OxfordAQA International A Level Physics 3.4 Electricity Exam Questions

About OxfordAQA International A Level Physics 3.4 Electricity

Electricity is the first section of Unit 2 (PH02), Electricity, waves and particles, on the OxfordAQA International AS course. You define current, potential difference and resistance, interpret I-V characteristics for an ohmic conductor, a diode and a filament lamp, and use resistivity, including the behaviour of thermistors and superconductors. You then analyse series and parallel circuits, potential dividers with sensors, and cells with internal resistance. These ideas return in capacitor circuits and electromagnetic induction at A2. Examiners like multi-step circuit calculations, sensor-based potential divider explanations, and the required practical in which emf and internal resistance are found from a graph of terminal pd against current.

What you need to know

• Basic definitions Current I = delta Q / delta t, potential difference V = W / Q, resistance R = V / I. Power P = VI = I^2 R = V^2 / R and energy E = VIt.
• I-V characteristics An ohmic conductor at constant temperature gives a straight line through the origin. A filament lamp's resistance increases as it heats up. A semiconductor diode conducts in the forward direction only, above a threshold of roughly 0.6 V for silicon.
• Resistivity R = rho L / A, with rho in ohm metres. Metal resistance rises with temperature; an NTC thermistor's resistance falls as temperature rises. A superconductor has zero resistivity below its critical temperature, useful for strong magnetic fields and low-loss power cables.
• Series and parallel Series: R total = R1 + R2 + ..., same current everywhere. Parallel: 1 / R total = 1 / R1 + 1 / R2 + ..., same pd across each branch. These follow from conservation of charge and of energy.
• Potential dividers Vout = Vin x R2 / (R1 + R2). Replacing a resistor with a thermistor or LDR makes the output depend on temperature or light level, which is the basis of sensing circuits.
• emf and internal resistance emf is the energy transferred per unit charge by the source. epsilon = I(R + r), so terminal pd V = epsilon - Ir. In the required practical, plot V against I: the y-intercept is the emf and the gradient is -r.

Common mistakes in 3.4 Electricity

• Saying that current is used up in a circuit. Charge is conserved, so the current into a junction equals the current out.
• Describing a filament lamp as obeying Ohm's law. Its resistance increases with temperature, so the I-V graph curves.
• Confusing emf with terminal pd. The terminal pd is less than the emf when current flows because of the lost volts, Ir.
• Using the diameter instead of the cross-sectional area in R = rho L / A, or forgetting to convert mm^2 to m^2.
• Explaining that an NTC thermistor's resistance falls because electrons move faster. The key point is that more charge carriers are released as temperature rises.

Try three 3.4 Electricity exam-style questions