Electromagnetic Induction Flashcards

All 44 cards in this deck

What are the steps to demonstrate electromagnetic induction in the laboratory?

e.g. using a bar magnet, a coil of wire and a sensitive voltmeter

1. Connect the voltmeter across the two ends of the coil.
2. Hold the magnet with its axis parallel to the axis of the coil.
3. Move the magnet into the coil — the voltmeter shows a reading.

True or false? A potential difference is induced while a magnet is held still inside a coil of wire.

False. There must be relative movement (a changing magnetic field) for a p.d. to be induced.

Apart from moving the magnet, how else can a current be induced in a coil connected in a circuit?

By moving the coil relative to the magnet — only the relative movement matters.

How is the generator effect used in the large-scale generation of electrical energy?

A turbine spins a magnet inside coils (or coils in a magnetic field), so the changing magnetic field induces a potential difference and current.

What four factors affect the size of an induced potential difference?

The speed of the relative movement, the number of turns on the coil, the strength of the magnetic field, and the area of the coil.

What two changes reverse the direction of an induced potential difference in a coil?

Reversing the direction of the movement, or reversing the poles of the magnet.

How does the magnetic field produced by an induced current relate to the original change?

It opposes the original change.
e.g. it repels a magnet being pushed in and attracts it as it is pulled out.

True or false? Moving a magnet into a coil more quickly increases the induced potential difference.

True. A faster rate of change of magnetic field gives a larger induced p.d.

True or false? The magnetic field made by an induced current acts in a direction that helps the movement causing it.

False. It always opposes the change that produced it, so work must be done to keep the movement going.

True or false? Using a coil with more turns gives a larger induced potential difference.

True. More turns means a larger induced p.d. for the same movement.

What type of current does an alternator generate?

Alternating current (a.c.) — current that repeatedly changes direction.

What type of current does a dynamo generate?

Direct current (d.c.) — current in one direction only.

What is the job of the slip rings in an alternator?

They keep each end of the coil connected to the same side of the circuit, so the induced current alternates in direction.

What is the job of the split-ring commutator in a dynamo?

It swaps the coil's connections to the circuit every half turn, so the output current stays in the same direction.

What does the current–time graph for an alternator output look like?

A sine-shaped curve going alternately above and below zero (current reverses direction each half turn).

What does the current–time graph for a dynamo output look like?

A series of humps all on the same side of the time axis — the current varies in size but never reverses direction.

True or false? A loudspeaker works by electromagnetic induction.

False. It works by the force on a current-carrying coil in a magnetic field; a microphone is the one that uses induction.

What determines the frequency of the sound produced by a loudspeaker or headphone?

The frequency of the alternating current supplied to its coil.

What is the useful energy transfer in a moving-coil microphone?

Energy carried by the sound wave is transferred to electrical energy in the circuit (sound to electrical).

What does a transformer do?

It changes the size of an alternating potential difference (voltage).

What are the three main parts of a transformer?

A primary coil, a secondary coil, and an iron core.

How does the current in the primary coil of a transformer induce a current in the secondary coil?

The alternating current in the primary coil makes a changing magnetic field in the core, which induces an alternating p.d. (and current) in the secondary coil.

True or false? A transformer will work when connected to a battery.

False. A battery gives a steady direct current, so the magnetic field does not change and no p.d. is induced.

How can you tell from its coils that a transformer is a step-up transformer?

It has more turns on the secondary coil than on the primary coil, so the output p.d. is larger.

What is the job of the iron core in a transformer?

It links the changing magnetic field from the primary coil to the secondary coil.

What is the turns ratio equation for a transformer?

$\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$

What are the steps to find the potential difference across the secondary coil of a transformer?

e.g. $V_p = 12$ V, $N_p = 10$ turns, $N_s = 20$ turns

1. Write $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$: $\dfrac{12}{V_s} = \dfrac{10}{20}$
2. Rearrange: $V_s = \dfrac{12 \times 20}{10}$
3. $V_s = 24$ V

What are the steps to find the number of turns on the secondary coil of a transformer?

e.g. $V_p = 200$ V, $V_s = 20$ V, $N_p = 500$ turns

1. Write $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$: $\dfrac{200}{20} = \dfrac{500}{N_s}$
2. Rearrange: $N_s = \dfrac{500 \times 20}{200}$
3. $N_s = 50$ turns

In the transformer equation, what do $N_p$ and $N_s$ stand for, and what units do they have?

The number of turns on the primary and secondary coils — they are just counts, with no units.

True or false? In a transformer the ratio of the potential differences equals the ratio of the numbers of turns.

True. $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$

What is the national grid?

The network of cables, pylons and transformers that transfers electrical power from power stations to consumers.

Where in the national grid is a step-up transformer used, and why?

Between the power station and the transmission lines, to raise the p.d. so the current is smaller and less energy is wasted as heat.

Where in the national grid is a step-down transformer used, and why?

In each locality before homes and businesses, to lower the p.d. to a safe, usable value (about 230 V).

True or false? Electrical energy is transferred across the national grid at high current to improve efficiency.

False. It is transferred at high potential difference and low current, which reduces heating losses.

What is a typical potential difference used in national grid transmission lines?

About $400\,000$ V (compared with 230 V for domestic supply).

What is the equation linking the potential differences and currents of a 100% efficient transformer?

$V_p \times I_p = V_s \times I_s$

What assumption is made when using $V_p \times I_p = V_s \times I_s$?

That the transformer is 100% efficient — the power output equals the power input.

What are the steps to find the secondary current of a 100% efficient transformer?

e.g. $V_p = 200$ V, $I_p = 3$ A, $V_s = 20$ V

1. Write $V_p \times I_p = V_s \times I_s$: $200 \times 3 = 20 \times I_s$
2. Rearrange: $I_s = \dfrac{600}{20}$
3. $I_s = 30$ A

True or false? A step-up transformer increases both the potential difference and the current.

False. As the p.d. is stepped up the current is stepped down, since the power output cannot exceed the power input.

Why is electrical power transmitted at high potential difference?

For a given power, $P = I \times V$ means the current is small, and $P = I^2 \times R$ means less energy is wasted as heat in the cables.

Which equation gives the power wasted as heat in a transmission cable?

$P = I^2 \times R$, where $I$ is the current and $R$ the resistance of the cable.

True or false? Doubling the current in a transmission line doubles the power wasted as heat.

False. Power wasted is proportional to $I^2$, so it becomes four times greater.

What are the steps to calculate the power wasted as heat in a transmission cable?

e.g. current 20 A, cable resistance 5 $\Omega$

1. Use $P = I^2 \times R$.
2. Substitute: $P = 20^2 \times 5$
3. $P = 2000$ W

For the same power transmitted, what happens to the current when the potential difference is stepped up ten times?

The current becomes ten times smaller, because $P = I \times V$.