Define electromagnetic induction .

Electromagnetic induction (the generator effect ) is when a potential difference is induced in a conductor or coil as it moves through a magnetic field, or as the magnetic field through it changes. If the conductor is part of a complete circuit, a current is also induced.

Define the motor effect .

The motor effect is when a current-carrying conductor in a magnetic field experiences a force . It is the opposite of the generator effect, in which movement induces a current rather than a current producing movement.

What condition must be met for a potential difference to be induced in a conductor?

There must be relative movement between the conductor and the magnetic field, so that the conductor cuts through the magnetic field lines.

State four factors that affect the size of the potential difference induced in a coil.

Speed of movement of the wire, coil or magnet; number of turns on the coil; size (area) of the coil; strength of the magnetic field. Increasing any of these increases the induced potential difference.

Increasing the ______ of the magnetic field increases the size of the induced potential difference.

Increasing the strength of the magnetic field increases the size of the induced potential difference.

True or False? Reversing the direction in which a magnet is moved through a coil has no effect on the induced potential difference.

False. Reversing the direction of movement reverses the direction of the induced potential difference — the direction depends on the orientation of the magnet's poles and the direction of movement.

An alternator is a generator that converts mechanical energy into electrical energy as alternating current . It consists of a rotating coil in a magnetic field, connected to the circuit by slip rings and brushes.

A dynamo is a direct current generator. It is built the same way as an alternator, except it uses a split-ring commutator instead of slip rings.

Why does an alternator produce an alternating current?

The rotating coil cuts through the magnetic field lines, and the slip rings maintain the same connection to the coil throughout rotation, so the induced potential difference (and current) repeatedly reverses direction as the coil turns.

How does a split-ring commutator keep a dynamo's output current flowing in one direction?

The split-ring commutator swaps the connections between the coil and the brushes every half turn (each time the coil is perpendicular to the field), so the current leaving the dynamo stays always positive (or always negative), varying from zero to a maximum twice each cycle.

Besides rotating a coil within a magnetic field, how else can an alternating current be generated?

By rotating a magnet within a stationary coil. What matters is the relative motion between the coil and the magnet.

A dynamo produces ______ current, while an alternator produces ______ current.

A dynamo produces direct current, while an alternator produces alternating current.

True or False? The current output from a dynamo periodically reverses polarity, in the same way as an alternator's output.

False. A dynamo's output current never reverses polarity — it varies between zero and a maximum twice per cycle but stays always positive (or always negative), unlike an alternator's output, which alternates between positive and negative.

A transformer is a device that uses the generator effect to change the value of an alternating potential difference or current. It consists of a primary coil , a secondary coil , and an iron core .

Define step-up transformer .

A step-up transformer increases the potential difference of a power source. It has more turns on the secondary coil than on the primary coil.

Define step-down transformer .

A step-down transformer decreases the potential difference of a power source. It has fewer turns on the secondary coil than on the primary coil.

Why is iron used for the core of a transformer?

Iron is easily magnetised , so the changing magnetic field produced by the primary coil passes through the core efficiently and reaches the secondary coil.

Explain how an alternating potential difference in the primary coil induces a potential difference in the secondary coil.

The alternating current in the primary coil produces a changing magnetic field . The iron core carries this changing field to the secondary coil, where it cuts through the turns and induces an alternating potential difference with the same frequency as the primary supply.

Why is electricity transmitted at high voltage using step-up transformers in the national grid?

For the same power, a higher voltage means a lower current , and a lower current reduces the energy lost as heat in the transmission cables.

A step-down transformer is placed ______ buildings to reduce mains voltage to the level used by domestic appliances.

A step-down transformer is placed before buildings to reduce mains voltage to the level used by domestic appliances.

True or False? A transformer works when connected to a constant direct current supply.

False. A transformer only works with an alternating current supply. A constant direct current produces a magnetic field that does not change, so no potential difference is induced in the secondary coil.

In the transformer equation, the ratio of the potential differences across the two coils is equal to the ratio of the number of ______ on each coil.

In the transformer equation, the ratio of the potential differences across the two coils is equal to the ratio of the number of turns on each coil.

IGCSEPhysicsEdexcelPhysics (Modular)Unit 2Flashcards7. Magnetism & ElectromagnetismElectromagnetic Induction

