Define induced e.m.f.

An e.m.f. induced in a conductor when there is relative movement between the conductor and a magnetic field , or when the conductor is stationary in a changing magnetic field.

The direction of an induced e.m.f. (or current) always opposes the change that produces it.

Why is no e.m.f. induced when a bar magnet is held stationary inside a coil?

There is no relative movement between the magnet and the coil, so no magnetic field lines are cut , and therefore no e.m.f. is induced.

True or False? Moving a magnet towards a coil in an open circuit induces a current in the coil.

False. An e.m.f. is induced in the coil, but a current only flows if the coil is part of a complete circuit .

A bar magnet is pushed, north pole first , into a coil of wire. Using Lenz's law, explain why the end of the coil nearest the magnet becomes a north pole .

The induced e.m.f. must oppose the change producing it (the magnet being pushed in), so the coil's near end becomes a north pole to repel the approaching north pole of the magnet.

When a magnet is pulled away from a coil, the end of the coil nearest the magnet becomes a ______ pole, so that it attracts the magnet and opposes its removal.

When a magnet is pulled away from a coil, the end of the coil nearest the magnet becomes a south pole, so that it attracts the magnet and opposes its removal.

What do the First finger , thu*M*b and se*C*ond finger represent in the Right-Hand Dynamo rule ?

First finger = F ield Thumb = M otion (direction the wire is moving) Second finger = C urrent (the induced e.m.f.)

In the magnet-and-coil experiment, what happens to the voltmeter reading as the magnet is moved faster through the coil?

The reading increases — a greater e.m.f. is induced.

Name two ways to increase the induced e.m.f. in the magnet-and-coil experiment.

Any two from: move the magnet faster ; add more turns to the coil; use a stronger magnet.

True or False? In the wire-between-magnets experiment, a current already flows in the wire before it starts moving.

False. There is no current flowing through the wire to start with — an e.m.f. is only induced once the wire moves and cuts through the magnetic field.

State three factors that increase the induced e.m.f. in the wire-between-magnets experiment.

Increasing the length of the wire; moving the wire faster between the magnets; increasing the strength of the magnets.

Increasing the ______ of the magnetic field increases the magnitude of the induced e.m.f.

Increasing the strength of the magnetic field increases the magnitude of the induced e.m.f.

What determines the direction of an induced e.m.f.?

The orientation of the poles of the magnet (i.e. the direction in which the wire, coil or magnet is moved).

What happens to the direction of the induced e.m.f. if the direction of movement of the magnet (or wire) is reversed ?

The direction of the induced e.m.f. also reverses .

Define an alternator .

A type of generator that produces an alternating current by rotating a coil in a uniform magnetic field (or rotating a magnet inside a coil).

What is the function of the slip rings and brushes in an alternator?

They provide a continuous electrical connection between the rotating coil and the external circuit (meter).

Why does the pointer on the meter connected to an alternator deflect first one way, then the opposite way?

The induced e.m.f., and therefore the current , repeatedly changes direction as the coil spins through the magnetic field.

State two ways an alternating e.m.f. can be generated using a coil and a magnet.

A coil rotating in a magnetic field; a magnet rotating within a coil.

True or False? The induced e.m.f. of an a.c. generator is at its maximum when the number of magnetic field lines through the coil is at its maximum.

False. When the number of field lines through the coil is at a maximum , the induced e.m.f. is at a minimum (zero); e.m.f. is maximum when the number of field lines through the coil is at a minimum .

At which coil positions in an a.c. generator's cycle is the induced e.m.f. at its maximum ?

At positions 2 and 4 , where no field lines pass through the centre of the coil (the rate of change of flux is greatest).

A ______ takes in motion and generates electricity, while a motor takes in electricity and produces motion.

A generator takes in motion and generates electricity, while a motor takes in electricity and produces motion.

A coil of wire that, when carrying a current, produces a magnetic field similar in shape to that of a bar magnet .

Define an electromagnet .

A solenoid with a soft iron core that produces a magnetic field only while a current flows through it, and can be switched on and off.

What shape is the magnetic field around a straight current-carrying wire?

A series of concentric circles centred on the wire, showing that the field around the wire has no poles .

Which rule is used to find the direction of the magnetic field around a current-carrying wire?

The right-hand thumb rule : point the thumb in the direction of the current ; the curled fingers show the direction of the magnetic field .

True or False? The magnetic field around a straight current-carrying wire gets stronger as the distance from the wire increases.

False. The field gets weaker with increasing distance from the wire (the field lines get further apart).

How can the polarity of each end of a current-carrying solenoid be found?

View the solenoid from the end: if the current flows clockwise , that end is a south pole ; if anticlockwise , it is a north pole .

