Define electromagnetic induction .

A voltage is induced in a conductor or coil when it moves through a magnetic field, or when the magnetic field through it changes.

How does the generator effect differ from the motor effect ?

In the motor effect, a current-carrying conductor experiences a force in a magnetic field. In the generator effect, there is no initial current, but a potential difference is induced when the conductor moves through the field.

What condition is needed for a potential difference to be induced in a conductor?

There must be relative movement between the conductor and the magnetic field.

A potential difference is induced in a conductor when it ______ through the magnetic field lines.

A potential difference is induced in a conductor when it cuts through the magnetic field lines.

How can the induced potential difference be measured in the laboratory?

Using a sensitive voltmeter . If the conductor is part of a complete circuit, the induced current can be measured with an ammeter .

How is electricity generated on a large scale using the generator effect?

A coil is spun rapidly in a magnetic field so it cuts through the field lines, inducing an alternating potential difference and current. This device is called an alternator .

True or False? Alternators generate direct current.

False. Alternators generate alternating current (a.c.), which is used in the mains supply of a building.

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

Any three from: the speed the magnet or coil is moved; the number of turns on the coil; the area of the coil; the strength of the magnetic field.

How does increasing the speed of movement affect the induced potential difference?

It increases the rate at which the magnetic field lines are cut, which increases the induced potential difference.

What factor determines the direction of the induced potential difference?

The orientation of the poles of the magnet (the direction it is moved).

What rule determines the direction of an induced potential difference and the magnetic field it produces?

The induced potential difference (and the resulting magnetic field) always opposes the change that produces it.

A magnet is pushed north pole first into a coil. What polarity does the near end of the coil become, and why?

It becomes a north pole , so it repels the magnet, opposing it being pushed in.

Pulling a magnet away from a coil makes the near end of the coil a ______ pole, which attracts the magnet to oppose its removal.

Pulling a magnet away from a coil makes the near end of the coil a south pole, which attracts the magnet to oppose its removal.

True or False? Increasing the number of coils increases the induced potential difference.

False. It is the number of turns , not the number of coils, that should increase the induced potential difference — these are not the same thing.

A generator that produces alternating current (a.c.) using a coil that spins in a magnetic field, connected to the circuit by slip rings and brushes.

A generator that produces direct current (d.c.); identical to an alternator except it uses a split-ring commutator instead of slip rings.

Why does an alternator produce alternating current?

The coil cuts through the magnetic field lines and the induced potential difference reverses direction every half turn, so the current alternates .

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

It swaps the connections between the coil and the brushes every half turn, so the potential difference never reverses polarity, though it still varies from zero to a maximum twice per rotation.

What component induces a potential difference in a bicycle dynamo?

A rotating magnet , driven by the bicycle wheel, placed inside or next to the coil.

Despite its name, a bicycle dynamo produces ______ current because the rotating magnet's field constantly changes direction.

Despite its name, a bicycle dynamo produces alternating current because the rotating magnet's field constantly changes direction.

True or False? The output current from a dynamo is only ever positive (or only ever negative), never both.

True. The split-ring commutator prevents the current from reversing polarity, unlike in an alternator.

Define moving coil microphone .

A device that uses electromagnetic induction to convert pressure variations in sound waves into variations in current in an electrical circuit.

Describe how a moving coil microphone generates a current from sound.

Sound wave pressure variations make the diaphragm vibrate, moving the attached coil through a magnetic field. This induces an alternating potential difference (and current) in the coil.

Describe how a loudspeaker produces sound from an alternating current.

The alternating current creates a changing magnetic field around the coil, which interacts with the permanent magnet's field to exert a constantly changing force (direction given by Fleming's left-hand rule ), making the coil and speaker cone oscillate and produce sound waves.

A loudspeaker's coil is wrapped around one pole of a ______ magnet.

A loudspeaker's coil is wrapped around one pole of a permanent magnet.

True or False? A d.c. motor and a loudspeaker work in exactly the same way, using the same type of current.

False. The explanations are similar, but a d.c. motor uses direct current while a loudspeaker or headphone uses alternating current .

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 .

Why is a transformer's core made of iron?

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

Describe how a changing current in the primary coil induces a current in the secondary coil.

The alternating current in the primary coil produces a changing magnetic field, which passes through the iron core and cuts through the secondary coil, inducing an alternating potential difference (and current) at the same frequency as the primary supply.

A ______ transformer has more turns on the secondary coil than the primary coil and increases the potential difference.

