How to affect electric current - OCR 21st CenturyInvestigating I-V characteristics of electrical components

Investigating resistance and potential difference experiment

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

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.

Analysis

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

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

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:

When the power supply is reversed:

Analysis

Plot a graph of current against potential difference.

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