P4.2 - Uses Of Magnetism

Question 1

What is the motor effect?

Answer 1

When a current carrying wire, in a magnetic field, experiences a force.

Question 2

What does the size of the force for the motor effect depend on?

Answer 2

The size of the force exerted, depends upon the strength of the magnetic field, the current in the wire and the length of the wire.

Question 3

What is the equation for the motor effect?

Answer 3

• F=BIL
• F - Force on conductor - N
• B - Magnetic flux density - T
• I - Current - A
• L - Length of conductor (in field) - m

Question 4

What can you use to calculate the direction of the force exerted on a wire?

Answer 4

• Fleming’s Left Hand Rule
• Thumb - Direction of the force.
• Index finger - Direction of the magnetic field (N to
  S).
• Middle finger - Direction of the current.

-The direction of the force exerted, is always
perpendicular to both the direction of the magnetic
field, and the direction of the current in the wire.

Question 5

What does an electric motor use?

Answer 5

An electric motor uses the motor effect to transfer electrical energy to kinetic energy.

Question 6

Describes what happens to a coil in an electric motor.

Answer 6

• An electric motor (in effect) consists of a
  loop/length of wire, with a current, in a magnetic
  field.
• The wire will experience a force, given by F=BIL.
• As current flows in opposite directions in the
  opposite sides of the coil, forces act in opposite
  directions on opposite sides of the coil, causing
  the coil to rotate.
• Using Flemings Left Hand Rule - Left side of the
  coil experiences a force up and the right hand side
  experiences a force downwards as the current is
  running in the opposite direction - The coil rotates
  clockwise.
• Once the coil rotates to the vertical position, the
  coil experiences no force on its sides because the
  current no longer runs perpendicular to the
  magnetic field lines but it continues to move past
  this position due to its momentum.
• As the two sides of the coil are now switched over
  and the current is back to running perpendicular to
  the field lines, but in an opposite direction the coil
  will experience a force again and turns in the
  same direction with the help of a split ring
  commutator.

Question 7

Why do you need a split ring commutator in an electric motor?

Answer 7

• If the current continued to run in the same
  direction, the direction of the force would keep
  on being in the opposite direction every half
  turn of the coil.
• The coil would end up flip-flopping backwards
  and forwards about the vertical position.
• A split ring commutator is used to reverse the
  direction of the current in the coil every half turn
  of the coil.

This reverses the direction of the forces on the sides of the coil every half turn, ensuring the coil continues to rotate in the same direction

Question 8

What do you need to do in order to increase the speed of a motor?

Answer 8

• Increase the size of the force acting.
• F=BIL
• More loops/Longer length of wire.
• Increase the strength of the magnet.
• Increase the current in the coil of wire

Question 9

What is electromagnetic induction?

Answer 9

• Sometimes called the generator effect.
• When a conductor cuts the magnetic field lines
  of a magnet, a potential difference is induced
  across the ends of the conductor.
• Opposite of the motor effect.

Question 10

How does electromagnetic induction work?

Answer 10

• When a wire cuts magnetic field lines, electrons
  are moved to one side of the conductor, which
  generates a p.d.
• If the wire is part of a complete circuit, a current
  will flow.

Question 11

What does the size of the induced potential difference in a conductor depend on?

Answer 11

The size of the induced p.d. in a conductor depends upon the rate at which you cut field lines.

Question 12

What are the three ways to increase the size of the induced potential difference?

Answer 12

You need to increase the rate of cutting field lines by either:

• Increasing the number of coils in the wire.
• Using a stronger magnet.
• Move the wire quicker through the magnetic field.

Question 13

What happens when you reverse the direction of the wire (electromagnetic induction)?

Answer 13

Reversing the direction of the wire reverses the direction of the induced p.d. in the wire.

Question 14

Explain the terms of field lines why using a stronger magnetic field induces a larger potential difference in a wire or coil.

Answer 14

• A stronger magnetic had a greater magnetic
  flux density/greater density of field lines around
  it.
• Thus, when a wire is moved at any given
  velocity through the magnetic field of a stronger
  magnet, the rate at which the wire cuts field lines
  is greater, so a larger potential difference is
  induced.

Question 15

What is Lenz’s Law and what does it ensure?

Answer 15

• “The direction of the induced p.d. is such that it
  opposes the change producing it”.
• Lenz’s Law ensures that the law of conservation
  of energy isn’t violated.

Question 16

Give an example of how Lenz’s Law works?

Answer 16

• Magnet approaching a coil of wire with its north
  pole.
• The coil cuts magnetic field lines.
• A potential difference is induced in the coil of
  the wire and will give rise to a north pole on the
  end the magnet is approaching.
• The north pole of the magnet and north pole of
  the coil will then repel, opposing the motion
  inducing the p.d.

Question 17

You need to be able to slow down a falling lift if the cable breaks.
Suggest and explain how you can use electromagnetic induction to slow down a falling lift.

Answer 17

-The lift is fitted with permanent magnets, and made
to travel inside a metal tunnel.

