Circuit Calculations & Behaviour
Electron Flow in Circuits
Electric current in a conductor is caused by the movement of electrons. In a metal wire, free electrons drift from the negative terminal of the supply towards the positive terminal — this is called electron flow. Conventional current, used in circuit diagrams and calculations, is defined as flowing from positive to negative, which is the opposite direction. For all OCR calculations you use conventional current direction, but you should be aware that actual electron flow goes the other way.
Series and Parallel Circuits
In a series circuit, components are connected in a single loop. The same current flows through every component, but the supply voltage is shared between them. The total resistance of a series circuit is simply the sum of all individual resistances: R_total = R1 + R2 + R3. If three resistors of 10 Ω, 20 Ω, and 30 Ω are connected in series, the total resistance is 60 Ω.
In a parallel circuit, components are connected across the same two nodes. Every component receives the full supply voltage, and the total current is the sum of the currents through each branch. The total resistance of a parallel circuit is lower than any individual resistor. For two resistors: 1/R_total = 1/R1 + 1/R2. For three resistors: 1/R_total = 1/R1 + 1/R2 + 1/R3. Always find 1/R_total first, then take the reciprocal to get R_total.
Series: R_total = R1 + R2 + R3 (resistances add directly). Parallel (up to three resistors): 1/R_total = 1/R1 + 1/R2 + 1/R3 — always take the reciprocal of your final sum to get R_total. The OCR specification covers a maximum of three resistors in either configuration.
Ohm's Law: V = I × R
Ohm's law states that the potential difference across a component is directly proportional to the current through it, provided temperature is constant. The formula is V = I × R, where V is voltage in volts (V), I is current in amps (A), and R is resistance in ohms (Ω). You must be able to rearrange this formula: I = V ÷ R to find current; R = V ÷ I to find resistance. For example, if a resistor has 12 V across it and a current of 0.5 A flows through it, its resistance is R = 12 ÷ 0.5 = 24 Ω.
Watt's Law: P = I × V
The power dissipated by a component equals the current through it multiplied by the voltage across it: P = I × V, where P is power in watts (W). This formula can be rearranged: I = P ÷ V; V = P ÷ I. For example, a motor operating at 6 V and drawing a current of 2 A has a power consumption of P = 2 × 6 = 12 W. P = I × V is the formula most directly tested; the specification only requires P = I × V and rearrangements of it.
Ohm's law: V = I × R (V in volts, I in amps, R in ohms). Watt's law: P = I × V (P in watts). Both formulas must be used with base units — convert any kΩ or mA to Ω or A before substituting.
AC and DC: Types of Current
Direct current (DC) flows in one direction only; its voltage remains constant over time. Batteries supply DC. Alternating current (AC) repeatedly reverses direction; the voltage rises and falls in a regular cycle. The UK mains supply is AC at 230 V, 50 Hz. Most electronic circuits use DC internally, but mains power is AC — a transformer and rectifier convert AC to DC in a mains adaptor.
Analogue and Digital Signals
An analogue signal varies continuously and can take any value within a range — like the output of a microphone or a temperature sensor. A digital signal takes only two discrete states: high (1) or low (0). An analogue signal varies continuously and can have different waveform shapes, including a sine wave. A digital signal can be represented by a square wave that switches between two voltage levels. Key waveform properties include: amplitude (the peak height of the wave), frequency (the number of complete cycles per second in Hz), and periodic time (the duration of one complete cycle, T = 1 ÷ f).
Three resistors — 10 Ω, 20 Ω, and 40 Ω — are connected in parallel. Find the total resistance. Step 1: 1/R_total = 1/10 + 1/20 + 1/40 Step 2: Convert to a common denominator of 40: 4/40 + 2/40 + 1/40 = 7/40 Step 3: R_total = 40 ÷ 7 ≈ 5.71 Ω Notice that the total resistance (5.71 Ω) is less than the smallest individual resistor (10 Ω) — this is always true in a parallel circuit. Show all three steps in exam working to maximise method marks.
Analogue signals capture the full continuous range of a measurement, which can mean higher fidelity — a microphone output faithfully follows every nuance of sound. However, analogue signals are susceptible to noise: any interference adds to the signal and cannot easily be removed. Digital signals can be regenerated perfectly from their two-state nature — if a signal is noisy but clearly above the threshold, a circuit can restore it to a clean 1. Digital signals are also easier to process, store, and transmit reliably, which is why almost all modern communication and computing systems use digital formats. The trade-off is that converting from analogue to digital (via an ADC) introduces a small quantisation error — the continuous signal is approximated in discrete steps.