IGCSE Physics / Electrical resistance
Practical: investigating how current varies with voltage revision notes
Edexcel International GCSE Physics (4PH1), chapter 8: Electrical resistance, specification point 2.9. Free lesson.
Watch the video lesson · IGCSE Physics past papers
An I-V investigation measures how current changes as voltage changes for a component. A variable resistor lets you collect enough readings to plot a graph and compare components.
How to connect the circuit
The circuit is a series loop, with the voltmeter connected across the component. The ammeter measures the current through the component, while the voltmeter measures the voltage across it. Connect the ammeter in series with the component. Connect the voltmeter in parallel, across the component. Add a variable resistor in series. Adjusting it changes the current, allowing you to take many readings. Include a battery or power supply and a switch so you can turn the circuit off between readings.
How to take and check the readings
Each reading is a pair of voltage and current values. Collect at least six pairs across a range so you can see the relationship clearly. Switching off between readings prevents heating from changing the component's resistance.
How to adapt the circuit for a lamp or diode
The same basic circuit can test a filament lamp or a diode, but each needs care for a different reason. For a filament lamp, switch off between readings because it heats up. Reversing its connections gives the same curve on the negative side. For a diode, add a protective resistor in series. When the diode conducts, its resistance is very low, so the current could become large enough to damage it. Take small steps in voltage where the diode current starts to increase. When the diode is reversed, the current is zero.
How to plot and read the results
Plot current against voltage: voltage goes along the horizontal axis and current up the vertical axis. Plot each pair as a point, then draw one ruled line or curve of best fit. Use the voltmeter reading, not the power supply setting, for the voltage. Label both axes with the quantity and its unit: voltage / V and current / A. For a fixed resistor, draw a straight line of best fit through the origin. Do not join the dots. A lamp gives a curve that flattens. A diode conducts in one direction only.
Why the variable resistor matters
Connecting a component straight to a battery and reading the meters gives only one pair of values. A single point is not enough to draw a graph, so you must include a way to vary the current. State that a variable resistor in series is used, then explain that adjusting it changes the current.