Static electricity

Rub two insulators together and electrons (negative) are transferred from one to the other. The one that gains electrons becomes negative; the one that loses them is left positive.

Watch out: a positive object is not made by moving positive charge in — it is made by electrons leaving. Static only builds up on insulators.

Electric fields & field lines

A charged object is surrounded by an electric field — the region where it exerts a force on another charge. We draw field lines that point in the direction a positive test charge would be pushed: away from + and towards –.

Sparking: if charge builds up enough, the field can become strong enough to push electrons through the air — you see a spark as the charge suddenly flows.

What makes it positive?

Current is a flow of charge

An electric current is the rate of flow of charge. Charge needs a source of potential difference and a complete (closed) circuit to flow.

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

Your turn — charge flow

Potential difference & resistance

Potential difference (p.d., in volts) is the energy transferred per unit charge — it pushes the current round. Resistance opposes the current:

Required practical: connect a component in series with an ammeter and a variable resistor, with a voltmeter in parallel across it. Vary the p.d., record V and I, and find R = V ÷ I. Investigate how the length of a wire changes its resistance.

Where the meters go

Two rules you must never mix up:

• An ammeter measures current, so it goes in series (in the loop).
• A voltmeter measures p.d., so it goes in parallel (across the component).

Your turn — resistance

I–V graphs of components

Plotting current against p.d. reveals whether a component's resistance is constant (ohmic) or changes:

Why the lamp bends: as current rises the filament gets hotter, so its resistance increases — the curve flattens. Both the resistor and lamp lines pass through the origin.

Read the graph

Tap the component that matches each I–V description.

Sensing components

Two components change resistance in response to their surroundings:

• An LDR (light-dependent resistor): resistance falls as light gets brighter.
• A thermistor: resistance falls as temperature rises (it's hotter → lower R).

Sensing the surroundings

Series vs parallel circuits

Series rules

Electrical power & energy

Power is the rate at which a device transfers energy. In a circuit it depends on p.d. and current:

The energy transferred over a time is then:

Your turn — electrical power

Your turn — energy transferred

Magnets & poles

Every magnet has a north and a south pole. Like poles repel; unlike poles attract — a non-contact force.

• A permanent magnet produces its own magnetic field all the time.
• An induced magnet only becomes magnetic when placed in a field (e.g. a steel paperclip near a magnet) — and the force between them is always attractive. It loses most of its magnetism when removed.

Earth's field: a freely-suspended dipping compass lines up with Earth's magnetic field, which is evidence that Earth's core is magnetic. The geographic and magnetic poles are not in the same place.

Field around a bar magnet

Magnetic field lines always run from North to South outside the magnet. They are closest together at the poles — that is where the field is strongest. Arrows point the way a compass north would point.

Which way do the lines point?

A current makes a magnetic field

When a current flows through a wire it creates a magnetic field of concentric circles around it. The field is stronger with a bigger current and closer to the wire.

Wind the wire into a coil — a solenoid — and the fields add up to give a strong, uniform field inside, just like a bar magnet. Add an iron core and you have an electromagnet (used in scrapyard cranes, relays and bells), whose strength you can switch on and off.

The motor effect & F = BIL

Put a current-carrying wire in a magnetic field and the two fields interact, exerting a force on the wire (the motor effect). The force is greatest when the wire is at right angles to the field:

Fleming's left-hand rule gives the direction. Hold your left hand so:

This force spins a coil in a d.c. motor. A split-ring commutator swaps the current direction every half-turn so the coil keeps rotating the same way.

Your turn — force on a wire (HT)

Induction & transformers

The generator effect is the reverse of the motor effect: move a magnet near a coil (or a wire through a field) and a potential difference is induced across it, which can drive a current. This is used in an alternator to generate a.c. and a dynamo to generate d.c.

A transformer uses this on an iron core: an alternating current in the primary coil makes a changing field that induces a p.d. in the secondary coil. The turns ratio sets the voltages:

Crucial misconception: a transformer only works with a.c. A steady d.c. current makes a steady field, which induces nothing — you need a changing field.

Your turn — transformer (HT)

Match the rule to the idea

Tap a term on the left, then its match on the right.

The equations to know

You've covered P3 Electricity (static, current, p.d., resistance, I–V graphs, circuits, power) and P4 Magnetism (magnets, fields, electromagnets, the motor effect and transformers). Press Finish to see your score.