Electromagnetism

What is a magnetic field?

A region of space in which moving charged particles experience a force

What produces a magnetic field?

Moving electric charges or magnetised materials

What magnetic field pattern is produced by a straight current-carrying wire?

Concentric circular field lines around the wire

How can the direction of the magnetic field around a straight wire be found?

Using the right-hand grip rule

State the right-hand grip rule for a straight wire

Thumb points in direction of current, fingers curl in direction of magnetic field

What is a permanent magnet?

A magnet made from magnetically hard material that produces its own magnetic field

How can magnetic field patterns be visualised?

Using iron filings

Why do iron filings align in a magnetic field?

They become magnetised and align with the field

What do magnetic field lines show?

Field direction and relative strength

What does the spacing of magnetic field lines represent?

Field strength

In which direction do magnetic field lines point?

From north pole to south pole

What is a uniform magnetic field?

A field with parallel, equally spaced field lines

How can a near-uniform magnetic field be produced?

Between the poles of a horseshoe magnet

What is an electromagnet?

A magnet created by a current-carrying coil

How does a solenoid behave when current flows?

It behaves like a bar magnet

How can the strength of an electromagnet be increased?

Increasing current, number of turns, or adding a soft iron core

Why is soft iron used as a core in electromagnets?

It magnetises and demagnetises easily

What is the difference between magnetically hard and soft materials?

Hard materials retain magnetisation, soft materials do not

What causes Earth’s magnetic field?

Movement of charged particles in the molten iron core

Why does a compass point towards geographic north?

It aligns with Earth’s magnetic field

Where is Earth’s magnetic south pole located?

Close to the geographic North Pole

What is the role of Earth’s magnetic field?

It protects Earth from solar wind

What causes the aurora borealis and australis?

Charged solar particles interacting with Earth’s magnetic field

Do like magnetic poles attract or repel?

Repel

Do unlike magnetic poles attract or repel?

Attract

What shape are magnetic field lines around a straight wire?

Concentric circles

How does magnetic field strength vary with distance from a wire?

It decreases with increasing distance

What is Fleming’s left-hand rule used for?

Determining the direction of force on a current-carrying conductor

State Fleming’s left-hand rule

First finger: field, second finger: current, thumb: force

When is the force on a current-carrying wire maximum?

When the current is perpendicular to the magnetic field

What is the equation for force on a current-carrying wire?

F = BIL sinθ

What does B represent in F = BIL sinθ?

Magnetic flux density

What is the SI unit of magnetic flux density?

Tesla (T)

Define one tesla

A field producing a force of 1 N per metre on a 1 A current at 90°

What is magnetic flux?

The magnetic field passing through an area

What is the equation for magnetic flux?

Φ = BA cosθ

What is the unit of magnetic flux?

Weber (Wb)

What is magnetic flux linkage?

Total magnetic flux through all turns of a coil

What is the equation for magnetic flux linkage?

NΦ = BAN cosθ

What is the equation for the force that acts on a charged particle moving in a magnetic field?

F = Bqv

Why does a magnetic field not change the speed of a charged particle?

The force is perpendicular to the velocity

What motion does a charged particle undergo in a uniform magnetic field?

Circular motion

What provides the centripetal force for circular motion in a magnetic field?

The magnetic force

What is electromagnetic induction?

The production of an emf due to a change in magnetic flux linkage

State Faraday’s law of electromagnetic induction

Induced emf is proportional to the rate of change of flux linkage

State Lenz’s law

The induced current opposes the change that produced it

Why does Lenz’s law conserve energy?

Work must be done against induced forces to produce electrical energy

How can an induced emf be produced in a wire?

Moving the wire through a magnetic field

What happens to induced current when motion stops?

It falls to zero

What is a search coil used for?

Measuring magnetic flux density

What is the principle of an AC generator?

Rotating a coil in a magnetic field induces an alternating emf

When is induced emf maximum in a generator?

When flux linkage changes most rapidly

What is the principle of an electric motor?

A current-carrying coil in a magnetic field experiences a turning force

What produces torque in a motor?

Equal and opposite forces on opposite sides of the coil

What is the function of a split-ring commutator?

Reverses current every half-turn to maintain rotation

What is the function of a transformer?

To change the voltage of an AC supply

Why do transformers require AC?

A changing magnetic flux is required for induction

What is the transformer turns ratio equation?

Vs/Vp = Ns/Np

What is a step-up transformer?

A transformer that increases voltage

What is a step-down transformer?

A transformer that decreases voltage

Why are step-up transformers used in power transmission?

To reduce power loss due to heating

What causes energy loss in transformers?

Resistive heating and magnetic heating

How can transformer efficiency be improved?

Using laminated soft iron cores and low-resistance wire