State three ways to increase the strength of the magnetic field produced by a solenoid.

Increasing the current ; increasing the number of turns (coils); adding a soft iron core through the centre.

Explain how a relay circuit uses an electromagnet to control a second circuit.

Current in circuit 1 creates a magnetic field around the electromagnet, which attracts a switch, closing the contacts in circuit 2 and allowing current to flow; when the current in circuit 1 stops, the magnetic field collapses and the switch opens circuit 2.

A loudspeaker converts electrical signals into sound using the ______ effect.

A loudspeaker converts electrical signals into sound using the motor effect.

Define the motor effect .

The phenomenon whereby a current-carrying conductor experiences a force when placed in an external magnetic field, provided the current is perpendicular to the field.

Under what condition does a current-carrying conductor experience a force in a magnetic field?

Only when the current through it is perpendicular to the direction of the magnetic field lines.

State two ways to reverse the direction of the force on a current-carrying rod in a magnetic field.

Reverse the direction of the current ; reverse the direction of the magnetic field .

What does each finger/thumb represent in Fleming's left-hand rule ?

First finger = F ield Se*C*ond finger = C urrent Thu*M*b = Th rust (force)

True or False? In Fleming's left-hand rule, the First finger points in the direction of the current.

False. The First finger points in the direction of the magnetic Field ; the seCond finger points in the direction of the Current .

In Fleming's left-hand rule, the thumb points in the direction of the ______ , also called the thrust.

In Fleming's left-hand rule, the thumb points in the direction of the force , also called the thrust.

What determines the direction of the magnetic field produced around a current-carrying wire?

The direction of the current flowing through the wire, found using the right-hand thumb rule .

What happens to two parallel current-carrying conductors when their currents flow in the same direction?

They attract each other, because the magnetic field lines between the conductors cancel out.

The magnitude of the force between two parallel current-carrying conductors depends on the amount of ______ and the length of the wire.

The magnitude of the force between two parallel current-carrying conductors depends on the amount of current and the length of the wire.

True or False? Two parallel wires carrying currents in opposite directions will attract each other.

False. Currents in opposite directions cause the wires to repel , since the magnetic field lines between them push each other apart.

Define split-ring commutator .

A split-ring commutator is a component in a d.c. motor that periodically reverses the direction of the current in the coil, keeping the current leaving the motor flowing in the same direction and keeping the coil rotating continuously.

Define Fleming's left-hand rule .

Fleming's left-hand rule is used to determine the direction of the force on a current-carrying conductor in a magnetic field: the F irst finger points along the F ield, the se C ond finger points along the C urrent, and the th umb points along the th rust (force).

State three ways of increasing the turning effect on the coil of a d.c. motor.

Increasing the number of turns on the coil, increasing the current , and increasing the strength of the magnetic field .

Why do the two sides of the coil in a d.c. motor experience forces in opposite directions?

Because the current flows in opposite directions on each side of the coil, so one side is pushed up and the other side is pushed down , causing the coil to rotate.

What keeps the coil of a d.c. motor rotating as it passes through the vertical position?

Momentum keeps the coil turning until the magnetic force takes over again.

The direction of rotation of the coil in a d.c. motor can be changed by reversing the current, or by reversing the ______ of the magnet.

The direction of rotation of the coil in a d.c. motor can be changed by reversing the current, or by reversing the poles of the magnet.

True or False? The split-ring commutator reverses the direction of the current in the coil every half turn.

True. This keeps the coil rotating continuously as long as current is flowing.

A transformer is an electrical device that uses the generator effect to increase or decrease the potential difference of an alternating current.

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 ( N s > N p ).

O LevelPhysicsCambridge (CIE)Flashcards4. Electricity & Magnetism4.5 Electromagnetic Effects

4.5 Electromagnetic Effects (Cambridge (CIE) O Level Physics): Flashcards

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Define induced e.m.f.

