Current electricity revision notes

Current electricity questions should begin by naming the physical quantity being asked for. Current is the rate of flow of charge, so it is measured in amperes and linked to charge and time by Q = It. Charge is measured in coulombs and describes how much charge has passed a point or been transferred. Potential difference is different: it is the energy transferred per unit charge between two points, so answers about voltage should mention energy transfer rather than simply saying that voltage is current.

Resistance describes how strongly a component opposes current. For an ohmic conductor at constant temperature, potential difference is proportional to current and V = IR can be used directly. For non-ohmic components, such as filament lamps, diodes, thermistors and LDRs, the current-potential difference relationship changes with temperature, light intensity, direction of connection, or component behaviour. When interpreting an I-V graph, identify the axes first, then explain whether the line is straight, curved, symmetrical, or directional.

Series and parallel circuits must be kept separate. In series, the same current flows through every component and resistances add. In parallel, each branch has the same potential difference and the total resistance is found using reciprocal relationships. Adding a parallel branch provides another route for charge and reduces the total resistance, which is the opposite of simply adding another resistor in series.

Power is the rate of energy transfer. Use P = VI when potential difference and current are known, P = I?R when current and resistance are known, P = V?/R when potential difference and resistance are known, and P = E/t when energy and time are known. Energy questions should finish in joules, kilojoules or kilowatt-hours, not volts or amps. For E = QV, charge and potential difference together determine the energy transferred. For E = Pt, power and time together determine the energy transferred by an appliance or component.

A-Level electricity also includes ideas such as resistivity, potential dividers, electromotive force and internal resistance. Resistivity is a material property, so it must be separated from the measured resistance of a particular wire. A potential divider uses the ratio of resistances to set an output potential difference. Electromotive force describes energy supplied per unit charge by a source, while terminal potential difference can fall when current flows because energy is transferred inside the source through internal resistance.

In exam answers, write the equation before substituting values, keep units beside numbers, and explain the physical meaning of the result. If the answer asks for a comparison, state which circuit quantity is being compared and why. If the answer asks for a graph interpretation, refer to gradient, proportionality, intercepts, curvature or direction as appropriate. The safest route is to keep current, charge, potential difference, resistance, power and energy in separate mental boxes, then choose the equation that links exactly the quantities in the question.

Worked examples should also show why the chosen formula is appropriate. If a question gives current and time, the target is usually charge, so the answer should describe charge flow before calculating. If a question gives charge and potential difference, the target is energy transferred, so the answer should link the result to work done on charges. If a question gives power and time, the answer should describe a rate acting over an interval, not a new electrical quantity. In circuit-comparison questions, a sentence explaining the rule is often as important as the calculation: series components share current, parallel branches share potential difference, and component behaviour can change when temperature or light level changes.

Common mistakes include writing the right number with the wrong unit, using resistance units for potential difference, treating electromotive force as identical to terminal potential difference, and using a series rule in a parallel circuit. Another common error is to quote an equation without explaining the symbols. A high-quality answer defines each symbol, substitutes values carefully, carries units through the calculation, and then interprets the result in words. For sensors and potential dividers, describe how a changing resistance changes the output potential difference. For resistivity, describe how length and cross-sectional area affect a wire while the material property remains the same.

For revision, practise sorting questions by the quantity required before attempting the arithmetic. Ask: is the answer a charge, current, potential difference, resistance, power, energy, or material property? Then choose the equation that contains that quantity and the values supplied. This prevents most formula-selection errors and makes explanations clearer for AQA A-Level Physics marking.