaluation

A diode only allows current to pass in one direction. In one direction, the diode's resistance is very large and there is no current. In the other direction, the diode's resistance is smaller and current will pass. In this direction, the diode's resistance is very large at low potential differences. At higher potential differences, the resistance quickly drops and a current passes.

Hazards and control measures

HazardConsequenceControl measures
Heating of the resistance wireBurns to the skin Do not touch the resistance wire whilst the circuit is connected and allow it time to cool
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