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Define induced e.m.f.
An e.m.f. induced in a conductor when there is relative movement between the conductor and a magnetic field, or when the conductor is stationary in a changing magnetic field.
Define Lenz's law.
The direction of an induced e.m.f. (or current) always opposes the change that produces it.
Why is no e.m.f. induced when a bar magnet is held stationary inside a coil?
There is no relative movement between the magnet and the coil, so no magnetic field lines are cut, and therefore no e.m.f. is induced.
True or False?
Moving a magnet towards a coil in an open circuit induces a current in the coil.
False.
An e.m.f. is induced in the coil, but a current only flows if the coil is part of a complete circuit.
A bar magnet is pushed, north pole first, into a coil of wire. Using Lenz's law, explain why the end of the coil nearest the magnet becomes a north pole.
The induced e.m.f. must oppose the change producing it (the magnet being pushed in), so the coil's near end becomes a north pole to repel the approaching north pole of the magnet.
When a magnet is pulled away from a coil, the end of the coil nearest the magnet becomes a ______ pole, so that it attracts the magnet and opposes its removal.
When a magnet is pulled away from a coil, the end of the coil nearest the magnet becomes a south pole, so that it attracts the magnet and opposes its removal.
What do the First finger, thu*M*b and se*C*ond finger represent in the Right-Hand Dynamo rule?
First finger = Field
Thumb = Motion (direction the wire is moving)
Second finger = Current (the induced e.m.f.)
In the magnet-and-coil experiment, what happens to the voltmeter reading as the magnet is moved faster through the coil?
The reading increases — a greater e.m.f. is induced.
Name two ways to increase the induced e.m.f. in the magnet-and-coil experiment.
Any two from: move the magnet faster; add more turns to the coil; use a stronger magnet.
True or False?
In the wire-between-magnets experiment, a current already flows in the wire before it starts moving.
False.
There is no current flowing through the wire to start with — an e.m.f. is only induced once the wire moves and cuts through the magnetic field.
State three factors that increase the induced e.m.f. in the wire-between-magnets experiment.
Increasing the length of the wire; moving the wire faster between the magnets; increasing the strength of the magnets.
Increasing the ______ of the magnetic field increases the magnitude of the induced e.m.f.
Increasing the strength of the magnetic field increases the magnitude of the induced e.m.f.
What determines the direction of an induced e.m.f.?
The orientation of the poles of the magnet (i.e. the direction in which the wire, coil or magnet is moved).
What happens to the direction of the induced e.m.f. if the direction of movement of the magnet (or wire) is reversed?
The direction of the induced e.m.f. also reverses.
Define an alternator.
A type of generator that produces an alternating current by rotating a coil in a uniform magnetic field (or rotating a magnet inside a coil).
What is the function of the slip rings and brushes in an alternator?
They provide a continuous electrical connection between the rotating coil and the external circuit (meter).
Why does the pointer on the meter connected to an alternator deflect first one way, then the opposite way?
The induced e.m.f., and therefore the current, repeatedly changes direction as the coil spins through the magnetic field.
State two ways an alternating e.m.f. can be generated using a coil and a magnet.
A coil rotating in a magnetic field; a magnet rotating within a coil.
True or False?
The induced e.m.f. of an a.c. generator is at its maximum when the number of magnetic field lines through the coil is at its maximum.
False.
When the number of field lines through the coil is at a maximum, the induced e.m.f. is at a minimum (zero); e.m.f. is maximum when the number of field lines through the coil is at a minimum.
At which coil positions in an a.c. generator's cycle is the induced e.m.f. at its maximum?
At positions 2 and 4, where no field lines pass through the centre of the coil (the rate of change of flux is greatest).
A ______ takes in motion and generates electricity, while a motor takes in electricity and produces motion.
A generator takes in motion and generates electricity, while a motor takes in electricity and produces motion.
Define a solenoid.
A coil of wire that, when carrying a current, produces a magnetic field similar in shape to that of a bar magnet.
Define an electromagnet.
A solenoid with a soft iron core that produces a magnetic field only while a current flows through it, and can be switched on and off.
What shape is the magnetic field around a straight current-carrying wire?
A series of concentric circles centred on the wire, showing that the field around the wire has no poles.
Which rule is used to find the direction of the magnetic field around a current-carrying wire?
The right-hand thumb rule: point the thumb in the direction of the current; the curled fingers show the direction of the magnetic field.
True or False?
The magnetic field around a straight current-carrying wire gets stronger as the distance from the wire increases.
False.
The field gets weaker with increasing distance from the wire (the field lines get further apart).
How can the polarity of each end of a current-carrying solenoid be found?
View the solenoid from the end: if the current flows clockwise, that end is a south pole; if anticlockwise, it is a north pole.
State three ways to increase the strength of the magnetic field produced by a solenoid.
Increasing the current; increasing the number of turns (coils); adding a soft iron core through the centre.
Explain how a relay circuit uses an electromagnet to control a second circuit.
Current in circuit 1 creates a magnetic field around the electromagnet, which attracts a switch, closing the contacts in circuit 2 and allowing current to flow; when the current in circuit 1 stops, the magnetic field collapses and the switch opens circuit 2.
A loudspeaker converts electrical signals into sound using the ______ effect.
A loudspeaker converts electrical signals into sound using the motor effect.
Define the motor effect.
