Magnetism and Electromagnetism

At which part of a magnet are the magnetic forces strongest?

At the poles of the magnet.

What happens when two magnets are brought close together?

They exert a non-contact force on each other.

What happens when two like poles of permanent magnets are brought together?

They repel each other.

What happens when two unlike poles of permanent magnets are brought together?

They attract each other.

What is a permanent magnet?

A magnet that produces its own magnetic field.

What is an induced magnet?

A material that becomes magnetic when placed in a magnetic field.

What force does induced magnetism always cause?

A force of attraction.

What happens to an induced magnet when it is removed from a magnetic field?

It loses most or all of its magnetism quickly.

What is a magnetic field?

The region around a magnet where a force acts on another magnet or magnetic material.

Which materials are magnetic?

Iron, steel, cobalt and nickel.

What force acts between a magnet and a magnetic material?

An attractive force.

How does magnetic field strength change with distance from a magnet?

The magnetic field becomes weaker as the distance from the magnet increases.

Where is a magnetic field strongest around a bar magnet?

At the poles.

What is the direction of a magnetic field at any point?

The direction of the force that would act on a north pole placed at that point.

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

From the north-seeking pole to the south-seeking pole outside the magnet.

How can the magnetic field pattern of a bar magnet be plotted?

Use a small compass to determine the field direction at different points and mark the direction of the compass needle.

What does a magnetic compass contain?

A small bar magnet.

Why does a compass needle point in a particular direction?

The needle aligns with the Earth's magnetic field.

What evidence suggests that the Earth's core must be magnetic?

A compass needle aligns with the Earth's magnetic field, showing that the Earth produces a magnetic field.

What is produced around a conducting wire when current flows through it?

A magnetic field.

What factors affect the strength of the magnetic field around a current-carrying wire?

The current through the wire and the distance from the wire.

How does increasing the current in a wire affect its magnetic field?

It increases the strength of the magnetic field.

How does increasing the distance from a current-carrying wire affect the magnetic field?

The magnetic field becomes weaker.

What is a solenoid?

A coil of wire that produces a magnetic field when current flows through it.

Why does shaping a wire into a solenoid increase the magnetic effect?

The magnetic fields around the individual turns combine to produce a stronger magnetic field.

What is the magnetic field like inside a solenoid?

Strong and uniform.

What is the shape of the magnetic field around a solenoid?

It has a similar shape to the magnetic field of a bar magnet.

What is an electromagnet?

A solenoid with an iron core.

Why does adding an iron core increase the strength of a solenoid's magnetic field?

Iron is easily magnetised and increases the strength of the magnetic field.

How can the magnetic effect of a current be demonstrated?

Place a compass near a current-carrying wire; the compass needle deflects because the current produces a magnetic field.

What is the motor effect? (HT)

The force produced when a current-carrying conductor is placed in a magnetic field.

What rule is used to determine the direction of the force in the motor effect? (HT)

Fleming's left-hand rule.

In Fleming's left-hand rule, what does the first finger represent? (HT)

The direction of the magnetic field.

In Fleming's left-hand rule, what does the second finger represent? (HT)

The direction of conventional current.

In Fleming's left-hand rule, what does the thumb represent? (HT)

The direction of the force or motion of the conductor.

What factors affect the size of the force on a current-carrying conductor in a magnetic field? (HT)

The current, magnetic flux density and length of conductor in the magnetic field.

What happens to the force if the current is increased? (HT)

The force increases.

What happens to the force if the magnetic field is stronger? (HT)

The force increases.

What happens to the force if the length of conductor in the magnetic field is increased? (HT)

The force increases.

What happens to the direction of the force if the current direction is reversed? (HT)

The direction of the force is reversed.

What happens to the direction of the force if the magnetic field direction is reversed? (HT)

The direction of the force is reversed.

What equation links force, magnetic flux density, current and length? (HT)

F = BIl.

What do the symbols mean in F = BIl? (HT)

F is force in newtons (N), B is magnetic flux density in tesla (T), I is current in amperes (A), and l is length in metres (m).

When can F = BIl be used? (HT)

When the conductor is at right angles to the magnetic field.

What is the unit of magnetic flux density? (HT)

Tesla (T).

How does an electric motor work? (HT)

A current-carrying coil is placed in a magnetic field. Forces act in opposite directions on opposite sides of the coil, causing a turning effect that makes the coil rotate.

Why do the forces on opposite sides of a motor coil cause rotation? (HT)

The forces act in opposite directions on opposite sides of the coil, producing a turning effect.

What is the basis of an electric motor? (HT)

The motor effect: a current-carrying conductor in a magnetic field experiences a force.

