Induced Potential, Transformers & the National Grid Flashcards

All 40 cards in this deck

What is the generator effect?

When a conductor moves relative to a magnetic field, or the magnetic field around a conductor changes, a potential difference is induced across the ends of the conductor.

What extra condition is needed for an induced potential difference to produce an induced current?

The conductor must be part of a complete circuit.

What three factors increase the size of the induced potential difference in a coil?

Moving faster (faster rate of change of field), a stronger magnetic field, and more turns on the coil.

What two things determine the direction of an induced potential difference or current?

The direction the conductor moves and the direction (polarity) of the magnetic field. Reversing either one reverses the induced current.

What is the direction of the magnetic field produced by an induced current?

It opposes the original change — either the movement of the conductor or the change in the magnetic field.

True or false? A generator becomes harder to turn when a lamp is connected across it.

True. The induced current produces a magnetic field that opposes the movement of the coil.

True or false? A potential difference is induced in a coil when a magnet is held still inside it.

False. The magnetic field must be changing (or the conductor moving) for a potential difference to be induced.

What type of current does an alternator generate, and which component allows this?

Alternating current (ac), using slip rings.

What type of current does a dynamo generate, and which component allows this?

Direct current (dc), using a split-ring commutator.

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

They provide a continuous, moveable contact between the rotating coil and the external circuit, so the wires do not twist.

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

It swaps the connections between the coil and the circuit every half turn, so the output current is always in the same direction.

What are the steps by which an alternator generates an alternating current?

e.g. turning the handle of a hand-cranked alternator

1. The rotating coil cuts magnetic field lines, so a potential difference is induced across it.
2. The coil is part of a complete circuit, so a current is induced.
3. Every half turn the induced potential difference reverses, so the current changes direction — ac.

True or false? Both an alternator and a dynamo work by the generator effect.

True. Both induce a potential difference by rotating a coil in a magnetic field; they differ only in how the coil connects to the circuit.

What shape is the potential difference–time graph produced by an alternator?

A sine wave that alternates between positive and negative values, crossing zero twice each turn.

What shape is the potential difference–time graph produced by a dynamo?

A series of humps all on the same side of the time axis (all positive or all negative), touching zero twice each turn.

How does the potential difference–time graph change if the generator coil is turned faster?

The peaks become higher and closer together (larger amplitude, shorter period / higher frequency).

How does the potential difference–time graph change if a stronger magnet is used, with the coil turned at the same speed?

The peaks become higher, but the period and frequency stay the same.

True or false? The output graph of a dynamo goes below the time axis.

False. A dynamo produces dc, so the graph stays on one side of the time axis.

What does a microphone convert sound waves into?

The pressure variations in sound waves are converted into variations in current (or potential difference) in a circuit.

Which effect does a moving-coil microphone use?

The generator effect (electromagnetic induction).

What are the steps by which a moving-coil microphone produces a current?

e.g. someone speaking into a microphone

1. The pressure variations of the sound wave make the diaphragm vibrate.
2. The diaphragm makes the attached coil vibrate near/over a magnet.
3. The coil cutting the magnetic field induces an alternating current in the circuit.

True or false? A louder sound produces a larger current in a moving-coil microphone.

True. The diaphragm and coil move further and faster, so a larger potential difference is induced.

What are the three main parts of a basic transformer?

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

Why is the core of a transformer made of iron?

Iron is easily magnetised, so it carries the changing magnetic field from the primary coil to the secondary coil.

What are the steps by which a transformer induces a potential difference in the secondary coil?

e.g. an alternating input p.d. of $230\ \text{V}$ across the primary

1. The alternating current in the primary coil produces a changing magnetic field.
2. The iron core carries this changing magnetic field to the secondary coil.
3. The changing field induces an alternating potential difference across the secondary coil.

In a step-up transformer, how do the potential differences and the numbers of turns compare?

$V_s > V_p$, and the secondary coil has more turns than the primary.

In a step-down transformer, how do the potential differences and the numbers of turns compare?

$V_s < V_p$, and the secondary coil has fewer turns than the primary.

True or false? A transformer will step up the potential difference of a battery (dc) supply.

False. A steady dc gives a constant magnetic field, so no potential difference is induced in the secondary coil.

What is the transformer equation linking potential differences and numbers of turns?

$\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}$

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

$V_s \times I_s = V_p \times I_p$ (power output = power input).

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

e.g. $V_p = 1.5\ \text{V}$, $V_s = 5.0\ \text{V}$, $n_p = 150$

1. Write the equation: $\dfrac{V_p}{V_s} = \dfrac{n_p}{n_s}$, so $\dfrac{1.5}{5.0} = \dfrac{150}{n_s}$.
2. Rearrange: $n_s = \dfrac{150}{0.3}$.
3. $n_s = 500$ turns.

What are the steps to find the current drawn from the input supply for a given power output?

e.g. output power $60\ \text{W}$, input p.d. $230\ \text{V}$, 100% efficient

1. Power input = power output: $V_p I_p = 60\ \text{W}$.
2. Rearrange: $I_p = \dfrac{60}{230}$.
3. $I_p = 0.26\ \text{A}$.

In a 100% efficient transformer, what does $V_s \times I_s$ give?

The power output at the secondary coil, in watts.

True or false? In a step-up transformer the current in the secondary coil is larger than in the primary coil.

False. If the pd is stepped up, the current is stepped down, because the power output equals the power input.

Which equation gives the power wasted as heating in transmission cables?

$P = I^2R$

Why is electrical power transmitted across the National Grid at high potential difference?

For the same power, a high pd means a small current, and power loss $= I^2R$, so less energy is transferred to the surroundings from the cables.

What does a step-up transformer do to the potential difference and current before transmission?

It increases the potential difference and decreases the current, increasing the efficiency of transmission.

Why is a step-down transformer used before electricity reaches homes?

It decreases the potential difference to a safe value for consumers.

True or false? Transformers are one reason the National Grid uses ac rather than dc.

True. Transformers only work with a changing (alternating) current, which produces a changing magnetic field.

True or false? Transmitting at high potential difference saves energy because it lowers the resistance of the cables.

False. The resistance is unchanged; the high pd lowers the current, and losses depend on $I^2R$.