P4 Magnetism

Where are the magnetic forces strongest on a magnet?

At the poles of the magnet.

What happens when two magnets are brought close to each other?

They exert a force on each other.

What type of force is exerted if two like poles of a magnet are brought near each other?

A repulsive, non-contact force.

What type of force is exerted if two unlike poles of a magnet are brought near each other?

An attractive, non-contact force.

What is a permanent magnet?

A magnet that produces its own magnetic field.

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

A permanent magnet produces its own magnetic field.

What is an induced magnet?

An induced magnet becomes magnetic when placed in a magnetic field.

What type of force does induced magnetism always cause?

A force of attraction.

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

The induced magnet loses most/all of its magnetism.

What is a magnetic field?

The region surrounding a magnet where another magnet or magnetic material experiences a non-contact force.

Give four examples of magnetic elements.

Iron, Cobalt, Nickel.

What can always be said about the force between a magnet and a magnetic material?

It is always attractive.

How does the strength of a magnetic field alter as you move further away from the magnet producing it?

The magnetic field strength decreases the further you move away.

In what direction does a magnetic field point?

In the direction that a north pole would experience a force if placed in the field; from the north seeking pole to the south seeking pole of a magnet.

What does a magnetic compass contain?

A small bar magnet that points in the direction of the Earth's magnetic field.

What is produced when current flows through a conducting wire?

A magnetic field is produced around the wire.

Describe the magnetic field produced around a current-carrying wire.

Uniform rings (Concentric circles) along the whole length of wire.

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

The magnitude of the current flowing through the wire and the distance from the wire.

What is a solenoid?

A coil of wire which when current passes through creates a strong magnetic field.

Describe the magnetic field found inside a solenoid.

Strong and uniform.

What does a magnetic compass provide evidence of?

The Earth's core being magnetic.

What is an electromagnet?

A solenoid with an added iron core.

What is the motor effect?

When a force is exerted between a magnetic field and a current-carrying conductor placed in that field.

What rule is used to determine the force experienced due to the motor effect?

Fleming's Left-Hand Rule.

What does the first finger represent in Fleming's left hand rule (Finger 2 Piano)?

The forefinger points in the direction of the magnetic field.

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

The second finger points in the direction of current flow in the conductor.

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

The thumb points in the direction of the force produced, and therefore the motion produced by the force.

What factors affect the size of the force on a current-carrying wire in a magnetic field?

The magnitude of the current flowing through the conductor and the strength of the magnetic field that the conductor is placed in.

What happens if the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed?

The direction of the force is reversed.

What happens if the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased?

The strength of the force is increased.

What criteria must be met for the equation linking force, magnetic flux density, current and length to hold?

The conductor must be at right-angles to the magnetic field it is placed in.

What is the unit used for magnetic flux density?

Tesla, T.

What equation is used to calculate the force that a current carrying wire experiences in a field?

Force (N) = Magnetic Flux Density (T) x Current (A) x Length (m) or F = BIL.

How does an electric motor work?

A coil of wire, carrying a current, is placed in a magnetic field, causing a rotational effect due to forces on the two sides perpendicular to the field.

How do loudspeakers make use of the motor effect?

The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound.

Explain how a loudspeaker works.

A cone with a wire wrapped around it is connected to an a.c power supply and is placed in a permanent magnetic field. When current flows through the wire, it creates a second magnetic field, which interacts with the permanent field, producing a force that causes the cone to vibrate.

How is the pitch of the sound from a loudspeaker changed?

The frequency of the a.c current is altered, creating a different frequency of vibration in the cone.

What happens when an electrical conductor moves relative to a magnetic field?

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

What happens to an electrical conductor when there is a change to the magnetic field that it is placed in?

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

What is the requirement for an induced potential difference to cause a current flow?

The conductor must form a closed loop or be part of a complete circuit.

What can be said about the direction of the magnetic field produced by an induced current?

The direction of the field is such that it opposes the change that induced the current.

If there is a change in the magnetic field around a conductor, what changes occur in the conductor?

A potential difference is induced across its ends, driving a current that creates a magnetic field opposing the change in the existing field.

What are two ways that the generator effect is used to generate different types of current?

In an alternator to produce alternating-current and in a dynamo to produce direct-current.

What electromagnetic effect does a microphone take advantage of and how?

The generator effect; it converts the pressure variations in sound waves into alternating current in a circuit.

Describe the makeup of a basic transformer.

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

Why is iron used as the core for a transformer?

It is easily magnetised.

Explain how a transformer works.

An alternating current flows through the primary coil, inducing a changing magnetic field in the core, which induces a current to flow in the secondary coil.

Why must the current flowing through the primary coil of a transformer be alternating?

For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing.

What can be said about the electrical power input and output of a 100% efficient transformer?

The electrical power input is equal to the electrical power output.

What equation is used to link the voltage and number of turns on the coils of a transformer?

VpNs= NpVs