Electromagnetic Induction (Edexcel IGCSE Physics (Modular): Unit 2): Flashcards

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Define electromagnetic induction.
Electromagnetic induction (the generator effect) is when a potential difference is induced in a conductor or coil as it moves through a magnetic field, or as the magnetic field through it changes. If the conductor is part of a complete circuit, a current is also induced.
Define the motor effect.
The motor effect is when a current-carrying conductor in a magnetic field experiences a force. It is the opposite of the generator effect, in which movement induces a current rather than a current producing movement.
What condition must be met for a potential difference to be induced in a conductor?
There must be relative movement between the conductor and the magnetic field, so that the conductor cuts through the magnetic field lines.
State four factors that affect the size of the potential difference induced in a coil.
Speed of movement of the wire, coil or magnet; number of turns on the coil; size (area) of the coil; strength of the magnetic field. Increasing any of these increases the induced potential difference.
What piece of equipment can be used to measure the size of an induced potential difference?
A sensitive voltmeter.
Increasing the ______ of the magnetic field increases the size of the induced potential difference.
Increasing the strength of the magnetic field increases the size of the induced potential difference.
True or False?
Reversing the direction in which a magnet is moved through a coil has no effect on the induced potential difference.
False.
Reversing the direction of movement reverses the direction of the induced potential difference — the direction depends on the orientation of the magnet's poles and the direction of movement.
Define alternator.
An alternator is a generator that converts mechanical energy into electrical energy as alternating current. It consists of a rotating coil in a magnetic field, connected to the circuit by slip rings and brushes.
Define dynamo.
A dynamo is a direct current generator. It is built the same way as an alternator, except it uses a split-ring commutator instead of slip rings.
Why does an alternator produce an alternating current?
The rotating coil cuts through the magnetic field lines, and the slip rings maintain the same connection to the coil throughout rotation, so the induced potential difference (and current) repeatedly reverses direction as the coil turns.
How does a split-ring commutator keep a dynamo's output current flowing in one direction?
The split-ring commutator swaps the connections between the coil and the brushes every half turn (each time the coil is perpendicular to the field), so the current leaving the dynamo stays always positive (or always negative), varying from zero to a maximum twice each cycle.
Besides rotating a coil within a magnetic field, how else can an alternating current be generated?
By rotating a magnet within a stationary coil. What matters is the relative motion between the coil and the magnet.
A dynamo produces ______ current, while an alternator produces ______ current.
A dynamo produces direct current, while an alternator produces alternating current.
True or False?
The current output from a dynamo periodically reverses polarity, in the same way as an alternator's output.
False.
A dynamo's output current never reverses polarity — it varies between zero and a maximum twice per cycle but stays always positive (or always negative), unlike an alternator's output, which alternates between positive and negative.
Define transformer.
A transformer is a device that uses the generator effect to change the value of an alternating potential difference or current. It consists of a primary coil, a secondary coil, and an iron core.
Define step-up transformer.
A step-up transformer increases the potential difference of a power source. It has more turns on the secondary coil than on the primary coil.
Define step-down transformer.
A step-down transformer decreases the potential difference of a power source. It has fewer turns on the secondary coil than on the primary coil.
Why is iron used for the core of a transformer?
Iron is easily magnetised, so the changing magnetic field produced by the primary coil passes through the core efficiently and reaches the secondary coil.
Explain how an alternating potential difference in the primary coil induces a potential difference in the secondary coil.
The alternating current in the primary coil produces a changing magnetic field. The iron core carries this changing field to the secondary coil, where it cuts through the turns and induces an alternating potential difference with the same frequency as the primary supply.
Why is electricity transmitted at high voltage using step-up transformers in the national grid?
For the same power, a higher voltage means a lower current, and a lower current reduces the energy lost as heat in the transmission cables.
A step-down transformer is placed ______ buildings to reduce mains voltage to the level used by domestic appliances.
A step-down transformer is placed before buildings to reduce mains voltage to the level used by domestic appliances.
True or False?
A transformer works when connected to a constant direct current supply.
False.
A transformer only works with an alternating current supply. A constant direct current produces a magnetic field that does not change, so no potential difference is induced in the secondary coil.
Define an ideal transformer.
An ideal transformer is 100% efficient, so the input power equals the output power:
$V_{p} \times I_{p} = V_{s} \times I_{s}$
State the transformer equation, defining each term.
$\frac{V_{p}}{V_{s}} = \frac{n_{p}}{n_{s}}$
Where V~p~ = potential difference across the primary coil (V), V~s~ = potential difference across the secondary coil (V), n~p~ = number of turns on the primary coil, n~s~ = number of turns on the secondary coil.
A transformer has 20 turns on its primary coil and 800 turns on its secondary coil. The primary potential difference is 500 V. Calculate the secondary potential difference.
$V_{s} = \frac{n_{s}}{n_{p}} \times V_{p} = \frac{800}{20} \times 500 = 20000 V$
A step-up transformer in a travel adapter steps 115 V up to 230 V to supply a hairdryer drawing 5 A. Assuming 100% efficiency, calculate the current drawn from the 115 V mains supply.
$I_{p} = \frac{V_{s} \times I_{s}}{V_{p}} = \frac{230 \times 5}{115} = 10 A$
In the transformer equation, the ratio of the potential differences across the two coils is equal to the ratio of the number of ______ on each coil.
In the transformer equation, the ratio of the potential differences across the two coils is equal to the ratio of the number of turns on each coil.
True or False?
An ideal transformer that steps up the potential difference also increases the power delivered to the secondary circuit.
False.
By conservation of energy, an ideal transformer cannot increase power: input power equals output power ( $V_{p} \times I_{p} = V_{s} \times I_{s}$ ), so an increase in potential difference is matched by a proportional decrease in current.

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