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4.5 Electromagnetic effects QS
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Complete the following sentence: A d.c. motor requires a …………………-ring commutator to keep the turning force generated in the same direction during one full revolution of the armature.
1 point
Your answer
Complete the following sentence: Step-up transformers are used to increase voltage and decrease ………………… before transmission to reduce energy loss via heating in the transmission wires.
1 point
Your answer
Complete the following sentence: A wire moving in a perpendicular direction to a magnetic field will ………………… an e.m.f. in the wire.
1 point
Your answer
A magnet is positioned near a wire as shown in the diagram. The wire runs perpendicular to the screen.
The magnet is pivoted in the centre and rotates clockwise.
Which statement describes the current induced in the wire?
1 point
No current is induced in the wire.
There is a steady current into the screen.
There is a steady current out of the screen.
There is a varying current in the wire, sometimes into the screen and sometimes out of the screen.
Clear selection
a)
This diagram shows a right hand being used to determine the relationship between a magnetic field, current and the movement of a wire through the field.
Complete the sentence to describe how the fingers help to show the relationship between these three factors.
The ………………… finger points in the direction of the magnetic field.
1 point
Your answer
b)
This diagram shows a right hand being used to determine the relationship between a magnetic field, current and the movement of a wire through the field.
Complete the sentence to describe how the fingers help to show the relationship between these three factors.
The …………………
finger points in the direction of the current.
1 point
Your answer
A wire is placed in a magnetic field as shown in the diagram.
The current in the wire is flowing out of the screen.
In which direction will the wire move: A, B, C or D?
1 point
A
B
C
D
Clear selection
A wire is placed in a magnetic field as shown in the diagram.
The current in the wire is flowing into the screen.
In which direction will the wire move: A, B, C or D?
1 point
A
B
C
D
Clear selection
a)
The diagram below shows a simplified d.c. motor.
The motor rotates in an anticlockwise direction.
Complete the sentences to describe the magnetic poles in the diagram:
To the left of the coil is a magnetic ………………… pole.
1 point
Your answer
b)
To the right of the coil is a magnetic ………………… pole.
1 point
Your answer
a)
The diagram below shows a simplified d.c. motor.
Complete the sentences to describe the function of the split-ring commutator:
The split-ring commutator reverses the direction of the
…………………
every
…
1 point
Your answer
b)
…
every
…………………
revolution. This ensures the forces on each side of the coil always act in the same direction.
1 point
Your answer
a)
The diagram below shows a simplified d.c. motor.
Complete the sentences to describe the forces acting on the coil.
The forces acting on each side of the coil will cause the direction of rotation to be
………………… .
1 point
Your answer
b)
The diagram below shows a simplified d.c. motor.
Complete the sentences to describe the forces acting on the coil.
To increase the size of the forces, increase the size of the
…………………
in the coil.
1 point
Your answer
The diagram shows a simplified d.c. motor. Which change will not increase the size of the turning effect?
1 point
Adding more turns to the coil of wire
Increasing the current to the coil
Reversing the direction of the current to the split-ring commutator
Using stronger magnets to produce the field
Clear selection
The motor shown in the diagram rotates anticlockwise.
In which direction will the motor rotate if the direction of the current and the direction of the magnetic field are reversed?
1 point
Your answer
A direct current passes through a rectangular coil of wire inside a uniform magnetic field.
Will the forces acting on the left side of the coil and the right side of the coil be in the same direction or in opposite directions?
1 point
Your answer
The diagram shows the path of a beam of positively-charged particles moving through a magnetic field.
What is the direction of the magnetic field? Choose your answer from
into the screen, out of the screen, upwards, l
eft, right or downwards.
1 point
Your answer
The diagram shows the force acting on a current-carrying wire placed in a magnetic field. The force on the wire acts downwards. Which of the following changes would increase the size of the force and reverse the direction of the force?
1 point
Increase the current in the wire only.
Reverse the direction of the magnetic field and reverse the direction of the current.
Increase the strength of the magnetic field and reverse the direction of the current.
Increase the strength of the magnetic field and increase the current.
Clear selection
a)
A teacher places a wire between two magnets as shown in the diagram.
Initially, the current in the wire is 0.0A.
The teacher gradually increases the current travelling through the wire in the direction X to Y.
Complete the sentence to describe the changes to the force acting on the wire:
As the current increases, the force acting ………………… on the wire …
1 point
Your answer
b)
… on the wire gradually ………………… in size.
1 point
Your answer
The diagram shows a straight wire placed between two magnets. There is a current travelling upwards through the wire. The wire experiences a force due to the current and the magnetic field.
In which direction will this force act?
1 point
into the screen or page
out of the screen or page
towards the north pole of the magnet
towards the south pole of the magnet
Clear selection
a)
The diagram shows the magnetic field around a solenoid with a current passing through it. Complete the sentences describing the magnetic poles of the solenoid:
End X of the solenoid is acting as a …………………
magnetic pole.
