Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equation: V = IR | Linked practicals: RP 4: I–V Characteristics, RP 3: Resistance
Notes
An I–V graph shows how the current through a component changes as the potential difference across it changes. The shape tells you how its resistance behaves.
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- Ohmic conductor (e.g. a resistor at constant temperature): current is directly proportional to p.d. The graph is a straight line through the origin, and the resistance is constant.
- Filament lamp: as the current increases, the filament gets hotter and its resistance increases. The graph is an S-shaped curve that gets flatter.
- Diode: current flows in one direction only. It has a very high resistance in the reverse direction.
- Thermistor: resistance decreases as temperature increases. Used in thermostats.
- LDR (light-dependent resistor): resistance decreases as light intensity increases. Used in automatic night lights.
Finding resistance from an I–V graph
Read off a pair of values for V and I, then use R = V ÷ I. For a curved graph, the resistance is different at each point, so always read the values at the point the question asks about.
Questions
Q1 (F) Which component only lets current flow in one direction?
Show answer
A diode.
Q2 (F) What happens to the resistance of an LDR when it gets darker?
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It increases.
Q3 (F) Give one use of a thermistor.
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A thermostat (e.g. in central heating or a fridge), or any temperature sensor.
Q4 (F) Describe the I–V graph for a resistor at constant temperature.
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A straight line through the origin, because current is directly proportional to p.d.
Q5 (F/H) At one point on an I–V graph, the p.d. is 6.0 V and the current is 0.30 A. Calculate the resistance.
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F: V = I R
I: 6.0 = 0.30 × R
F: R = 6.0 ÷ 0.30
A: 20 Ω
Q6 (F/H) Explain why the I–V graph for a filament lamp is curved.
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As the current increases, the filament gets hotter. This increases its resistance, so the current increases less for each extra volt and the graph gets flatter.
Q7 (H) An LDR is used in a circuit to switch on a street light. Explain how the LDR’s resistance changes at dusk and what this does to the current through it.
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As the light intensity falls, the LDR’s resistance increases. For the same p.d., the current through it decreases. The circuit uses this change to switch the light on.