A step-up transformer has more turns on the secondary coil than the primary coil and increases the potential difference.

State two uses of transformers in the supply of electricity.

Any two from: increasing the potential difference before transmission across the national grid ; lowering high-voltage electricity to household voltages; lowering mains voltage in adapters for electronic devices.

True or False? A step-down transformer has more turns on its secondary coil than its primary coil.

False. A step-down transformer has fewer turns on the secondary coil than the primary coil, which decreases the potential difference.

Define the transformer equation (in words).

The ratio of the potential differences across the primary and secondary coils of a transformer equals the ratio of the number of turns on each coil.

In the worked example, the transformer has more turns on the secondary coil than the primary coil, so it is a ______ transformer.

In the worked example, the transformer has more turns on the secondary coil than the primary coil, so it is a step-up transformer.

True or False? Rearranging the transformer equation is easier when the variable you are solving for is on the denominator.

False. There is less rearranging to do when the variable you are trying to find is on the numerator (top line) of the fraction.

Why does transmitting electricity at a high current cause energy loss ?

A high current flowing through the power lines heats the wires, transferring electrical energy to heat and reducing efficiency.

Why is electricity transmitted at high voltage rather than high current?

Since P* = *IV , transmitting at a high voltage allows the same power to be delivered with a smaller current , which reduces heating and energy loss in the cables.

Define a step-up transformer .

A transformer that increases the potential difference of electricity, placed after the power station before transmission.

Define a step-down transformer .

A transformer that decreases the potential difference of electricity, placed before buildings for domestic use.

Name three roles of transformers in the supply of electricity.

Increasing the potential difference before transmission across the national grid, lowering the high voltage in power lines to the voltages used in houses, and lowering mains voltage in adapters for electronic devices.

Electricity is transmitted at ______ voltage and ______ current to reduce energy loss in the power lines.

Electricity is transmitted at high voltage and low current to reduce energy loss in the power lines.

True or False? Step-down transformers are placed after the power station.

False. Step- up transformers are placed after the power station; step-down transformers are placed before buildings .

Define an ideal transformer .

A transformer that is 100% efficient , so the input power equals the output power .

Why can a transformer not increase the power output of a supply?

By the law of conservation of energy , a transformer can only change the voltage and current , not increase the total power .

For a 100% efficient transformer, V ~ p ~ × I ~ p ~ = V ~ s ~ × ______

For a 100% efficient transformer, V ~ p ~ × I ~ p ~ = V ~ s ~ × I ~ s ~

Write the equation linking electrical power , potential difference and current .

P* = *VI where P is power in watts (W), V is potential difference in volts (V) and I is current in amps (A).

A transformer steps up a 100 V supply to 400 V. If the current in the primary coil is 8 A, calculate the current in the secondary coil, assuming the transformer is 100% efficient.

V ~ p ~ × I ~ p ~ = V ~ s ~ × I ~ s ~ 100 × 8 = 400 × I ~ s ~ I ~ s ~ = 2 A

True or False? In an ideal transformer, increasing the voltage on the secondary coil increases the current on the secondary coil.

False. Since V ~ p ~ × I ~ p ~ = V ~ s ~ × I ~ s ~ is fixed by the input power, increasing V ~ s ~ decreases I ~ s ~.

Write the equation for the power lost in a wire due to its resistance.

P* = *I*^2^*R where P is power in watts (W), I is current in amps (A) and R is resistance in ohms (Ω).

GCSEPhysicsEdexcelFlashcards13. Electromagnetic InductionElectromagnetic Induction

Electromagnetic Induction (Edexcel GCSE Physics): Flashcards

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Define electromagnetic induction.

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Cards in this collection (58)

