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EnglandCombined ScienceSyllabus dot point

What are current, voltage and resistance, and how are they related?

Electric current, potential difference and resistance, the equation V = IR, Ohm's law, and the current-voltage characteristics of resistors, filament lamps and diodes.

A focused answer to Edexcel GCSE Combined Science Topic 10 (CP10), covering electric current, potential difference and resistance, the equation V = IR, Ohm's law, and the current-voltage characteristics of resistors, filament lamps and diodes.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. Current, potential difference and resistance
  3. The equation V = IR
  4. Ohm's law and the resistor
  5. The filament lamp and the diode
  6. Try this

What this dot point is asking

Edexcel wants you to define electric current, potential difference and resistance, use the equation V=IRV = IR, state Ohm's law, and describe the current-voltage characteristics of a fixed resistor, a filament lamp and a diode.

Current, potential difference and resistance

A larger potential difference drives a larger current; a larger resistance reduces the current. You can picture potential difference as the push that moves the charge, current as how much charge flows each second, and resistance as how hard it is for the charge to get through.

The equation V = IR

Ohm's law and the resistor

The filament lamp and the diode

  • Filament lamp: as the current increases, the filament gets hotter, so the metal ions vibrate more and the resistance increases. Its current-voltage graph is an S-shaped curve that levels off.
  • Diode: only lets current flow in one direction (the forward direction). In reverse it has a very high resistance, so almost no current flows. Its graph is flat for reverse voltage and rises sharply in the forward direction.

To investigate these characteristics, you set up a circuit with the component, an ammeter in series to measure the current and a voltmeter in parallel across the component to measure the potential difference. By changing the supply (often with a variable resistor) you record pairs of current and voltage readings, then plot current against voltage. The shape of the graph identifies the component: a straight line for an ohmic resistor, an S-shaped curve for a filament lamp, and a one-way characteristic for a diode.

Two other components are worth knowing. A thermistor's resistance decreases as its temperature rises, so it is used in temperature sensors and thermostats. A light-dependent resistor (LDR) has a resistance that decreases as the light gets brighter, so it is used in light sensors such as automatic street lights and camera light meters. Both are examples of how resistance can change with conditions, just as a filament lamp's resistance changes with temperature. In a sensing circuit, the change in resistance changes the current or the share of the voltage, which can be used to switch something on or off automatically.

Try this

Q1. State the equation linking potential difference, current and resistance. [1 mark]

  • Cue. V=IRV = IR.

Q2. A current of 2 A2\,A flows through a 5 Ω5\,\Omega resistor. Calculate the potential difference. [2 marks]

  • Cue. V=IR=2×5=10 VV = IR = 2 \times 5 = 10\,V.

Exam-style practice questions

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

Edexcel 20193 marksA component has a potential difference of 6 V6\,\text{V} across it and a current of 0.4 A0.4\,\text{A} through it. Calculate its resistance, stating the equation you use.
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A 3-mark calculation using V = IR.

Use V=IRV = IR, rearranged to R=VIR = \dfrac{V}{I} (1 mark). Substitute: R=60.4R = \dfrac{6}{0.4} (1 mark). So R=15 ΩR = 15\,\Omega (1 mark).

Markers reward stating and rearranging V=IRV = IR, the substitution, and the correct value with the unit ohms. Dividing the wrong way round is the usual error.

Edexcel 20214 marksExplain why the resistance of a filament lamp increases as the current through it increases, and describe how its current-voltage graph differs from that of a fixed resistor.
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A 4-mark question on current-voltage characteristics.

As the current increases, the filament gets hotter (1 mark). The metal ions in the filament vibrate more, so they collide more with the moving electrons, making it harder for the current to flow, which increases the resistance (1 mark). The current-voltage graph for a filament lamp is therefore a curve (an S-shape) that becomes less steep as the voltage increases (1 mark), whereas a fixed resistor at constant temperature gives a straight line through the origin (it obeys Ohm's law) (1 mark).

Markers reward the heating and increased vibration explanation, and the curved graph for the lamp versus the straight line for the resistor.

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