# Required practical - investigate current - voltage graphs

There are different ways to investigate the relationship between and . 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 , bulb and

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 and .
4. Use the 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 with a bulb.
9. Plot a graph of current against potential difference for each .

### 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
......

### 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 and is true because the of the resistor is fixed and does not change. A resistor is an .

### Filament bulb

For a 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
......

### 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 of the in the filament because of the increased temperature make it harder for the 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

### Evaluation

A 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.