Electrical current transfers energy around circuits. There are two types of current: direct and alternating.
There are different ways to investigate the relationship between current and potential difference. In this practical activity, it is important to:
To investigate the relationship between current and potential difference for a resistor, bulb and diode.
For a 10 Ohm fixed resistor the results may look like this:
Potential difference (V) | Current (A) |
---|---|
0.5 | 0.05 |
1.0 | 0.10 |
1.5 | 0.15 |
2.0 | 0.20 |
... | ... |
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 running 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.
For a filament bulb, the results may look like this:
Potential difference (V) | Current (A) |
---|---|
0.5 | 0.10 |
1.0 | 0.20 |
1.5 | 0.35 |
2.0 | 0.50 |
2.5 | 0.65 |
3.0 | 0.78 |
3.5 | 0.90 |
4.0 | 1.00 |
4.5 | 1.08 |
5.0 | 1.15 |
5.5 | 1.20 |
6.0 | 1.25 |
6.5 | 1.31 |
7.0 | 1.36 |
7.5 | 1.38 |
8.0 | 1.41 |
8.5 | 1.44 |
9.0 | 1.46 |
9.5 | 1.48 |
10.0 | 1.50 |
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 higher temperature increases the vibrations of the ions in the filament, which makes it harder for the electrons to get past.
Potential difference (V) | Current (mA) |
---|---|
0.2 | 0 |
0.4 | 0 |
0.6 | 0 |
0.8 | 0 |
1.0 | 0 |
1.2 | 0 |
1.4 | 1 |
1.6 | 3 |
1.8 | 8 |
2.0 | 20 |
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 | Consequence | Control measures |
---|---|---|
Heating of the resistance wire | Burns to the skin | Do not touch the resistance wire whilst the circuits are connected and allow it time to cool |