A metal wire at constant temperature passes 0.30 A at 1.5 V. Calculate its resistance. [1 mark]

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How does current vary with potential difference for different components?

The definition of potential difference and resistance, Ohm's law, the I-V characteristics of an ohmic conductor, a filament lamp and a diode, and how the resistance of a thermistor and LDR varies.

A focused answer to AQA A-Level Physics 3.5.1.2, covering potential difference and resistance, Ohm's law, the current-voltage characteristics of an ohmic conductor, a filament lamp and a diode, and how the resistance of a thermistor and a light-dependent resistor changes.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Potential difference and resistance
Ohm's law
I-V characteristics
Thermistors and LDRs
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What this dot point is asking

AQA specification point 3.5.1.2 wants you to define potential difference and resistance, state Ohm's law, sketch and explain the I-V characteristics of an ohmic conductor, a filament lamp and a diode, and describe how the resistance of a thermistor and an LDR changes with conditions.

Potential difference and resistance

The potential difference between two points is the work done per coulomb in moving charge between them, so $1 \text{ V} = 1 \text{ J C}^{-1}$ . Resistance measures how strongly a component opposes current; a high resistance means a large voltage is needed to drive a given current. Resistance arises microscopically from collisions between drifting electrons and the vibrating lattice ions, which transfer the electrons' kinetic energy to thermal energy in the lattice.

Ohm's law

The key condition is constant temperature. Many components only obey Ohm's law over a limited range, and any device whose temperature changes appreciably as current flows will depart from it.

I-V characteristics

For all three, the resistance at any point is $\dfrac{V}{I}$ for that point, which is not the same as the gradient of the curve unless the line is straight and through the origin. On a curving characteristic the resistance is different at every point.

Thermistors and LDRs

A thermistor of the negative temperature coefficient (NTC) type has a resistance that decreases as temperature increases, because thermal energy releases more charge carriers from the semiconductor lattice, raising the number density $n$ . This makes thermistors useful as temperature sensors. A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases, because incident photons free additional charge carriers. Both are common in potential-divider sensing circuits, where their changing resistance shifts an output voltage.

Reading resistance from an I-V graph at two points

A filament lamp gives a current of $0.20 \text{ A}$ at $2.0 \text{ V}$ and $0.50 \text{ A}$ at $8.0 \text{ V}$ . Show that its resistance increases, and find the resistance at each point.

step 1: Resistance at the lower voltage

$R_1 = \dfrac{V}{I} = \dfrac{2.0}{0.20} = 10 \text{ }\Omega$ .

step 2: Resistance at the higher voltage

$R_2 = \dfrac{V}{I} = \dfrac{8.0}{0.50} = 16 \text{ }\Omega$ .

step 3: Compare

Because $R_2 > R_1$ , the resistance has increased with voltage (and hence with current and temperature), confirming the lamp is non-ohmic.

step 4: Interpret the graph

On the characteristic, the line from the origin to the higher point is less steep, so the larger $\dfrac{V}{I}$ matches the bending-over shape.

Try this

Q1. Define resistance. [1 mark]

Cue. The ratio of the potential difference across a component to the current through it.

Q2. State how the resistance of an NTC thermistor changes as it gets hotter. [1 mark]

Cue. Its resistance decreases.

Q3. A metal wire at constant temperature passes $0.30 \text{ A}$ at $1.5 \text{ V}$ . Calculate its resistance. [1 mark]

Cue. $R = \dfrac{V}{I} = \dfrac{1.5}{0.30} = 5.0 \text{ }\Omega$ .

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksExplain the shape of the current-voltage characteristic of a filament lamp.

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As the current increases, the filament heats up and its temperature rises.

The positive metal ions in the lattice vibrate with greater amplitude, so they collide with and impede the flow of conduction electrons more strongly. The mean drift velocity for a given field falls, so the resistance increases.

On the I-V graph this means the gradient (which equals $\dfrac{1}{R}$ only at the origin) decreases as the voltage rises, so the curve bends over towards the voltage axis and is not a straight line. The lamp is therefore non-ohmic.

Markers reward linking the temperature rise to increased ion vibration, to increased resistance, and to the curving (non-ohmic) shape.

AQA 20213 marksA semiconductor diode is connected in a circuit. Describe its current-voltage characteristic and explain what is meant by its threshold voltage.

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In reverse bias the diode conducts almost no current, so the characteristic lies along the voltage axis with $I \approx 0$ . In forward bias the current stays very small until the potential difference reaches the threshold (around $0.6 \text{ V}$ for silicon), after which the current rises sharply for small increases in voltage.

The threshold (or turn-on) voltage is the minimum forward potential difference needed before the diode conducts appreciably; below it the diode has a very high resistance and above it a very low one.

Markers reward describing both bias directions and defining the threshold as the forward pd needed for significant conduction.

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Sources & how we know this

AQA A-level Physics (7408) specification — AQA (2017)