IGCSE Physics Electricity & circuits notes

There are two kinds of charge, positive and negative. Like charges repel; opposite charges attract. Rubbing insulating materials together transfers electrons only — the negative charge. A polythene rod rubbed with a dry cloth gains electrons and goes negative; an acetate rod rubbed loses electrons and goes positive. The charges are detected with an electroscope or by the attraction/repulsion of suspended charged rods.

Conductors let charge flow through them because they contain free (delocalised) electrons — all metals, plus graphite. Insulators hold their electrons tightly, so charge stays put where it was rubbed — plastic, dry air, glass, rubber. A quick test: place a material in a circuit with a lamp and a battery; the lamp lights only for conductors.

Supplement: charge is measured in coulombs (C). An electric field is a region in which a charge experiences a force; field lines point the way a positive charge would be pushed — away from positive charges, towards negative ones, and straight from the positive to the negative plate between oppositely charged parallel plates.

Current is the flow of charge: I = Q/t (Supplement) — charge in coulombs per second, current in amperes (A). An ammeter measures current and is connected in series, so the flow passes through it; analogue meters need the right range selected before connecting, digital ones switch ranges automatically. In a metal wire the charge is carried by free electrons; conventional current is drawn from + to − around the circuit even though the electrons actually drift from − to + (Supplement). Direct current (d.c.) flows one way only; alternating current (a.c.) reverses direction repeatedly — the mains is a.c.

Electromotive force (e.m.f.) is the electrical work done by a source moving unit charge around the complete circuit — the push of a battery, in volts. Potential difference (p.d.) is the work done by unit charge passing through a component — also in volts. A voltmeter measures p.d. and is connected in parallel across the component. Supplement: both come from the same equation, E = W/Q for a source and V = W/Q for a component, with W the work done in joules and Q the charge in coulombs.

Measuring resistance: connect the component in a circuit with a battery, an ammeter in series and a voltmeter in parallel across the component. Read V and I, then calculate R = V/I. Repeating with different currents (using a variable resistor) and averaging improves the result.

For a metallic wire, resistance grows with length (a longer tunnel of jostling ions) and falls as the cross-sectional area grows (more parallel paths). Supplement: R ∝ L and R ∝ 1/A — doubling the length doubles R; doubling the area halves it.

Supplement: the I–V graphs tell three stories. A fixed resistor: a straight line through the origin — current is proportional to voltage, constant R. A filament lamp: a curve that flattens at higher voltage — the hot filament’s resistance grows, because the ions vibrate more and impede the electrons. A diode: no current one way (high resistance), then current flows easily above a small threshold voltage the other way — the diode conducts in one direction only, which is also how LEDs behave while emitting light.

Series circuit (one loop): the current is the same at every point; the p.d.s across the components add up to the source p.d.; resistances add (R = R₁ + R₂ + …); and the combined e.m.f. of several cells in series, all the same way round, is their sum. One break anywhere and everything stops.

Parallel circuit (branches): each branch gets the full source p.d.; the branch currents add at the junctions — current in equals current out (Supplement: charge is conserved at the junction); the combined resistance of two resistors in parallel is less than either alone (Supplement: R = R₁R₂/(R₁ + R₂) for two). More branches mean more total current from the source.

Why lamps go in parallel: every lamp gets the full mains voltage and shines at full brightness; each can be switched independently; and one blown lamp does not break the loop for the others — in series, one failure kills the whole string.

Supplement — the potential divider: two resistors in series share the source voltage in the ratio of their resistances: R₁/R₂ = V₁/V₂. A variable potential divider (a track with a sliding contact) taps off anywhere from 0 to the full input voltage — smoother control than a variable resistor in some circuits, because it can deliver zero volts.

Mains hazards: damaged insulation exposes live conductors; overheating cables (current too large for the wire) melt insulation and start fires; damp conditions let current leak through the moisture to people; and overloading plugs, extension leads and sockets draws more current than the wiring can carry. Each hazard is really the same danger — too much current through the wrong path, often through a person.

A mains circuit has three wires: the live wire at high potential, the neutral wire that completes the loop near earth potential, and the earth wire, a safety connection to the ground through the metal casing. The switch must sit in the live wire: switched off, it then disconnects the appliance from the high potential — a switch in the neutral would leave the appliance live even when “off”. The fuse (a thin wire that melts) or the trip switch (an electromagnetic breaker that flips open) sits in the live wire too: a fault that makes the current too large melts the fuse or trips the breaker, cutting the circuit. Choose the fuse rating just above the appliance’s normal current — a 3 kW heater at 230 V draws about 13 A, needing a 13 A fuse, while a lamp needs only 3 A.

If a live wire touches a metal casing, the casing becomes live and anyone touching it completes a circuit to earth. An earth wire bolted to the casing offers that current an easy low-resistance path: the surge blows the fuse instantly, before the casing can harm anyone. Appliances with non-conducting (plastic) casings cannot go live — they are double-insulated and need no earth wire; the fuse alone still protects their circuit and cabling.