4 4 electromagnetic effects

Electromagnetic induction is the effect of inducing e.m.f. in a conductor due to the relative motion between a conductor and a magnetic field.

An e.m.f. be induced in a wire by moving it through a magnetic field.

False. An e.m.f. can also be induced by placing a stationary conductor in a changing magnetic field.

The direction of an induced e.m.f. can be reversed by:

Lenz's Law states that the direction of an induced e.m.f. always opposes the change producing it.

Lenz's Law can be demonstrated using a bar magnet and a coil of wire. When the magnet moves into or out of a coil of wire, the induced e.m.f. creates a magnetic field that opposes the motion of the magnet.

False. When the north pole of a bar magnet is pushed towards a coil, the end of the coil begins to act as a north pole. As a result, the two like poles repel in order to oppose the motion of the magnet.

The Right-Hand Dynamo rule is used to determine the direction of an induced e.m.f in a wire moving through a magnetic field. Where:

When using the Right-Hand Dynamo rule, the first finger represents the direction of the magnetic field from the north pole to the south pole.

True. When using the Right-Hand Dynamo rule, the second finger always indicates the direction of positive charge carriers or the flow of current from the positive terminal to the negative terminal of the battery.

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

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

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

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

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:

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:

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

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:

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:

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:

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:

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:

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

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

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

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

A simple transformer consists of:

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:

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: