Uses of Magnetism (OCR GCSE Physics A (Gateway)): Flashcards

Fleming's left-hand rule is a method used to find the direction of the force (thrust) on a current-carrying wire in a magnetic field, using the first finger, second finger, and thumb

The first finger points in the direction of the magnetic field (N to S). The second finger points in the direction of the current. The thumb points in the direction of the force (thrust).

False.

The thumb points in the direction of the force (thrust). It is the second finger that points in the direction of the current.

In Fleming's left-hand rule, the first finger points in the direction of the magnetic field and the thumb points in the direction of the force (thrust).

Thrust is the term used for the force acting on the current-carrying wire -- it is the direction the wire will move as a result of the motor effect

The left-hand rule applies to the motor effect, where a current-carrying wire experiences a force in a magnetic field. The right-hand rule is used for a different situation (the generator effect), so using the correct hand gives the correct direction of force.

True.

The force (thrust), magnetic field, and current are all at 90 degrees to each other -- they act in three different perpendicular directions.

The magnetic field runs from north to south -- always from the north pole of the magnet to the south pole. This is the direction used for the first finger in Fleming's left-hand rule.

A d.c. electric motor is a device that uses the motor effect to convert electrical energy into rotational (kinetic) energy, using a current-carrying coil in a magnetic field.

In a d.c. motor, forces act in opposite directions on each side of the coil, causing it to rotate.

The force can be increased by:

1. Increasing the current in the coil

2. Increasing the strength of the magnetic field

3. Adding more turns to the coil

Complete the table showing how to change d.c. motor behaviour.

False.

Reversing the poles of the magnet reverses the direction of rotation, not the speed. To increase speed, you need to increase the current or use a stronger magnet.

Current flows in opposite directions through the two sides of the coil. Since the magnetic field is the same on both sides, applying Fleming's left-hand rule gives forces in opposite directions, causing the coil to rotate.

Electromagnetic induction is the process by which a potential difference is induced across a conductor when there is a change in the magnetic field around it — this can drive a current in a complete circuit.

False.

A potential difference is induced whenever there is relative movement between the conductor and the magnetic field. This can happen by moving the conductor, or by moving the magnetic field relative to a fixed conductor.

The generator effect is the production of an induced potential difference (and current) when a conductor moves through a magnetic field. It is the opposite of the motor effect — there is no initial current, but one is induced by movement.

If a magnet is stationary inside a coil of wire, the induced potential difference is zero, because there is no relative movement between the conductor and the magnetic field.

The induced potential difference is increased by: greater speed of movement, more turns on the coil, larger area of the coils, and a stronger magnetic field.

The nearest end becomes a north pole. This is because the induced potential difference opposes the change that produces it — the coil repels the incoming magnet, so its nearest end must be a north pole to oppose the north pole being pushed in.

(Higher Tier Only)

True.

This is a key rule of electromagnetic induction: the induced potential difference (and the magnetic field it creates) always acts to oppose the original change, such as repelling an incoming magnet.

In the motor effect, a current already flows in a conductor and a force is produced. In electromagnetic induction (the generator effect), there is no initial current — instead, movement through a magnetic field induces a potential difference and current.

When a magnet is pulled away, a potential difference is still induced, but in the opposite direction — the nearest end of the coil becomes a south pole to attract and oppose the magnet moving away.

An alternator is a generator that converts mechanical energy into electrical energy in the form of alternating current (a.c.).

False.

A dynamo uses a split-ring commutator, not separate slip rings. The commutator swaps connections every half turn to keep the current always in the same direction.

The coil spins in a uniform magnetic field and cuts through the field lines, inducing a potential difference. As the coil rotates, the induced potential difference repeatedly changes direction, producing an alternating current.

A dynamo is the same as an alternator except that it uses a split-ring commutator instead of slip rings, which means the current leaving the dynamo is always in the same direction.

A split-ring commutator is a device in a dynamo that swaps the coil connections every half turn to keep the output current flowing in the same direction at all times.

The split-ring commutator swaps the coil connections every half turn. This reverses the circuit connections each time the induced potential difference changes direction, so the current always flows in the same direction.

False.

The output of a dynamo varies between zero and a maximum value but is always positive (or always negative). The output of an alternator alternates between positive and negative values.

Complete the table comparing an alternator and a dynamo.

A dynamo is a direct-current (d.c.) generator that uses a split-ring commutator to keep the output current flowing in the same direction.

A transformer is a device used to change the value of an alternating potential difference (or current) using the principle of electromagnetic induction.

The current must be alternating so that it produces a changing magnetic field in the iron core. A changing magnetic field is needed to induce a potential difference in the secondary coil.

False.

Iron is used because it is easily magnetised (not permanently magnetised). This allows the changing magnetic field from the primary coil to pass through the core efficiently.

In a transformer, an alternating current in the primary coil creates a changing magnetic field, which passes through the iron core and induces a potential difference in the secondary coil.

A basic transformer has three main parts: a primary coil, a secondary coil, and an iron core.

The induced alternating potential difference in the secondary coil drives an alternating current through the circuit. The current has the same frequency as the alternating current in the primary coil.

True.

The frequency of the induced potential difference in the secondary coil matches the frequency of the alternating current in the primary coil because the magnetic field changes at the same rate.

The changing magnetic field in the iron core cuts through the turns of the secondary coil. Any changing magnetic field cutting through a conductor will induce a potential difference in it.

The transformer equation links the potential differences and number of turns on each coil:

where V_p and V_s are the primary and secondary potential differences, and N_p and N_s are the number of turns on each coil.

A step-up transformer increases the output potential difference.

False.

A step-down transformer has fewer turns on the secondary coil than on the primary coil (N_s < N_p). This means the output potential difference is lower than the input potential difference.

A transformer with 50 turns on the primary coil and 200 turns on the secondary coil will increase the potential difference by a factor of 4.

Complete the table of transformer equation symbols.

True.

In a step-up transformer, N_s > N_p. This means the ratio V_s/V_p = N_s/N_p is greater than 1, so the output potential difference is higher than the input.

Using the transformer equation:

Rearrange for V_s:

V_s = 500 × 800 ÷ 20

V_s = 20 000 V

A dynamic microphone (moving coil microphone) converts pressure variations in sound waves into variations in current in an electrical circuit, using the principle of electromagnetic induction.

Sound waves cause the diaphragm to vibrate, which moves the coil back and forth through a magnetic field. The coil cuts through field lines, inducing an alternating potential difference and therefore an alternating current.

False.

A loudspeaker converts electrical signals into sound using the motor effect, not the generator effect. It works in the opposite way to a microphone.

In a dynamic microphone, sound waves cause the diaphragm to vibrate, which moves the coil through the magnetic field, inducing an alternating current.

A loudspeaker (or headphone) converts electrical signals into sound waves, using the motor effect.

The alternating current creates a changing magnetic field around the coil. This interacts with the field from the permanent magnet, exerting a force that constantly changes direction, causing the coil to oscillate.

False.

A loudspeaker uses alternating current, not direct current. The alternating current produces a changing magnetic field that constantly reverses the force on the coil, making it oscillate and produce sound.

(Higher Tier Only) Complete the table comparing a dynamic microphone and a loudspeaker.

The oscillating coil causes the speaker cone to oscillate. The oscillating cone causes the surrounding air to oscillate, creating sound waves.