Required practical

Investigating circuit elements

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

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

Aim of the experiment:

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

Method:

  1. Connect the circuit as shown in the first 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 on the voltmeter and ammeter.
  6. Repeat steps three to four, each time increasing the potential difference slightly.
  7. Reverse the power supply connections and repeat steps two to six.
  8. Plot a graph of current against potential difference for each component.
  9. Repeat the experiment but replace the fixed resistor with a bulb.

Fixed resistor

Results:

Use the circuit diagram below to correctly complete this part of the investigation.

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

For a 10 ohm fixed resistor the results may look like this:

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

Analysis:

Graph plotting potential difference against current for a fixed resistor. Line is directly proportional.

When a graph of current against potential difference is plotted, it shows a linear relationship passing through the origin. These graphs are indicators of direct proportionality. Such a relationship means that both variables change in the same way, ie if the potential difference is doubled, the current doubles as a result.

Evaluation:

For a fixed resistor, the potential difference is directly proportional to the current. Doubling the amount of energy into the resistor results in a current twice as fast through the resistor. 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.

Filament bulb

Use the circuit diagram below to correctly complete this part of the investigation.

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

Results:

For a filament bulb, the results may look like this:

Potential difference (V)Current (A)
0.50.10
1.00.20
1.50.35
2.00.50
2.50.65
3.00.78
3.50.90
4.01.00
4.51.08
5.01.15
5.51.20
6.01.25
6.51.31
7.01.36
7.51.38
8.01.41
8.51.44
9.01.46
9.51.48
10.01.50

Analysis:

Graph 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.

In this case, the graph shows a non-linear relationship. Current is not proportional to potential difference.

Evaluation:

In a filament bulb, the current does not increase as fast as the potential difference. Doubling the amount of energy does not cause a current twice as fast.

The more energy that is put into the bulb, the harder it is for the current 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. The increased vibrations of the ions in the filament because of the increased temperature make it harder for the electrons to get past.

Semiconductor diode

Use the circuit diagram below to correctly investigate the experiment.

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

Method:

  1. Connect the circuit as shown in the diagram having chosen a suitable protective resistor (between 100 Ω and 500 Ω).
  2. Set the variable resistor to give the lowest potential difference and record the readings on the voltmeter and milliammeter.
  3. Alter the variable resistor to increase the potential difference by 0.2 V.
  4. Record the new readings on the voltmeter and milliammeter.
  5. Repeat steps - three to four, each time increasing the current slightly.
  6. Reverse the power supply connections and repeat steps two to six.
  7. Plot a graph of current against potential difference for the diode.

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

Analysis:

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.

When the graph is plotted this time, it shows that the diode does not allow any current to flow until the potential difference reaches a certain value (usually around 0.7V).

Evaluation:

A semiconductor diode only allows current to flow in one direction. If the potential difference is arranged to try and push the current the wrong way (also called reverse-bias) no current will flow as the diode's resistance remains very large. Current will only flow if the diode is forward-biased. When forward-biased, the diode's resistance is very large at low potential differences but at higher potential differences, the resistance quickly drops and current begins to flow.

Hazards and control measures

HazardConsequenceControl measures
Heating of the resistance wireBurns to the skinDo not touch the resistance wire whilst the circuit is connected and allow time for the wire to cool