Topic 7 Electromagnetism (F + H Qs) Flashcards
Explain how the electromagnet and conveyor belt are used to separate the steel cans from
the aluminium cans
steel cans are attracted to the electromagnet and are transferred to the container (by
the conveyor belt)
aluminium cans are not attracted to the electromagnet and are left behind on the
table
A bar magnet is an example of __________
a permanent magnet
At the top of the table the strength of the magnetic field is only just enough to pick the cans
up.
Describe two ways to increase the strength of magnetic field at the top of the table.
rRaise the height of the table
use a larger potential difference / current
or
use a stronger electromagnet
The floating magnet was placed near to the north pole of the fixed magnet. The floating
magnet then moved along the path shown in the photograph.
Explain why. (2)
(the north pole of the floating magnet is) repelled from the north pole (of the fixed magnet)
and attracted to the south pole (of the fixed magnet)
The student replaced the floating magnet with a piece of iron.
What happened to the piece of iron?
it was attracted (to the fixed magnet)
Describe how to use a compass to plot the magnetic field pattern around a bar magnet.
* mark where the compass points on the paper
* move the compass to the marked point
* repeat until you go back to the magnet
* join up the points
* add an arrow pointing from the north pole to the south pole
* repeat for positions (above and below the bar magnet)
Which type of rod would make the magnetic field of the coil stronger?
steel rod
Why does the spring get longer when the electromagnet is switched on?
electromagnet exerts a downwards force on the iron bar
The student measured the strength of the electromagnet by counting the number of paper
clips the electromagnet could hold.
Explain why it was important that the paper clips were all the same size
so the paper clips have the same weight / mass
which allows the results for different numbers of turns to be compared
The student wrote the hypothesis:
‘Increasing the current through the wire will make the electromagnet stronger.’
Describe how the student should change the investigation to test this hypothesis.
paperclip
keep number of turns constant
allow a specific number of turns
(use the variable resistor to) change the current (several times)
change the p.d. is insufficient
(for each current value) count how many paper clips the electromagnet will hold
Describe how to plot the magnetic field pattern of a bar magnet
place a (plotting) compass near the (north / south) pole of the
magnet and mark the direction that the compass points
move the (plotting) compass around the bar magnet (to the other
pole) marking at (regular) intervals the direction the compass points
join the points up and add an arrow pointing from the north pole to
the south pole
The student gives the steel ball bearing a gentle push in the direction of the iron rod.
At the same time the student closes the switch S.
Explain the effect on the motion of the ball bearing when the switch S is closed.
(closing switch S) causes a current in the coil
allow switches on the electromagnet
1
a magnetic field is created
1
a force of attraction acts on the ball bearing
1
so the ball bearing accelerates (towards the iron rod)
A student investigated the strength of different fridge magnets by putting small sheets of
paper between each magnet and the fridge door.
The student measured the maximum number of sheets of paper that each magnet was
able to hold in place.
Why was it important that each small sheet of paper had the same thickness?
so the results for each magnet can be compared
or
so there is only one independent variable
Describe how you could show that a magnetic field has been produced around the wire.
move a (magnetic / plotting) compass around the wire
the changing direction of the compass needle shows a magnetic field has been produced
sprinkle iron filings onto the card (1)
tapping the card will move the filings to show the magnetic field (pattern) (1)
Explain how the ignition circuit works
- closing the (ignition) switch causes a current to pass through the electromagnet
- the iron core (of the electromagnet) becomes magnetised
- the electromagnet / iron core attracts the (short side of the ) iron arm
- the iron arm pushes the (starter motor) contacts (inside the electromagnetic switch)
together - the starter motor circuit is complete
- a current flows through the starter motor (which then turns)
Explain how Fleming’s left hand rule can be used to predict the direction in which the
copper rod will move
hold thumb first finger and second finger (of left hand) at right angles to each other
allow first two fingers/index and middle for first and
second finger throughout
second finger represents the current pointing out of the paper
first finger represents the field pointing downwards
thumb points in the direction of the force / thrust / acceleration
(therefore) the rod moves left to right
Suggest two changes to the equipment that would increase the force on the copper rod.
decrease the resistance of the variable resistor
allow increase the current/pd
use a stronger magnet
What is the name of the effect used by the moving-coil loudspeaker to produce sound
waves?
motor (effect)
Explain how a moving-coil loudspeaker produces a sound wave
current creates a magnetic field (around the coil)
(which) interacts with the permanent magnet field
producing a (resultant) force causing the coil/cone to move
(when the) direction of the current reverses, the direction of the (resultant) force
reverses (producing a sound wave)
The visitor moves further away from the security lens in the door.
