Magnetism and the Motor Effect

What happens between unlike and like magnetic poles?

Unlike magnetic poles attract; like magnetic poles repel.

What are permanent magnetic materials?

Permanent magnetic materials retain their magnetism and are difficult to magnetise and demagnetise. Steel and cobalt are examples.

What are temporary magnetic materials?

Temporary magnetic materials are easily magnetised but lose their magnetism easily. Iron and nickel are examples.

What is the difference between a permanent magnet and an induced magnet?

A permanent magnet remains magnetised and produces its own magnetic field. An induced magnet becomes magnetised when placed in a magnetic field and usually loses its magnetism when removed.

What is a magnetic field?

A region where a magnetic material or another magnet experiences a force.

What do magnetic field lines show?

They show the direction and shape of a magnetic field. The closer the lines are together, the stronger the magnetic field.

What is the direction of magnetic field lines around a bar magnet?

Outside a bar magnet, magnetic field lines go from the north pole to the south pole.

How is the strength of a magnetic field shown on a field-line diagram?

A greater concentration of field lines represents a stronger magnetic field.

How can plotting compasses be used to show the magnetic field around a magnet?

Place small plotting compasses at different positions around the magnet. The compass needles show the direction of the magnetic field, allowing the field lines to be plotted.

How can plotting compasses be used to investigate Earth's magnetic field?

Place plotting compasses at different positions and record the direction in which their needles point. This shows the direction of Earth's magnetic field.

Why does a magnetic compass provide evidence that Earth's core must be magnetic?

A compass aligns with Earth's magnetic field, showing that Earth produces a magnetic field. This provides evidence that Earth's interior/core must contain magnetic material or act as a magnetic source.

How can you show that a current creates a magnetic field around a straight conductor?

Pass a current through a long straight wire and place plotting compasses around it. The compasses show that the magnetic field forms concentric circles around the wire.

What happens to the direction of the magnetic field around a wire when the current is reversed?

The direction of the magnetic field is reversed.

What determines the strength of the magnetic field around a long straight conductor?

The field becomes stronger when the current increases and weaker as the distance from the conductor increases.

What is a solenoid?

A solenoid is a coil of wire carrying an electric current and acts as an electromagnet.

Why is the magnetic field inside a solenoid strong?

The magnetic fields from the individual coils add together inside the solenoid, producing a strong, almost uniform magnetic field along its centre.

Why is the magnetic field outside a solenoid weaker?

The magnetic fields from the individual coils largely cancel outside the solenoid, producing a much weaker field.

What happens when a current-carrying conductor is placed near a magnet?

The conductor experiences a force, and an equal and opposite force acts on the magnet.

Why do magnetic forces occur?

Magnetic forces are caused by interactions between magnetic fields.

What is the motor effect?

The motor effect is the force experienced by a current-carrying conductor placed in a magnetic field.

What does Fleming's left-hand rule show?

It shows the relative directions of the force, current and magnetic field when they are mutually perpendicular.

What does the first finger represent in Fleming's left-hand rule?

The first finger represents the direction of the magnetic field.

What does the second finger represent in Fleming's left-hand rule?

The second finger represents the direction of the conventional current.

What does the thumb represent in Fleming's left-hand rule?

The thumb represents the direction of the force or motion.

What is the equation for the force on a current-carrying conductor at right angles to a magnetic field?

F = B × I × l

What does F represent in F = BIl?

F is the force on the conductor, measured in newtons (N).

What does B represent in F = BIl?

B is the magnetic flux density, measured in tesla (T), equivalent to N/(A m).

What does I represent in F = BIl?

I is the current in the conductor, measured in amperes (A).

What does l represent in F = BIl?

l is the length of conductor in the magnetic field, measured in metres (m).

What happens to the force if the current increases?

The force increases in direct proportion to the current, provided the magnetic flux density and conductor length remain constant.

What happens to the force if the magnetic flux density increases?

The force increases in direct proportion to the magnetic flux density, provided the current and conductor length remain constant.

What happens to the force if the length of conductor in the magnetic field increases?

The force increases in direct proportion to the length, provided the magnetic flux density and current remain constant.

How does the motor effect cause an electric motor to rotate?

A current-carrying coil in a magnetic field experiences forces in opposite directions on opposite sides of the coil. These forces produce a turning effect, causing the coil to rotate.

Why does the coil in an electric motor continue rotating?

The current in the coil is reversed every half-turn, so the forces continue producing a turning effect in the same direction.

What is magnetic flux density measured in?

Tesla (T), which is equivalent to newtons per ampere metre (N/(A m)).