The units you must use

Section 2 is built on a small set of SI units. Get these straight first — examiners drop easy marks for wrong units.

Energy at home is also metered in kilowatt-hours (kWh) — a much bigger unit than the joule, handy for billing. 1 kWh is the energy a 1 kW device uses in 1 hour.

Match the unit

Current is the flow of charge

Electric current is the rate of flow of charge. In a solid metal it is a flow of negatively charged electrons — but by convention we draw conventional current flowing from + to −, the opposite way to the electrons.

In a single loop the current is the same at every point: charge is not used up as it goes round.

Your turn — charge

Voltage, energy and resistance

Voltage (potential difference) is the energy transferred per unit charge. One volt is one joule per coulomb (1 V = 1 J/C).

Resistance opposes the current. The key relationship links voltage, current and resistance:

Rearranging: R = V ÷ I and I = V ÷ R. The bigger the resistance, the smaller the current for the same voltage.

Your turn — resistance

What does a bulb take from the charge?

How can more resistance mean less?

Series vs parallel circuits

How components are connected changes how current and voltage behave. Here is a correct series circuit and a correct parallel circuit:

• Series: current is the same everywhere; the supply voltage is shared between components, and the total resistance of two resistors is R₁ + R₂.
• Parallel: the voltage across components in parallel is the same; current splits between branches and is conserved at a junction. Adding another branch gives the current an extra path, so more current flows from the same supply: the total resistance goes down.

Domestic lighting is wired in parallel: each lamp gets the full mains voltage and can be switched on and off on its own, and if one lamp fails the others stay lit.

Reading a series circuit

Misconception buster: current is not "used up" as it goes round. In a series circuit the same current passes through every component — it is conserved. It is the voltage that is shared out.

How can more resistance mean less?

Think of a water circuit

A circuit is hard to see, so picture water pumped round pipes:

• The pump is the cell — it raises the pressure (the voltage).
• The flow rate of water is the current.
• A narrow pipe is a resistor — it makes the water harder to push, so less flows.

Like all analogies it isn't perfect, but it makes V = I × R feel natural: more pressure → more flow; narrower pipe → less flow.

What notices that it is getting dark?

How current varies with voltage

Plotting current (y) against voltage (x) for a component gives its I–V characteristic. The shape tells you how it behaves:

Why the lamp curves: as more current flows, the filament gets hotter, its resistance increases, so the current rises less steeply — the line bends over.

Name that graph

Resistance that changes

Two components have a resistance that changes with their surroundings:

Remember the direction: for both, more of the input (light / heat) means lower resistance — useful in light sensors and temperature sensors. Lamps and LEDs can be used to show when a current is flowing.

Sensing the dark

What notices that it is getting dark?

What saves you from a live kettle?

Mains electricity & safety

Mains electricity is alternating current (a.c.) — it repeatedly changes direction. A cell or battery supplies direct current (d.c.), which flows one way only.

• Earthing — a metal case is connected to earth, so a fault current flows safely to ground instead of through you.
• Fuses & circuit breakers — break the circuit if the current gets too large.
• Insulation & double insulation — plastic casing means no exposed metal can become live.

Misconception buster: a fuse protects the cable (and appliance) from too large a current — on its own it does not stop you getting a shock. If too much current flows, the thin fuse wire heats up and melts, breaking the circuit. It protects you together with the earth wire: if a fault makes a metal case live, a large current flows to earth and the fuse melts, cutting off the supply.

Wiring a three-pin plug

Know the colours and where the fuse goes. The fuse is always fitted on the live wire.

Plug colours & the fuse

What saves you from a live kettle?

Hairdryer or bulb: which transfers more energy?

Electrical power

A current in a resistor transfers energy and makes it warmer (useful in kettles and heaters). The power — the rate of transfer — is:

This is used to choose the right fuse: work out the normal current (I = P ÷ V) and pick a fuse rated just above it.

Your turn — power

Energy transferred & the cost

Over a time t, the energy transferred by an electrical device is:

Since P = I × V, this is the same as E = P × t. For home bills we use kilowatt-hours (kWh):

Your turn — running cost

Hairdryer or bulb: which transfers more energy?

Why can flowing fuel be dangerous?

Static electricity

Rub two insulators together and electrons are transferred from one to the other by friction:

• The material that gains electrons becomes negatively charged.
• The material that loses electrons is left positively charged.
• Like charges repel; unlike charges attract.

Misconception buster: objects are charged by moving electrons, never protons. Protons are locked in the nucleus and do not move. "Positive" just means a shortage of electrons.

Charging by friction

Static: dangers and uses

A build-up of static charge can discharge as a spark — sometimes dangerous, sometimes useful.

• Danger — refuelling: when fuel flows through a pipe into an aircraft or tanker, friction builds up charge. A spark could ignite the fuel vapour, so the pipe and vehicle are earthed (bonded) to let the charge flow away safely.
• Use — photocopiers & inkjet printers: in a photocopier, charged toner is attracted to oppositely charged parts of a drum and then onto the paper; in an inkjet printer, charged ink droplets are steered by charged plates to exactly the right place on the paper.

Sort it out next: decide whether each scenario is a danger or a use of static electricity.

Danger or use of static?

Tap a scenario, then tap the box it belongs in.

Why can flowing fuel be dangerous?

The equations to know

You've covered the key ideas across the four parts of 4PH1 Section 2 — units, mains electricity, energy & voltage in circuits, and electric charge (static). Press Finish to see your score.