• If the cable breaks, the lift will fall, and the
  magnets will move past the metal surfaces of the
  tunnel.
• The metal surfaces of the tunnel will cut magnetic
  field lines, in inducing a p.d. and current in them.
• The current carrying surface will now interact with
  the magnetic field of the magnets, give rise to a
  motor force.
• This motor force will acts upwards, opposing the
  motion of the lift, causing it to fall/accelerate at a
  lesser rate than it would under the force of just its
  weight.

Question 18

How does an alternator work?

Answer 18

• Coil of wire spins between the poles of a
  uniform magnetic field (equivalent to the
  magnet moving in and out).
• The brushes are not attached to the slip rings
  but brush against the slip rings so that the
  voltmeter is always connected to the ends of
  the coil but the coil does not become tangled.
• As the coil turns, each side of it cuts the field
  lines from below then above and so on.
• The induced potential difference is therefore
  repeatedly changing direction (and so therefore
  is the induced current).

Question 19

How does a dynamo work?

Answer 19

• Instead of two slip rings, a dynamo has a split ring
  commutator.
• The split ring reconnects the coil the other way
  round every half turn of the coil.
• Therefore, the induced p.d. does not reverse its
  direction but can only change in magnitude.

Question 20

Compare an alternator and a dynamo.

Answer 20

• Both use coils and magnets to induce a p.d.
  (and current).
• Alternators use slip-ring commutators to give
  rise to an a.c. current.
• Dynamos use split-ring commutators to give
  rise to a d.c. current.

Question 21

How does doubling the speed of rotation of a dynamo change the potential difference induced?

Answer 21

• The size of the peak p.d. induced will double.
• The rate at which field lines are cut is doubles
  which doubles the frequency as well the the
  peak p.d.

Question 22

Why is the potential difference zero when the coil is parallel with the poles of the magnet?

Answer 22

When the coil is parallel with the magnet, it will cut no field lines when it starts to move, thus inducing no p.d.

Question 23

A) Explain why the ammeter point deflects?
B) Explain why is deflects only once and returns
back to its position.

Answer 23

A) When the switch is closed a current flows
through the wire which gives rise to a magnetic
field.
-Coil Y will cut the magnetic field lines and a p.d.
will be induced in the conductor.
-If the conductor is a complete circuit, a current
will flow inside the conductor which is why the
ammeter pointer deflects.

B) When the current is constant, the field lines
around the magnet stop growing.
-Coil Y no longer cuts field lines so there is no
p.d. induced which means that no current flows,
so the ammeter does not deflect

Question 24

What are transformers?

Answer 24

• Transformers are devices that use an
  alternating p.d. (current) in a primary circuit to
  induce an alternating p.d. (current) in a
  secondary circuit.
• Transformers work by making use of
  electromagnetic induction.
• Transformers only work with an AC supply in
  the primary circuit.

Question 25

How do transformers work?

Answer 25

• An alternating p.d. across the primary coil
  produces an alternating current in the primary coil.
• An alternating current in the primary coil produces
  a magnetic field in the iron core that is always
  changing.
• The secondary coil experiences a constantly
  changing magnetic field.

- A p.d. (current) is continually induced in the 
  secondary coil (in alternating directions).

Question 26

What do step up and step down transformers do?

Answer 26

• Step up transformers - Increase the p.d. (higher
  voltage induces in the secondary circuit), because
  the secondary circuit has more turns.
• Step down transformers - Decrease the p.d. (lower
  voltage induced in the secondary circuit), because
  the secondary circuit has fewer turns.

Question 27

Explain why it is important for the laminated iron core of a transformer to be a complete loop with no gaps?

Answer 27

The magnetic field lines wouldn’t be transferred over the primary coil to the secondary coil efficiently, and power loss would occur in the secondary coil.

Question 28

How does a microphone work?

Answer 28

• A microphone is like a generator.
• A sound wave is like a pressure wave.
• A sound wave hits a diaphragm and causes it to
  move in and out - areas of high pressure,
  compressions, push the diaphragm in, and areas
  of low pressure, rarefactions, pull it out.
• As the diaphragm moves in and out, so does the
  coil.
• The coil moves inside a permanent’s magnet’s
  magnetic field and so cuts field lines.
• A p.d. (and therefore current) in now induced in
  the coil, producing an electric signal that is fed
  to a speaker.

Question 29

How does a loudspeaker work?

Answer 29

• A loudspeaker is like a motor.
• A changing p.d. in a coil of wire produces a
  changing current.
• The wire is inside a permanent magnet’s
  magnetic field and so experiences a force.
• A cone attached to the coil moves in and out.
• A sound wave is generated of the same
  frequency at which the diaphragm vibrates
  (moves in and out).

Question 30

How can you make a simple loudspeaker?

Answer 30

• You can make a simple loudspeaker with a paper
  cone, a piece of wire, and a pair of magnets.
• If you connect the ends of the wire to an
  alternating potential difference of a suitable
  frequency, you will hear a sound.