← Back to Lab
Physics · 4.2.5 Resistance · I–V characteristics · Paper 6

I–V Characteristics. Sweep it.

Vary the voltage across three components and plot current against voltage. A fixed resistor gives a straight line (ohmic); a filament lamp curves (resistance rises as it heats); a diode only conducts one way.

0625 Topic 4.2.5 — Resistance & I–V graphs Ohmic vs non-ohmic Paper 6 — ATP
Resistor — adjust the voltage and record I. The graph builds as you go.

Variables

2.0
10

Live readouts

Voltage V
2.00 V
Current I
0.20 A
Resistance V/I
10.0 Ω
Behaviour
ohmic (constant R)
An ohmic conductor gives a straight line through the origin: V ∝ I at constant temperature.
📋 Method (Cambridge ATP procedure)
  1. Connect the component in series with an ammeter and a variable resistor (rheostat) / power supply.
  2. Connect a voltmeter in parallel across the component.
  3. Adjust the supply to several voltages; record V and I at each, switching off between readings.
  4. Reverse the connections to obtain negative values (especially for the diode).
  5. Plot I (y-axis) against V (x-axis).

Shapes: resistor = straight line (ohmic); lamp = S-curve flattening (R rises with temperature); diode = current only for forward voltage above ~0.7 V.

⚠ Sources of error & precautions
  • Heating — take readings quickly and switch off between them so the component temperature is steady.
  • Meter zero errors — check both meters read zero with the supply off.
  • Take readings both ways through the origin for a full characteristic.
🎯 Syllabus reference (0625)
  • 4.2.5 Resistance — sketch and explain the I–V characteristics of a metallic conductor at constant temperature, a filament lamp and a diode; recall and use R = V/I; state Ohm's law and the conditions under which it applies.

Ask the lab assistant