Define electromagnetic induction.
A voltage is induced in a conductor or coil when it moves through a magnetic field, or when the magnetic field through it changes.
How does the generator effect differ from the motor effect?
In the motor effect, a current-carrying conductor experiences a force in a magnetic field. In the generator effect, there is no initial current, but a potential difference is induced when the conductor moves through the field.
What condition is needed for a potential difference to be induced in a conductor?
There must be relative movement between the conductor and the magnetic field.
A potential difference is induced in a conductor when it ______ through the magnetic field lines.
A potential difference is induced in a conductor when it cuts through the magnetic field lines.
How can the induced potential difference be measured in the laboratory?
Using a sensitive voltmeter. If the conductor is part of a complete circuit, the induced current can be measured with an ammeter.
How is electricity generated on a large scale using the generator effect?
A coil is spun rapidly in a magnetic field so it cuts through the field lines, inducing an alternating potential difference and current. This device is called an alternator.
True or False?
Alternators generate direct current.
False.
Alternators generate alternating current (a.c.), which is used in the mains supply of a building.
State three factors that affect the size of the potential difference induced in a coil.
Any three from: the speed the magnet or coil is moved; the number of turns on the coil; the area of the coil; the strength of the magnetic field.
How does increasing the speed of movement affect the induced potential difference?
It increases the rate at which the magnetic field lines are cut, which increases the induced potential difference.
What factor determines the direction of the induced potential difference?
The orientation of the poles of the magnet (the direction it is moved).
What rule determines the direction of an induced potential difference and the magnetic field it produces?
The induced potential difference (and the resulting magnetic field) always opposes the change that produces it.
A magnet is pushed north pole first into a coil. What polarity does the near end of the coil become, and why?
It becomes a north pole, so it repels the magnet, opposing it being pushed in.
Pulling a magnet away from a coil makes the near end of the coil a ______ pole, which attracts the magnet to oppose its removal.
Pulling a magnet away from a coil makes the near end of the coil a south pole, which attracts the magnet to oppose its removal.
True or False?
Increasing the number of coils increases the induced potential difference.
False.
It is the number of turns, not the number of coils, that should increase the induced potential difference — these are not the same thing.
Define alternator.
A generator that produces alternating current (a.c.) using a coil that spins in a magnetic field, connected to the circuit by slip rings and brushes.
Define dynamo.
A generator that produces direct current (d.c.); identical to an alternator except it uses a split-ring commutator instead of slip rings.
Why does an alternator produce alternating current?
The coil cuts through the magnetic field lines and the induced potential difference reverses direction every half turn, so the current alternates.
How does a split-ring commutator keep a dynamo's output current flowing in one direction?
It swaps the connections between the coil and the brushes every half turn, so the potential difference never reverses polarity, though it still varies from zero to a maximum twice per rotation.
What component induces a potential difference in a bicycle dynamo?
A rotating magnet, driven by the bicycle wheel, placed inside or next to the coil.
Despite its name, a bicycle dynamo produces ______ current because the rotating magnet's field constantly changes direction.
Despite its name, a bicycle dynamo produces alternating current because the rotating magnet's field constantly changes direction.
True or False?
The output current from a dynamo is only ever positive (or only ever negative), never both.
True.
The split-ring commutator prevents the current from reversing polarity, unlike in an alternator.
Define moving coil microphone.
A device that uses electromagnetic induction to convert pressure variations in sound waves into variations in current in an electrical circuit.
Describe how a moving coil microphone generates a current from sound.
Sound wave pressure variations make the diaphragm vibrate, moving the attached coil through a magnetic field. This induces an alternating potential difference (and current) in the coil.
What effect allows a loudspeaker to convert an electrical signal into sound?
The motor effect.
Describe how a loudspeaker produces sound from an alternating current.
The alternating current creates a changing magnetic field around the coil, which interacts with the permanent magnet's field to exert a constantly changing force (direction given by Fleming's left-hand rule), making the coil and speaker cone oscillate and produce sound waves.
A loudspeaker's coil is wrapped around one pole of a ______ magnet.
A loudspeaker's coil is wrapped around one pole of a permanent magnet.
True or False?
A d.c. motor and a loudspeaker work in exactly the same way, using the same type of current.
False.
The explanations are similar, but a d.c. motor uses direct current while a loudspeaker or headphone uses alternating current.
Define transformer.
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.
Why is a transformer's core made of iron?
Iron is easily magnetised, so it lets the changing magnetic field from the primary coil pass through to the secondary coil.
Describe how a changing current in the primary coil induces a current in the secondary coil.
