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Home · IGCSE 0625 · Electricity and magnetism · I-V graphs · Practice
Extended · Practice questions · I-V graphs
Supplement (Extended) content

Reading the characteristic.

Six original Cambridge-style questions on I-V graphs: identifying the three characteristics, reading resistance from a graph, and explaining the filament lamp and the diode.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
The shapes that earn the marks

Line, curve, one-way.

01
Recall
[3 marks]

Describe the shape of the I-V graph for each of the following: a fixed resistor at constant temperature, a filament lamp, and a diode.

  • Fixed resistor: a straight line through the origin. ✓
  • Filament lamp: a curve through the origin that flattens as the voltage increases. ✓
  • Diode: zero (or near zero) current in reverse, then a steeply rising current once forward voltage passes about 0.6 V. ✓
02
Calculation
[2 marks]

From a resistor's I-V graph, a current of 0.50 A flows at a potential difference of 2.0 V. Calculate the resistance, and state what its constant value tells you about the graph.

R = V ÷ I = 2.0 ÷ 0.50

R = 4.0 Ω

A constant R means the graph is a straight line through the origin. ✓

03
Analysis
[3 marks]

Explain why the I-V graph of a filament lamp is a curve and not a straight line.

  • As the voltage increases, the current increases, heating the filament. ✓
  • The higher temperature increases the resistance of the filament. ✓
  • So the current rises less and less steeply, and the graph curves (flattens). ✓
04
Calculation
[3 marks]

For a filament lamp, the current is 1.0 A at 2.0 V and 2.0 A at 8.0 V. Calculate the resistance at each point and state whether the resistance has increased or decreased.

At 2.0 V: R = 2.0 ÷ 1.0 = 2.0 Ω At 8.0 V: R = 8.0 ÷ 2.0 = 4.0 Ω

The resistance has increased (from 2.0 Ω to 4.0 Ω). ✓

05
Analysis
[2 marks]

Describe how the current through a diode depends on the direction and size of the potential difference applied to it.

  • In the forward direction, almost no current flows until about 0.6 V, after which the current rises steeply. ✓
  • In the reverse direction, effectively no current flows (its resistance is very high). ✓
06
Analysis
[2 marks]

Two I-V graphs are drawn. Graph A is a straight line through the origin; graph B is a curve through the origin that becomes less steep. State which is the fixed resistor and which is the lamp, and give the reason.

  • Graph A is the fixed resistor: a straight line means constant resistance. ✓
  • Graph B is the lamp: the curve becoming less steep shows the resistance increasing as it heats. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.