1 point
Your answer
b)
E
nd Y of the solenoid is acting as a …………………
magnetic pole.
1 point
Your answer
The diagram shows the magnetic field lines around a solenoid with a current passing through it. Which statement describes how the pattern of the magnetic field lines will change if the direction of the current is reversed and the size of the current is increased?
1 point
The direction of the magnetic field lines will not change but the field lines will become closer together.
The direction of the magnetic field lines will not change but the field lines will become further apart.
The direction of the magnetic field lines will reverse and they will become closer together.
The direction of the magnetic field lines will reverse and they will become further apart.
Clear selection
a)
The diagram shows the magnetic field lines around a solenoid with a current passing through it.
Complete these sentences to describe the magnetic field:
The magnetic field is weakest at point
………………… .
1 point
Your answer
b)
The magnetic field is most uniform at point
………………… .
1 point
Your answer
A long straight wire carries a current.
Which statement describes the pattern of the magnetic field around the wire?
1 point
A circular field which is stronger further away from the wire.
A circular field which is weaker further away from the wire.
A field the same shape as the magnetic field surrounding a bar magnet.
A linear field with field lines in the same direction as the current flow in the wire.
Clear selection
a)
The diagram shows part of an electricity delivery grid.
Complete the sentences describing this system:
High voltage is used in transmission cables so that
…………………
can be used.
1 point
Your answer
b)
This reduces energy losses by heating due to the electrical
…………………
of the cables.
1 point
Your answer
a)
A transformer has a primary current of 150mA
and a primary voltage of 230V. The output voltage is 5.0V. Calculate the output current from this transformer.
1 point
Your answer
b)
What is the assumed percentage efficiency of this transformer?
1 point
Your answer
For a transformer, the relationship between the primary and secondary voltages and currents can be expressed by the equation:
𝐼ₚ
×
𝑉ₚ =
𝐼ₛ
×
𝑉ₛ
What assumption must be made for this equation to be correct?
1 point
Both the input and output currents are large.
Both the input and output currents are small.
The input voltage is less than the output voltage.
There are no energy losses in the transformer.
Clear selection
A transformer has an input voltage of 12V
, 2400
turns of wire on its primary coil and 750 turns of wire on its secondary coil.
Calculate the output voltage of this transformer. Give your answer to 2 significant figures.
1 point
Your answer
A transformer used for a laptop computer converts an input voltage of 240V
to an output voltage of 12V
.
The transformer has 50 turns of wire on the secondary coil.
How many turns of wire are on the primary coil?
1 point
Your answer
Which of the following is an advantage of transferring electricity at high voltage?
1 point
High-voltage transmission is more efficient then low-voltage transmission.
High-voltage transmission reduces the risk of electrocution.
High-voltage transmission uses larger currents than low-voltage transmission.
Transformers are not needed for high-voltage transmission.
Clear selection
A transformer in an electricity substation has 16 000 turns of wire on its primary coil and 4000 turns on its secondary coil.
Is this a step-up transformer or a step-down transformer?
1 point
Your answer
A student connects a transformer to a d.c. power supply and a lamp. The transformer has two coils and a soft iron core.
Which statement could explain why the lamp does not light up?
1 point
A transformer needs a harder iron core to increase its efficiency.
A transformer needs an alternating power supply so a changing magnetic field is created in the iron core.
A transformer needs an alternating power supply so a constant magnetic field is created in the iron core.
The direction of the current in the core is incorrect and needs to be reversed.
Clear selection
a)
Complete the following sentence about transformers:
A transformer is constructed from two ………………… of wire …
1 point
Your answer
b)
…
wire
wrapped around a
…………………
core.
1 point
Your answer
a)
Complete the following sentence about transformers:
A step-up transformer has
………………… t
urns of wire on the secondary coil than it has on the primary coil.
1 point
Your answer
b)
This means the output voltage is ………………… than the input voltage.
1 point
Your answer
a)
The diagram shows the coil of an a.c. generator as it rotates at a constant speed in a magnetic field. The arrows show the direction of the magnetic field.
Complete the sentences to describe the size of the e.m.f. induced when the coil is in different positions.
At positions A and C, the magnitude of the induced e.m.f. will be …………………
as the sides of the coil are moving parallel to the direction of the magnetic field.
1 point
Your answer
b)
At positions B and D, the magnitude of the induced e.m.f. will be …………………
because the sides of the coil are moving perpendicular to the direction of the magnetic field
.
1 point
Your answer
Which of the following graphs shows the output e.m.f. of an alternating current generator?
1 point
A
B
C
D
Clear selection
The output varies over each rotation in the shape of a sine wave. There are peaks, points where the output is 0.0V and regions where the output voltage is negative. Why are slip rings used to connect the coil to the external circuit?
1 point
To allow the generator to rotate continually while maintaining electrical contact with the external circuit
To insulate the generator from the external circuit
To make sure the output of the generator is direct current
To prevent the magnetic field produced by the generator from affecting the external circuit
Clear selection
A long straight wire is placed between two magnets. The wire is moved in the direction of the magnetic field (directly towards the south pole of the right-hand magnet) as shown. Which statement describes what happens in the wire?