How do loudspeakers use the motor effect? (Physics only, HT)

They convert variations in current in an electrical circuit into pressure variations in sound waves.

How does a moving-coil loudspeaker work? (Physics only, HT)

A coil attached to a cone is placed in the magnetic field of a permanent magnet. The varying current produces a varying force on the coil, causing the cone to vibrate and produce sound waves.

How is the pitch of sound from a loudspeaker changed? (Physics only, HT)

The frequency of the alternating current is changed, causing the frequency of vibration of the cone to change.

What happens when an electrical conductor moves relative to a magnetic field? (Physics only, HT)

A potential difference is induced across the ends of the conductor.

What happens when the magnetic field around a conductor changes? (Physics only, HT)

A potential difference is induced across the conductor.

What is the generator effect? (Physics only, HT)

The production of a potential difference across a conductor due to its movement relative to a magnetic field or a change in the magnetic field around it.

What must be true for an induced potential difference to produce an induced current? (Physics only, HT)

The conductor must form part of a complete circuit.

What factors increase the size of an induced potential difference/current? (Physics only, HT)

A faster movement, a stronger magnetic field and a greater number of turns in the coil increase the induced potential difference.

What determines the direction of an induced current? (Physics only, HT)

The direction of movement of the conductor and the direction of the magnetic field.

What is the direction of the magnetic field produced by an induced current? (Physics only, HT)

It opposes the change that caused the induced current.

How does an alternator use the generator effect? (Physics only, HT)

It uses the generator effect to produce alternating current (AC).

How does a dynamo use the generator effect? (Physics only, HT)

It uses the generator effect to produce direct current (DC).

What happens to the induced potential difference when a coil in a generator cuts through the magnetic field faster? (Physics only, HT)

The induced potential difference increases.

What happens to the induced potential difference when a stronger magnetic field is used in a generator? (Physics only, HT)

The induced potential difference increases.

What happens to the induced potential difference when a coil has more turns? (Physics only, HT)

The induced potential difference increases.

What does the graph of induced potential difference against time from an alternator show? (Physics only, HT)

An alternating potential difference that changes direction periodically.

How does a microphone use the generator effect? (Physics only, HT)

It converts pressure variations in sound waves into variations in current in an electrical circuit.

How does a moving-coil microphone work? (Physics only, HT)

Sound waves cause a diaphragm and attached coil to vibrate in a magnetic field, inducing a varying potential difference and current in the circuit.

What is a basic transformer made from? (Physics only, HT)

A primary coil and a secondary coil of wire wound around an iron core.

Why is iron used as the core of a transformer? (Physics only, HT)

Iron is easily magnetised.

How does a transformer work? (Physics only, HT)

An alternating current in the primary coil produces a changing magnetic field in the iron core. The changing magnetic field induces a potential difference across the secondary coil.

Why must the current in the primary coil of a transformer be alternating? (Physics only, HT)

The magnetic field in the core must continuously change to induce a potential difference in the secondary coil.

What determines whether a transformer is step-up or step-down? (Physics only, HT)

The relative number of turns on the primary and secondary coils.

What is a step-up transformer? (Physics only, HT)

A transformer in which the secondary potential difference is greater than the primary potential difference.

What is a step-down transformer? (Physics only, HT)

A transformer in which the secondary potential difference is less than the primary potential difference.

What equation links the potential differences and number of turns in a transformer? (Physics only, HT)

Vp / Vs = np / ns.

What do the symbols mean in Vp / Vs = np / ns? (Physics only, HT)

Vp is the primary potential difference, Vs is the secondary potential difference, np is the number of turns on the primary coil and ns is the number of turns on the secondary coil.

What is the power equation for an ideal transformer? (Physics only, HT)

VpIp = VsIs.

What does 100% efficiency mean for a transformer? (Physics only, HT)

The electrical power output equals the electrical power input.

Why are transformers used in the National Grid? (Physics only, HT)

They change the potential difference so electrical power can be transmitted efficiently.

What is the National Grid?

A system of cables and transformers linking power stations to consumers.

Why is electricity transmitted at a high potential difference in the National Grid?

A higher potential difference allows the required power to be transmitted at a lower current, reducing energy wasted as thermal energy in the cables.

How does a step-up transformer help the National Grid?

It increases the potential difference before electricity is transmitted through the cables, reducing the current needed for a given power.

How does a step-down transformer help the National Grid?

It decreases the potential difference to a much lower value suitable for domestic use.

What happens to the current when the potential difference is increased for the same power?

The current decreases.

Why does a lower current reduce energy losses in the National Grid?

The cables transfer less energy to the surroundings as thermal energy.

What is the relationship between power, potential difference and current?

P = VI.