The alternating current in the primary coil produces a changing magnetic field, which passes through the iron core and cuts through the secondary coil, inducing an alternating potential difference (and current) at the same frequency as the primary supply.
A ______ transformer has more turns on the secondary coil than the primary coil and increases the potential difference.
A step-up transformer has more turns on the secondary coil than the primary coil and increases the potential difference.
State two uses of transformers in the supply of electricity.
Any two from: increasing the potential difference before transmission across the national grid; lowering high-voltage electricity to household voltages; lowering mains voltage in adapters for electronic devices.
True or False?
A step-down transformer has more turns on its secondary coil than its primary coil.
False.
A step-down transformer has fewer turns on the secondary coil than the primary coil, which decreases the potential difference.
Define the transformer equation (in words).
The ratio of the potential differences across the primary and secondary coils of a transformer equals the ratio of the number of turns on each coil.
Write the transformer equation, defining each symbol.
$\frac{V_{p}}{V_{s}} = \frac{n_{p}}{n_{s}}$
where V~p~ and V~s~ are the potential differences (V) across the primary and secondary coils, and n~p~ and n~s~ are the number of turns on the primary and secondary coils.
A transformer has 20 turns on the primary coil and 800 turns on the secondary coil, with 500 V across the primary coil. Calculate the output potential difference.
$V_{s} = \frac{V_{p} \times n_{s}}{n_{p}} = \frac{500 \times 800}{20}$
$V_{s} = 20 000$ V (20 kV)
In the worked example, the transformer has more turns on the secondary coil than the primary coil, so it is a ______ transformer.
In the worked example, the transformer has more turns on the secondary coil than the primary coil, so it is a step-up transformer.
What term should be used for the individual loops of wire around each side of a transformer?
Turns, not coils.
True or False?
Rearranging the transformer equation is easier when the variable you are solving for is on the denominator.
False.
There is less rearranging to do when the variable you are trying to find is on the numerator (top line) of the fraction.
Why does transmitting electricity at a high current cause energy loss?
A high current flowing through the power lines heats the wires, transferring electrical energy to heat and reducing efficiency.
Why is electricity transmitted at high voltage rather than high current?
Since P* = *IV, transmitting at a high voltage allows the same power to be delivered with a smaller current, which reduces heating and energy loss in the cables.
Define a step-up transformer.
A transformer that increases the potential difference of electricity, placed after the power station before transmission.
Define a step-down transformer.
A transformer that decreases the potential difference of electricity, placed before buildings for domestic use.
Name three roles of transformers in the supply of electricity.
Increasing the potential difference before transmission across the national grid, lowering the high voltage in power lines to the voltages used in houses, and lowering mains voltage in adapters for electronic devices.
Electricity is transmitted at ______ voltage and ______ current to reduce energy loss in the power lines.
Electricity is transmitted at high voltage and low current to reduce energy loss in the power lines.
True or False?
Step-down transformers are placed after the power station.
False.
Step-up transformers are placed after the power station; step-down transformers are placed before buildings.
Define an ideal transformer.
A transformer that is 100% efficient, so the input power equals the output power.
Why can a transformer not increase the power output of a supply?
By the law of conservation of energy, a transformer can only change the voltage and current, not increase the total power.
For a 100% efficient transformer, V~p~ × I~p~ = V~s~ × ______
For a 100% efficient transformer, V~p~ × I~p~ = V~s~ × I~s~
Write the equation linking electrical power, potential difference and current.
P* = *VI
where P is power in watts (W), V is potential difference in volts (V) and I is current in amps (A).
A transformer steps up a 100 V supply to 400 V. If the current in the primary coil is 8 A, calculate the current in the secondary coil, assuming the transformer is 100% efficient.
V~p~ × I~p~ = V~s~ × I~s~
100 × 8 = 400 × I~s~
I~s~ = 2 A
True or False?
In an ideal transformer, increasing the voltage on the secondary coil increases the current on the secondary coil.
False.
Since V~p~ × I~p~ = V~s~ × I~s~ is fixed by the input power, increasing V~s~ decreases I~s~.
Write the equation for the power lost in a wire due to its resistance.
P* = *I*^2^*R
where P is power in watts (W), I is current in amps (A) and R is resistance in ohms (Ω).
Write the equation linking energy lost, power and time.
E* = *Pt
where E is energy in joules (J), P is power in watts (W) and t is time in seconds (s).
A cable of resistance 2 Ω carries a current of 5 A for 60 s. Calculate the energy lost as heat in the cable.
*P* = *I*^2^*R* = 5^2^ × 2 = 50 W
*E* = *Pt* = 50 × 60 = 3000 J
A step-up transformer has ______ turns on the secondary coil than on the primary coil.
A step-up transformer has more turns on the secondary coil than on the primary coil.
Why does increasing the transmission voltage reduce energy loss in cables?
Since I~p~V~p~ = I~s~V~s~, increasing the voltage lowers the current, and since power loss = I²R, a lower current greatly reduces the power lost as heat.
True or False?
Halving the current in a transmission cable halves the power lost as heat.
False.
Power loss = I*^2^*R, so halving the current reduces the power lost to a quarter of its original value.

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