Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Current, pd & resistance
Current (I)Current is the rate of flow of electric charge, measured in coulombs per second, or amperes. It is related to charge by Q = I t.
Potential difference (V)Potential difference is the energy transferred per unit charge between two points in a circuit, measured in joules per coulomb, or volts. It is related to energy and charge by E = Q V.
Resistance (R)Resistance is the opposition to current in a circuit, measured in ohms (Ω), and is related to current and potential difference by the equation V = I R.
Current is measured with an ammeter connected in series, so that the current flows through it. Potential difference is measured with a voltmeter connected in parallel across the component being tested.
Ohmic conductorAn ohmic conductor produces a straight I–V line through the origin, provided its temperature stays constant, which means its resistance also stays constant.
Filament lampIn a filament lamp, the current heats the wire, and hot metal has a higher resistance, so the I–V graph curves and flattens as the current increases.
Diode / LEDA diode, or LED, conducts current in one direction only, and has a very high resistance in the reverse direction.
LDRA light-dependent resistor (LDR) has a resistance that falls in bright light, which makes it useful in circuits such as automatic night lights.
ThermistorA thermistor has a resistance that falls as it gets hotter, which makes it useful in circuits such as thermostats.
🧪 Required practical: In the wire-resistance practical, resistance is found to be directly proportional to length: doubling the length of the wire doubles its resistance. The wire must be kept cool, using low currents and switching off the supply between readings, or heating changes its resistance and skews the results.
Series & parallel
Series rulesIn a series circuit, the current is the same everywhere, the potential difference is shared between the components, and the resistances simply add together: Rt = R1 + R2.
Parallel rulesIn a parallel circuit, each branch has the full supply potential difference across it, the current splits between the branches and then rejoins, and the total resistance is less than the smallest individual branch resistance, because an extra branch gives the current more paths to flow through.
🎯 Examiner tip: It can seem surprising that adding a resistor in parallel decreases the total resistance, but this happens because the extra branch gives the current an additional path to flow through. Examiners want that reasoning explained, not just the rule stated.
Mains, power & the Grid
Batteries supply direct current (dc), which flows in one direction only. UK mains electricity supplies alternating current (ac) at 230 V and 50 Hz, meaning the current changes direction 50 times every second.
Live wire (brown)The live wire carries the 230 V supply and is dangerous even when an appliance is switched off, because touching it lets current flow through a person's body to earth.
Neutral (blue)The neutral wire completes the circuit and is normally at a potential close to 0 V.
Earth (green/yellow)The earth wire is a safety wire that only carries current if there is a fault; it gives the fault current an easy path to flow through, so the fuse blows instead of a person being electrocuted.
P = V I; P = I² R; E = P t; E = Q V
The National Grid transmits electricity from power stations to homes. Step-up transformers raise the potential difference to around 400,000 V, which lowers the current; since the heating effect in the cables depends on I²R, a lower current means far less energy is wasted heating the cables. Step-down transformers then reduce the potential difference to a safe 230 V for use in homes.
Static electricity (Triple) Separate science only
When two insulating materials are rubbed together, electrons are transferred from one to the other. The material that loses electrons becomes positively charged, and the material that gains electrons becomes negatively charged; only electrons move, never protons. If enough charge builds up, it can jump as a spark, which is what causes the crackle from a jumper or a flash of lightning.
An electric field fills the space around any charged object, and another charge placed in that field feels a force: like charges repel, and opposite charges attract. This force is strongest close to the charge, and field lines are drawn pointing away from a positive charge. A strong enough electric field can pull electrons off air molecules, which makes the air conduct and produces a spark.
Turn these notes into memory. 23 flashcards cover this topic. Short daily sessions, spaced repetition up to your exam date, quizzes with friends. Free during the beta.Practise Electricity in the app ↗