magnetism and electromagnetism

Answer 1

Magnets
- North and South Poles
- Same Poles repel
- Opposite poles attract
Permanent Magnets
- Always magnetic, always have poles
Induced Magnets
- Materials that are “magnetic” but do not have fixed poles
- These can be made into temporary magnets by ‘stroking’ them
with a permanent magnet
o These align the domains in the material all in the same
direction, creating a temporary magnet
o Iron, Nickel, Cobalt

Answer 2

Magnetic Fields
- Field Lines point from North to South
- Strength decreases with distance from the magnet
- Direction always points to south pole and away from north pole, at any point
- Use Plotting Compasses
o Small compasses which show the direction of the magnetic field at a certain point

Answer 3

Earth’s Core
- The core is magnetic, and creates a large magnetic field around
the Earth
- We know this because a freely suspended magnetic
compass will align itself with the earth’s field lines and
point North
- It doesn’t point to the Geographic North pole – it is over
North Canada
- Also, the compass is effectively a suspended Bar Magnet,
with its own north pole lining up with Earth’s ‘North pole’
o However this cannot be right, as like poles repel
o So in fact, Earth’s magnetic pole above Canada is a
magnetic South Pole! (and the geographic south
pole is close to the Magnetic North Pole)

Question 4

What does a current produce around the wire?

Answer 4

• Current produces a magnetic field around the wire

Question 5

How is the direction of the current dictated?

Answer 5

• The direction is dictated by the “right hand grip rule”
• Plotting compasses on a piece of paper through which a
  wire is pierced shows this

Question 6

Describe the relationship between current and the strength of a magnetic field

Answer 6

• Greater current, stronger magnetic field

Question 7

Describe the relationship between distance and the strength of a magnetic field

Answer 7

• Greater distance from wire, weaker field

Question 8

What is the magnetic field shape of a solenoid like?

Answer 8

• Magnetic field shape is similar to a bar magnet

Question 9

Explain the function of a solenoid

Answer 9

• It enhances the magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to the direction of the current

Question 10

What does having an iron core in the centre of a solenoid do?

Answer 10

• Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air

Question 11

Name the factors that affect the strength of a magnetic field of a solenoid

Answer 11

• Size of current
• Length
• Cross sectional area
• Number of turns (coils)
• Using a soft iron core

Question 12

Explain the motor effect?

Answer 12

• Two magnets will interact, feeling a magnetic force of attraction/repulsion
• So a magnet and a wire will also exert a force, as the two magnetic fields
  (generated by the magnet and the current in the wire) will also interact

o The magnetic field around a wire is circular, but the magnetic field
between two magnets is straight
o When the two interact, the wire is pushed away from the field between
the poles (at right angles to the wire direction and the field direction)
- To visualise
o Fixed permanent magnets have field lines along the x axis, as the
magnets are at A and B and the field lines are shown
o Wire is along y axis, where current is moving up from C to
D
o The Force felt on the wire is at right angles to both the direction of the
current and magnetic field lines
 Along the z axis

Question 13

Explain how to use Fleming’s Left Hand Rule

Answer 13

• Each direction is 90° to each other
• Use this to work out the unknown factor out of the
  three (usually the direction of the force felt)
• Remember current is conventional current, which moves
  in opposite direction to the electrons

Question 14

Explain how electric motors work

Answer 14

• Permanent Magnets lie in fixed positions
• In between, a coil of current-carrying wire lies on an axis
• Force on one side moves that side up
• Force on the other side (where current is flowing in opposite direction) moves down
• This can be verified using Fleming’s Left Hand Rule
• Hence it rotates

Answer 15

Electromagnetic Induction (Physics only)
- When there is a relative movement between a conductor and a magnetic field, a potential
difference is induced across the conductor.
- This happens if the magnetic field changes as well
- A current flows if the conductor forms a complete circuit.
- This current will produce its own magnetic field, which oppose the change inducing it

Question 16

Explain how electric generators (dynamos) work

Answer 16

• Same setup as a motor, with a coil of wire able to rotate between two permanent magnets
• A turbine spins turning the coil of wire
• The movement of the wire causes the wire to cut through the magnetic field
• It experiences a change in magnetic field
• This creates a potential difference
• If the coil of wire is connected to a complete circuit, an alternating current (AC) will flow – this is a basic alternator (picture)
• Direct current (DC) current is produced if the ends, A and D in diagram above, are connected to a split ring commutator
• This reverses the current each half-rotation so current remains positive – this system is called a dynamo

Answer 17

Transformers (Physics only)
- AC in first coil creates a changing magnetic field
- This changing magnetic field cuts through the secondary
coil
- This induces a current in the secondary coil
o Which is also AC
o If primary current was DC, magnetic field it
produces will be constant, not inducing anything
in the secondary coil
- More coils on secondary: Step up transformer, as
voltage will be increased, as changing field will cut
through more of the secondary wire inducing a larger pd
- Fewer coils on secondary: Step down transformer, as
smaller pd forms on secondary
number of coils on primary pd of primary
number of coils on secondary. pd of secondary
This only works with current too if the transformer is 100% efficient. Unless it states this,
assume not and just use this to find voltage

Question 18

Explain how dynamic microphones work

Answer 18

• They produce a current which is proportional to the sound signal
• Fixed magnet is at the centre, and the coil of wire around the
  magnet is free to move
• Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it cuts the magnetic field)
• This current is then sent to a loudspeaker

Question 19

Explain how loudspeakers work

Answer 19

• The setup is identical to the microphone, however, working in reverse
• The current flows into the coil
• The magnetic field from magnet and from current interact, causing the coil to move
• The cone therefore moves
• Producing pressure variations, making sound