The phenomenon whereby a current-carrying conductor experiences a force when placed in an external magnetic field, provided the current is perpendicular to the field.
Under what condition does a current-carrying conductor experience a force in a magnetic field?
Only when the current through it is perpendicular to the direction of the magnetic field lines.
State two ways to reverse the direction of the force on a current-carrying rod in a magnetic field.
Reverse the direction of the current; reverse the direction of the magnetic field.
What does each finger/thumb represent in Fleming's left-hand rule?
First finger = Field
Se*C*ond finger = Current
Thu*M*b = Thrust (force)
True or False?
In Fleming's left-hand rule, the First finger points in the direction of the current.
False.
The First finger points in the direction of the magnetic Field; the seCond finger points in the direction of the Current.
In Fleming's left-hand rule, the thumb points in the direction of the ______, also called the thrust.
In Fleming's left-hand rule, the thumb points in the direction of the force, also called the thrust.
What determines the direction of the magnetic field produced around a current-carrying wire?
The direction of the current flowing through the wire, found using the right-hand thumb rule.
What happens to two parallel current-carrying conductors when their currents flow in the same direction?
They attract each other, because the magnetic field lines between the conductors cancel out.
In which direction do the magnetic forces act on two parallel conductors when they attract each other?
Towards each other.
In which direction do the magnetic forces act on two parallel conductors when they repel each other?
Away from each other.
The magnitude of the force between two parallel current-carrying conductors depends on the amount of ______ and the length of the wire.
The magnitude of the force between two parallel current-carrying conductors depends on the amount of current and the length of the wire.
True or False?
Two parallel wires carrying currents in opposite directions will attract each other.
False.
Currents in opposite directions cause the wires to repel, since the magnetic field lines between them push each other apart.
Define split-ring commutator.
A split-ring commutator is a component in a d.c. motor that periodically reverses the direction of the current in the coil, keeping the current leaving the motor flowing in the same direction and keeping the coil rotating continuously.
Define Fleming's left-hand rule.
Fleming's left-hand rule is used to determine the direction of the force on a current-carrying conductor in a magnetic field: the First finger points along the Field, the seCond finger points along the Current, and the thumb points along the thrust (force).
State three ways of increasing the turning effect on the coil of a d.c. motor.
Increasing the number of turns on the coil, increasing the current, and increasing the strength of the magnetic field.
Why do the two sides of the coil in a d.c. motor experience forces in opposite directions?
Because the current flows in opposite directions on each side of the coil, so one side is pushed up and the other side is pushed down, causing the coil to rotate.
What keeps the coil of a d.c. motor rotating as it passes through the vertical position?
Momentum keeps the coil turning until the magnetic force takes over again.
The direction of rotation of the coil in a d.c. motor can be changed by reversing the current, or by reversing the ______ of the magnet.
The direction of rotation of the coil in a d.c. motor can be changed by reversing the current, or by reversing the poles of the magnet.
True or False?
The split-ring commutator reverses the direction of the current in the coil every half turn.
True.
This keeps the coil rotating continuously as long as current is flowing.
Define transformer.
A transformer is an electrical device that uses the generator effect to increase or decrease the potential difference of an alternating current.
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 (N_s > N_p).
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 (N_s < N_p).
What three components make up a basic transformer?
A primary coil, a secondary coil, and a soft iron core.
Why is soft iron used for the core of a transformer?
Because it is easily magnetised, allowing the changing magnetic field to pass through it.
What is induced in the secondary coil due to the changing magnetic field from the primary coil?
An alternating potential difference (e.m.f.) is induced in the secondary coil.
A transformer works using the ______ effect.
A transformer works using the generator effect.
True or False?
The alternating potential difference induced in the secondary coil has the same frequency as the current supplied to the primary coil.
True.
The induced p.d. is always alternating at the same frequency as the primary current.
State the equation linking the potential differences across the primary and secondary coils of a transformer to the number of turns on each coil.
$\frac{V_{p}}{V_{s}} = \frac{N_{p}}{N_{s}}$
where V_p = p.d. across the primary coil (V), V_s = p.d. 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 p.d. is 500 V. Calculate the secondary p.d.
$V_{s} = \frac{N_{s} V_{p}}{N_{p}} = \frac{800 \times 500}{20} = 20000 V$
Why is electricity transmitted at high potential difference across the national grid?
Because for a fixed power, P = IV, so increasing the p.d. reduces the current. A smaller current produces less heating in the cables, reducing energy loss.
A smaller ______ flowing through the power lines results in less heat being produced, reducing energy loss.
A smaller current flowing through the power lines results in less heat being produced, reducing energy loss.
True or False?
Transformers are used to increase the potential difference of electricity before transmission across the national grid, then decrease it again before it reaches homes.
True.
Both step-up and step-down transformers are used at different stages of electricity distribution.
What role do transformers play in adapters for electronic devices?
They lower the mains voltage to the lower voltages used by many electronic devices.

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