Electromagnetic Effects (Cambridge (CIE) IGCSE Physics)

The size of the induced potential difference in the wire in the diagram is determined by:

• The speed at which the wire is moved

• The strength of the magnetic field

The size of the induced potential difference in the coil in the diagram is determined by:

• The speed at which the magnet is moved

• The strength of the magnetic field

• The number of turns on the coils of wire

• The size of the coils

• The strength of the magnetic field

Electromagnetic induction be demonstrated using a wire and a U-shaped magnet by:

• Moving the wire up or down between the magnet poles

• Moving the magnet up or down while holding the wire still

Electromagnetic induction be demonstrated using a coil and a bar magnet by:

• Moving the magnet into or out of the coil

• Moving the coil towards or away from the magnet

A sensitive voltmeter or ammeter can be used to determine if an e.m.f. has been induced in a conductor.

True.

For electromagnetic induction to occur, there must be relative motion between the conductor and the magnetic field.

Therefore, an e.m.f. is induced when a wire moves up and down between the poles of a stationary magnet.

False.

For electromagnetic induction to occur, there must be relative motion between the conductor and the magnetic field.

Therefore, an e.m.f. is not induced when a coil and a bar magnet move in the same direction at the same speed.

Three factors that affect the magnitude of an induced e.m.f. are:

• The speed of motion of the magnet or coil

• The number of turns on the coil

• The strength of the magnet

True.

The size of the induced e.m.f. across a conductor is directly proportional to the rate at which it cuts magnetic field lines.

The induced e.m.f. can be increased using a wire and a U-shaped magnet by:

• increasing the length of the wire between the poles, or using a magnet with wider poles

• moving the wire between the magnet poles faster

• increasing the strength of the magnet

The induced e.m.f. can be increased using a coil and a bar magnet by:

• adding more turns to the coil

• moving the magnet into the coil faster

• increasing the strength of the magnet

False.

The number of paper clips attracted by a strong electromagnet can be increased by increasing the number of coils of wire.

An A.C. generator is a device which converts energy from a mechanical store to an electrical store.

The output of an A.C. generator is alternating current.

A simple A.C. generator consists of:

1. A rectangular coil of wire rotating in a magnetic field

2. Slip rings and brushes

The purpose of slip rings is to provide a connection between the rotating coil and the external circuit.

The direction of the current reverses every half rotation of the coil in the A.C. generator.

False.

A maximum value of current is induced when the coil is perpendicular to the magnetic field.

True.

No current is induced when the coil is parallel to the field.

For an A.C. generator, the shape of the graph of output current against time is a sine or cosine curve.

The frequency of an A.C. output is the number of complete cycles it makes each second e.g. 1 cycle per second = 1 Hz.

In an A.C. generator, the size of the induced current can be increased by:

• adding more turns to the coil

• rotating the coil faster

• increasing the strength of the magnet

The magnetic field due to a current in a straight wire forms concentric circles that get further apart with increasing distance from the wire.

The right-hand thumb rule can be used to determine the direction of the magnetic field around a current-carrying wire.

Inside a solenoid, the magnetic field is uniform, so the field lines are equally spaced straight lines pointing in the same direction.

False.

When viewed from the end of a solenoid, the pole is a south pole if the current travels clockwise around the coil.

The strength of the magnetic field around a solenoid can be increased by:

• increasing the current

• increasing the number of turns on the coil

• adding a soft iron core to the centre of the coil

Changing the direction of the current through a solenoid would change the direction of the magnetic field around it.

A current-carrying conductor in the presence of a magnetic field experiences a force, resulting in its movement or deflection.

False.

A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines.

Fleming's left-hand rule is used to determine the direction of the force acting on a current-carrying conductor in a magnetic field, as well as the direction of the current and magnetic field.

The quantity represented by each finger in Fleming's left-hand rule is:

• First finger is the direction of the magnetic field

• Second finger is the direction of the current in the wire

• Thumb is the direction of the force which shows the direction of the movement of the wire

True.

The direction of the force on a current-carrying wire depends on the direction of the current and the direction of the magnetic field, both of which should be perpendicular to each other.

The maximum force experienced by a charged particle entering a magnetic field occurs when the particle is traveling perpendicular to the field lines.

Charged particles in a magnetic field experience a force which causes them to deflect.

The direction of the force experienced by an electron in a magnetic field can be determined using Fleming's left-hand rule, where the second finger (representing current) points opposite to the direction of electron flow.

The motor effect can be used to create a simple d.c. electric motor.

A simple d.c. electric motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field.

The coil in a d.c. motor rotates due to experiencing a force (and therefore a turning effect) when an electric current flows through it in a magnetic field.

True.

Every half turn, the pair of split rings swap contact with the brushes.

The split-ring commutator reverses the direction of the current in the coil of a d.c. motor every half-turn, ensuring continuous rotation as long as the current flows.

The factors that affect the speed of rotation of a d.c. motor are:

• increasing the current through the rotating coil

• using a stronger magnet on either side of the rotating coil

The direction of rotation of the coil in a d.c. motor can be changed by:

• reversing the direction of the current

• reversing the poles of the magnet

The force supplied by a d.c. motor is increased by:

• increasing the current in the coil

• increasing the strength of the magnetic field

• adding more turns to the coil

You determine the direction of rotation of the coil in a d.c. motor by:

• drawing arrows for the direction of the magnetic field and current

• using Fleming's left-hand rule to find the force direction

• determining the direction of rotation of the coil based on the forces

The function of a transformer is to change the value of an alternating potential difference or current.

A simple transformer consists of:

• a soft iron core

• a primary coil

• a secondary coil

A step-up transformer increases the potential difference of a power source.

A step-down transformer decreases the potential difference of a power source.

True.

When an alternating current is supplied to the primary coil of a transformer, it produces a changing magnetic field in the iron core.

False.

A step-down transformer has fewer turns on the secondary coil than the primary coil.

True.

When a changing magnetic field is produced in the iron core of a transformer, it induces a changing potential difference in the secondary coil.

A step-down transformer has fewer turns on the primary coil than the secondary coil.

A step-up transformer has fewer turns on the primary coil than the secondary coil.

True.

To increase potential difference, a step-up transformer must have more turns on the secondary coil than on the primary coil.

The type of transformer in the diagram is a step-down transformer.

The primary voltage of the transformer in the diagram is 300 V.

The purpose of transformer calculations is to determine the output potential difference (voltage) of a transformer based on:

• Number of turns on the primary and secondary coils

• Input potential difference (voltage)

True.

The equation for the ratio of potential differences across the primary and secondary coils of a transformer is .

The equation for calculating the output potential difference of a transformer is

Where:

• = output potential difference, measured in volts (V)

• ​ = the number of turns on the secondary coil

• ​ = input potential difference, measured in volts (V)

• ​ = the number of turns on the primary coil