1 point
A small current travelling downwards is induced in the wire.
A small current travelling towards the north pole is induced in the wire.
A small current travelling upwards is induced in the wire.
No current is induced in the wire.
Clear selection
a)
A magnet is moved into and out of a coil of wire connected to an ammeter, as shown in the diagram below.
Complete these sentences to explain why there is a force acting on the magnet as it moves.
The movement of the magnet induces an
………………… which causes a current in the coil. The current in the coil turns the coil into an electromagnet, which produces a magnetic field..
1 point
Your answer
b)
The magnetic field from the coil always
…………………
the motion of the magnet: when the magnet moves into the field from the coil (downwards), the coil repels the magnet and when the magnet moves out of the field (upwards), the coil attracts the magnet.
1 point
Your answer
A student pushes a magnet into a coil of wire connected to an ammeter, as shown in the diagram below.
What is the name of the effect which produces a small current in the coil of wire when the magnet is moving?
1 point
Your answer
a)
A wire is moved downwards through a magnetic field.
The wire is connected to a sensitive ammeter, which deflects slightly to the left as the wire moves.
Which statement describes how the ammeter needle will move when the wire stops moving?
1 point
The needle will deflect further to the left.
The needle will deflect slightly to the right.
The needle will deflect strongly to the right.
The needle will show a current of 0.0A.
Clear selection
b)
A wire is moved downwards through a magnetic field.
The wire is connected to a sensitive ammeter, which deflects slightly to the left as the wire moves.
Which statement describes how the ammeter needle will move when the wire is pulled upwards through the magnetic field, twice as fast as before?
1 point
The needle will move exactly as it did before.
The needle will move half as far as before and to the left.
The needle will move the same amount as before and to the right.
The needle will move twice as far as before and to the right.
Clear selection
A student is investigating the relationship between current and force of an electromagnet. The student is recording the number of paper clips picked up by the electromagnet to represent force.
Which option correctly describes a way to improve the precision of the experiment?
1 point
Keep room temperature constant
Use a smaller current
Use heavier paper clips
Use lighter paper clips
Clear selection
Which equipment list would enable a student to build a transformer and measure the efficiency?
1 point
Iron core; insulated wire; power supply; lightbulb only
Iron core; insulated wire; power supply; variable resistor only
Iron core; insulated wire; power supply; voltmeter; amme
Iron nail; insulated wire; power supply; ammeter only
Clear selection
Which of the circuits below could be used to investigate induction of current?
1 point
A
B
C
D
Clear selection
A d.c. motor experiences a turning force equal to
𝐹
. The number of coils in the armature is doubled, the length of the armature is halved and the current in the armature is also halved. By what factor has
𝐹
changed?
1 point
Your answer
A magnetic field points right to left, a wire carries current into the page and the force experienced by the wire is upwards. If the direction of the current is reversed, what direction is the force on the wire now?
1 point
Your answer
The cross-sectional area of a current-carrying wire with a magnetic field is doubled. What factor does the strength of the magnetic field change by: a factor of 2 or 0.5?
1 point
Your answer
In the diagram, a compass is placed at position A, left of a wire carrying current into the page. In which direction would the compass needle point? (Answer with 'Up', 'Down', 'Left' or 'Right')
1 point
Your answer
The voltage on the primary coil of a transformer is four times larger than the voltage on the secondary coil. The number of turns of wire on the primary coil is 16. Calculate the number of turns of wire on the secondary coil.
1 point
Your answer
State whether the device shown is a 'step-up' or a 'step-down' transformer.
1 point
Your answer
The armature in the generator is rotated by 90°, as shown by the circular arrow in the diagram. State whether the induced e.m.f. is a maximum or a minimum.
1 point
Your answer
The wire in the diagram below is moved at constant velocity to the right in a magnetic field into the page. State the direction of the induced current.
1 point
Your answer
Look at the diagram. An electron is fired from left to right and encounters a uniform magnetic field. Which of the following statements describes the force experienced by the electron when it passes through the magnetic field?
1 point
Left
Right
Into the page
Out of the page
Clear selection
What is the direction of the force experienced by the current-carrying wire in the diagram?
1 point
Down
Up
Into the page
Out of the page
Clear selection
Which of the following diagrams correctly identifies the magnetic field around a wire with current travelling out of the page?
1 point
A
B
C
D
Clear selection
Study the generator. Which point on the e.m.f. against time graph correctly correlates with the position of the armature in the generator?
1 point
A
B
C
D
Clear selection
In the diagram, a bar magnet is dropped through a coil of wire and a current is induced. Which of the following options best describes the acceleration of the magnet as it falls?
1 point
Less than 𝑔
Equal to 𝑔
Greater than 𝑔
Less than 𝑔, then greater than 𝑔
Clear selection
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