CCEA A Level Physics
CCEA A Level Physics 1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity Exam Questions
Free CCEA A Level Physics revision for 1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity: what you need to know, common mistakes and exam-style questions with answers. Covers current, charge, potential difference, e.m.f., series and parallel circuits, resistance, resistivity, superconductivity, Ohm's law and I-V characteristics.
Practise 1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity with past papers
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About CCEA A Level Physics 1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity
These sections start the electricity half of Unit AS 1: Forces, Energy and Electricity. You need to define current as rate of flow of charge, define the volt in terms of energy per unit charge and distinguish e.m.f. from potential difference. Resistance then covers series and parallel combinations, power, resistivity, superconductivity, Ohm's law and the I-V characteristics of a metallic conductor, a filament lamp, a diode and an NTC thermistor. Examiners like precise definitions, resistivity calculations with careful conversion of wire diameter, and explanations of I-V graph shapes in terms of temperature and resistance.
What you need to know
- Current and charge Current is the rate of flow of charge, I = Q/t (ΔQ/Δt). One coulomb is the charge passing a point when a current of 1 A flows for 1 s.
- Potential difference and e.m.f. V = W/q. P.d. is the energy transferred from electrical to other forms per unit charge; e.m.f. is the energy transferred to electrical form per unit charge by a source. 1 V = 1 J C-1.
- Resistance and power R = V/I. Series: R = R1 + R2 + ... Parallel: 1/R = 1/R1 + 1/R2 + ... Power P = IV = I^2R = V^2/R.
- Resistivity R = ρL/A, so ρ = RA/L with unit Ω m. Measure the diameter of a wire with a micrometer at several points and use A = πd^2/4.
- Superconductivity Below a critical (transition) temperature some materials have zero resistance, so a current can flow with no energy loss. Uses include strong electromagnets in MRI scanners and particle accelerators.
- Ohm's law and I-V graphs Ohm's law: current is proportional to p.d. for a metallic conductor at constant temperature. A filament lamp curves because resistance rises with temperature. A diode conducts only above about 0.6 V in forward bias. An NTC thermistor's resistance falls as temperature rises.
Common mistakes in 1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity
- Defining current as the flow of charge rather than the rate of flow of charge.
- Using the diameter instead of the radius in A = πr^2, or forgetting to convert mm to m before squaring.
- Saying a non-ohmic component has no resistance; it still has R = V/I at each point, but R is not constant.
- Mixing up e.m.f. and p.d.; e.m.f. is energy given to charge by the source, p.d. is energy given out by charge in a component.
- Thinking resistance is the gradient of an I-V graph; R is V/I at a point, not the gradient.
Try three 1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity exam-style questions
1. Define the volt. (1 mark)
Show the mark scheme answer
The potential difference between two points when 1 J of energy is transferred per coulomb of charge passing between them.
2. A nichrome wire is 2.0 m long with diameter 0.50 mm. Resistivity of nichrome = 1.1 x 10^-6 Ω m. Calculate its resistance. (3 marks)
Show the mark scheme answer
A = π(0.25 x 10^-3)^2 = 1.96 x 10^-7 m^2; R = ρL/A = 1.1 x 10^-6 x 2.0 / 1.96 x 10^-7; = 11 Ω.
3. Sketch and explain the I-V characteristic of a filament lamp. (3 marks)
Show the mark scheme answer
Curve through origin with gradient decreasing as V increases, symmetrical in both directions; as current increases the filament gets hotter; ions vibrate more so electrons collide more often and resistance increases.
1.9 Current, Charge, P.D. and E.M.F. and 1.10 Resistance and Resistivity FAQs
What does CCEA A Level Physics 1.9 and 1.10 cover?
Current, charge, potential difference and e.m.f., series and parallel resistors, electrical power, resistivity, superconductivity, Ohm's law and I-V characteristics of common components.
Which unit are 1.9 and 1.10 examined in?
Both are in Unit AS 1: Forces, Energy and Electricity.
How do I measure the resistivity of a wire?
Measure the diameter with a micrometer at several points, then measure R for several lengths using an ammeter and voltmeter. Plot R against L; the gradient equals ρ/A, so ρ = gradient x A.
