Required practical - investigate current - voltage graphs

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

A resistor

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. Adjust the variable resistor so that the potential difference is very low or zero at the start.
  3. Record the reading on the voltmeter and ammeter.
  4. Use the variable resistor to increase 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. Repeat the experiment but replace the fixed resistor with a bulb.
  9. Plot a graph of current against potential difference for each component.

Results

Fixed 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.

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 big 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

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.

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 big.

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

If the above experiment was done for a diode, the results would look like the following:

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.

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 wire and bulbBurns to the skinDo not touch the resistance wire or bulb whilst the circuit is connected and allow time to cool

Further components

The above experiment could also be used to investigate the variation in resistance of a thermistor as the temperature changes, and the variation in resistance of a light-dependent resistor as the light level is altered.