4.7 Magnetism and electromagnetism

How does a transformer work?

1. An alternating current flows through the primary coil 2. This alternating current induces an alternating magnetic field 3. The alternating magnetic field in the iron core induces an alternating p.d. in the secondary coil 4. If the secondary coil is part of a complete circuit, an alternating current flows in the secondary coil

What shape is the magnetic field around a solenoid bar?

The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid. An electromagnet is a solenoid with an iron core.

What are the poles of a magnet?

The poles of a magnet are the places where the magnetic forces are strongest.

What is the ratio of the potential difference across the primary and secondary coils of a transformer dependent on?

The ratio of the potential differences across the primary and secondary coils of a transformer V and V depends on the ratio of the number of turns on each coil, n and n .V /V = N /N

What is the magnetic field?

The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.

Does a DC generator include a slip or split ring?

The split-ring commutator rotates with the coil and maintains a complete circuit with the external circuit. Each half turn the sides of the coil connect to the opposite side of the circuit. The direction of the induced potential difference between the ends of the coil reverses when the coil cuts through field lines in the opposite direction, the direction of the current induced in the circuit doesn't change.

What does the strength of a magnetic field depend on?

The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.

What is the right hand grip method?

The thumb points in the direction of the current. Fingers wrap in direction of magnetic field.

What do like and unlike poles do?

Two like poles repel each other. Two unlike poles attract each other.

What is the motor effect?

When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.

What happens when a current flows through a conducting wave?

When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire.

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

When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly

What happens when two magnets are brought together?

When two magnets are brought close together they exert a force on each other.

What is a transformer?

A basic transformer consists of a primary coil and a secondary coil wound on an iron core.

What is the basis of an electric motor?

A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.

What is a magnetic compass?

A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth's magnetic field.

What does a permanent magnet do?

A permanent magnet produces its own magnetic field

What is an AC generator called?

Alternator

What does an induced current generate?

An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.

What is an induced magnet?

An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction.

Why does the tinfoil in a loudspeaker move?

Current flows through it so the foil becomes a magnet and interacts with the permanent magnet and experiences a force

What does a step-down transformer do?

Decreases the voltage and increases the current

What affects the direction of the force (motor effect)?

Direction of current and direction of magnetic field

What is a DC generator called?

Dynamo

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

For a conductor at right angles to a magnetic field and carrying a current: force = magnetic flux density × current × length F = B I l

What is Lenz law?

Lenz's law helps us to work out the direction of the induced field. It states that an induced current generates a magnetic field that opposes the thing that caused it to exist in the first place.

What side of a cell is positive?

Long side positive Short side negative

How do loudspeakers and headphones work?

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.

What do microphones use?

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits

What direction does 'conventional current' flow in?

Positive to negative

What increases the strength of the magnetic field?

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.

What is the force between a magnet and a magnetic material?

The force between a magnet and a magnetic material is always one of attraction.

What is the generator effect used for?

The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.

How can the force experienced by a wire in a magnetic field be increased?

- Putting a greater length of wire in the field - Using a stronger magnetic field - Using a larger current

How does a moving-coil microphone work?

1. The sound waves hit the microphone 2. The changes in air pressure related to the sound wave cause the microphone diaphragm to oscillate 3. The microphone diaphragm and coil vibrate at the same freq. as the incoming sound wave 4. This induces a p.d. and current in the coil with the same freq. as the incoming sound wave and an amplitude dependent on the amplitude of the incoming wave

How does an AC generator induce a p.d?

As the coil rotates, cutting magnetic field lines, it generates a potential difference between the ends of the coil. The potential difference is greatest at the instant when the coil is horizontal, because this is when the moving coil is cutting through field lines.

What type of force does a magnet exert?

Attraction and repulsion between two magnetic poles are examples of non-contact force.

What is the generator effect?

If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.

What would be true if transformers were 100% efficient?

If transformers were 100% efficient, the electrical power output would equal the electrical power input. V × I = V × I Where V × I is the power output (secondary coil) and V × I is the power input (primary coil).

What is true in a step-down transformer?

In a step-down transformer V < V

What is true in a step-up transformer?

In a step-up transformer V > V

Does an AC generator include a slip or split ring?

In an alternator, slip rings are connected to the ends of the coil to allow the coil to spin without winding the wire around itself. The brushes are conducting contacts that touch the slip rings and complete the circuit.

What does a step-up transformer do?

Increases the voltage and decreases the current

What are transformers made of?

Iron is used as it is easily magnetised.

How does a DC generator induce a p.d?

It is used to generate a direct current by moving the coil in a magnetic field. As the coil turns it cuts the magnetic field lines. Doing this induces a potential difference between the ends of the coil. This causes an induced current to pass through the coil and through the circuit the coil is a part of.

What affects the size of the force (motor effect)?

Strength of magnet, size of current and number of coils

What is magnetic flux density measured in?

T, tesla

Why does the movement of the tinfoil vary in a loudspeaker?

The current is alternating- size of the current is varying, direction of the current is varying, frequency is changing

What is the direction of the magnetic field given by?

The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.