How does the size of the image change?
Decreases
Explain why the door unlocks when the switch is closed
there is a current in the solenoid / circuit
allow a charge flows through the solenoid / circuit
creating a magnetic field
allow the solenoid / coil is magnetised
attracting the bolt
Give two ways the resultant force on the bolt could be increased.
ncrease the current (in the solenoid / circuit)
allow any sensible suggestion for increasing the current such as
increasing the p.d. / power of the battery OR using lower resistance wire
in the solenoid
* add more turns to the solenoid
* use a spring with a lower spring constant
The portable power supply has an alternator connected to a transformer.
The transformer can be adjusted to have different numbers of turns on the secondary coil.
Suggest why
to vary the (output) potential difference
allow different devices require different potential differences
so that you don’t need a different generator for each type of device
The handle of the alternator is turned, causing the coil to rotate.
Explain why an alternating current is induced in the coil.
the coil moves through the magnetic field
or
the coil cuts magnetic field lines
a potential difference is induced (across the coil)
there is a complete circuit, so a current is induced (in the coil)
every half turn the potential difference reverses direction
so (every half turn) the current changes direction
Suggest the purpose of the slip rings
provides a continuous / moveable contact / connection (between the coil and the
transformer / contacts / brushes)
The alternator from the portable power supply is disconnected from the transformer and
lamp.
Explain why the handle of the alternator becomes much easier to turn
(after disconnection) there is no induced current
so no magnetic field (produced around / by the coil)
to oppose the movement of the coil
What is the function of a microphone?
to convert (the pressure variations in) sound (waves) into variations in current
Explain how a moving-coil microphone works
sound (waves) cause the diaphragm to vibrate
the diaphragm causes the coil / wire to vibrate
do not accept moves the coil / wire up and down
the coil / wire moves through the magnetic field
or
the coil / wire cuts magnetic field lines
a potential difference is induced
Explain why the iron rods move apart when the switch is closed.
the rods become (induced) magnets
with the same polarity (at each end)
Why does the movement of the bar magnet induce a potential difference across the coil?
magnetic field around the coil changes
Why is the induced potential difference across the coil alternating?
because the magnet changes direction
The seismometer cannot detect small vibrations.
Suggest two changes to the design of the seismometer that would make it more sensitive
to small vibrations
* stronger magnetic field
allow stronger magnet
allow heavier magnet
bigger magnet is insufficient
* more turns on the coil
bigger coil is insufficient
do not accept more coils of wire
* turns pushed closer together
* spring with a lower spring constant
what is the same as 4 microtesla?
4 × 10^−6
When there is a current in the coil, the coil rotates continuously.
Explain why.
the sides of the coil (parallel to the magnet) experience a force (in opposite directions)
allow the current creates a magnetic field
ignore Fleming’s Left Hand Rule
1
the forces cause moments that act in the same (clockwise / anticlockwise) direction
or
the moments cause the coil to rotate (clockwise / anticlockwise)
allow the magnetic fields interact to create a pair of
forces (acting in opposite directions)
or
allow the magnetic fields interact causing the coil to
rotate
1
(each half-revolution) the two halves of the (rotating) commutator swap from one
(carbon) brush to the other
1
(each half-revolution) the commutator reverses the current (in the coil)
or
keeping the forces in the same direction (keeping the coil rotating)
Why is iron a suitable material for the core of a transformer?
It is easily magnetised.
What is the name of the effect that causes the wire X to move downwards?
motor effect
