Core Practical: Investigating and Testing Circuits

Section 1 of 7Investigation 1: current-voltage characteristics

Aim

• Aim To find how the current through a fixed resistor and a filament lamp depends on the potential difference across them.

Variables

• Variables Independent: potential difference, V. Dependent: current, I.
• Controlthe supply voltage range, the same wires and meters, and the same room conditions.

Equipment

• Equipment Power supply, fixed resistor, filament lamp, variable resistor, ammeter (resolution 0.01 A), voltmeter (resolution 0.1 V), switch and leads.

Section 2 of 7Method for Investigation 1

1

Build a series circuit with the cell or supply, switch, ammeter, variable resistor and the fixed resistor, and place the voltmeter in parallel across the resistor.

2

Adjust the variable resistor to give a range of potential differences, collecting 8 to 10 different readings.

3

At each setting, read the current three times and calculate the mean current.

4

Switch the circuit off between readings so that the component and wires do not heat up.

5

Reverse the connections to the supply and repeat to obtain negative potential differences and currents.

6

Replace the resistor with the filament lamp and repeat the whole procedure.

Section 3 of 7Analysing the I-V graphs

Component	Graph shape	Explanation
Fixed resistor	Straight line through the origin	Current is directly proportional to potential difference because V = I × R with R constant.
Filament lamp	Curve that flattens at higher values	Greater current heats the filament, ions vibrate more, resistance increases, so current rises less quickly.
Diode (for comparison)	Current only in one direction	Very high resistance in the reverse direction.

Section 4 of 7Investigation 2: series and parallel

1Single component

• Single component Measure V and I for one resistor, then calculate R = V ÷ I. Repeat with a second resistor.

2Two in series

• Two in series Connect the resistors end to end, measure V and I, and calculate the total resistance.

3Two in parallel

• Two in parallel Connect them side by side, measure V and I, and calculate the total resistance.

4Conclusion

• Conclusion In series the total resistance equals the sum of the individual resistances.
• In parallel the total resistance is smaller than either individual resistance.

Section 5 of 7Evaluation and safety

Improving and doing it safely

Systematic errors

• Systematic errors Check that meters read zero before connecting;
  • a zero error would shift every reading by the same amount.

Random errors

• Random errors Repeat readings and take means.
• Control the temperature of the equipment, because resistance changes with temperature.
• Meters have some resistance of their own, so readings are slightly inaccurate.

Hot components

• Hot components Thin wires and lamps become hot at high current;
  • do not touch them while the circuit is on and let them cool between readings.

Other hazards

• Other hazards Keep liquids away from the equipment and switch off at once if you smell burning.

Section 6 of 7Key terms

Term	Meaning
Potential difference	The energy transferred per unit charge between two points, measured in volts.
Resistance	How strongly a component opposes current, calculated as R = V ÷ I, in ohms.
I-V graph	A graph of current against potential difference for a component.
Zero error	A fault where a meter does not read zero when nothing is being measured.
Resolution	The smallest change in a quantity that a measuring instrument can show.

Section 7 of 7Exam technique

1

Describing the lamp graph as showing constant resistance - the resistance increases because the filament gets hotter.

2

Leaving the circuit switched on between readings, which heats the components and changes their resistance.

3

Taking only one reading at each setting, so that random error cannot be reduced.

4

Saying that adding a resistor in parallel increases the total resistance - it decreases it.