Magnets and Electromagnetism

Question 1

In an electric motor, which way does the coil spin and why?

Answer 1

the forces are the same size
but in opposite directions/side AB goes up, side CD goes down
so the coil spins clockwise

Question 2

Why is there no force in an electric motor when the magnetic field is parallel to the current?

Answer 2

when the current is parallel to the magnetic field
Fleming’s Left Hand rule says that the current must be perpendicular to the field
for a force to act

Question 3

In an electric motor, when the coil is vertical, the contacts are no longer in contact with the battery and a current no longer flows in the coil.
Explain why the coil continues to move.

Answer 3

the coil has momentum
so continues to move until the coil is in contact with the battery again on the
other side

Question 4

Describe the difference between a permanent and induced magnet.

Answer 4

A permanent magnet is always magnetic.
An induced magnet is only magnetic when placed inside the magnetic field of another object.

Question 5

What does the magnetic flux density show?

Answer 5

• How many field lines there are in a region
• This shows the strength of the magnetic field

Question 6

What is the motor effect?

Answer 6

• When a current carrying wire is put between two magnetic poles, the magnetic field arounf the wire interacts with the magnetic field it has been placed in.
• This causes the magnet and the wire to exert a force on each other, causing the coil to move.

Question 7

What does the force acting on a conductor in a magnetic field depend on?

Answer 7

• Length of wire
• Magnetic flux density
• Current

Question 8

Force Equation (Electromagnetism)

Answer 8

Force= Current * Magnetic Flux Density * Length

Question 9

Fleming’s Left Hand Rule

Answer 9

First Finger = Direction of Field
Second Finger = Direction of Current
Thumb = Motion (direction of force)

Question 10

Uses of Electromagnets:

Answer 10

Loudspeakers
Headphones

Question 11

What is the Generator Effect?

Answer 11

• When there is relative motion between an electrical conductor and a magnetic field, a potential difference will be induced across
  the ends of the conductor.
• A current will flow if this conductor is part of a complete circuit.

Question 12

What do step-up transformers do?

Answer 12

• step the p.d up
• more turns on secondary coil than primary coil

Question 13

What do step-down transformers do?

Answer 13

• step the p.d down
• more turns on primary coil than secondary

Question 14

How efficient are transformers?

Answer 14

• almost 100%

Question 15

Both Transformer Equations

Answer 15

Input p.d/ Outpud P.d = Primary coil turns/ secondary coil turns

Pd * current secondary coil = P.d * current primary coil

Question 16

What do transformers do?

Answer 16

• change the size of the p.d of an a.c

Question 17

What are primary and secondary coils joined by?

Answer 17

• an iron core

Question 18

When an alternating p.d is applied across the primary coil…

Answer 18

the iron core magetises and demagnetises quickly.
This changing magnetic field induces an alternating p.d in the secondary coil.
If the second coil is part of a complete circuit, a current is induced.

Question 19

Fleming’s Right Hand Rule

Answer 19

Changing the direction of the current, changes the direction of the magnetic field.

Question 20

How do dynamos generate a d.c?

Answer 20

• Use a split ring commutator to swap the connection every half trun and keep the current flowing in the same direction.

Question 21

How do alternators generate an a.c?

Answer 21

• rotate a coil in a magnetic field
• as the coil spins a current is induced in the coil
• this current changes direction every half turn
• the slip rings make sure the contacts are not swapped every half turn.
• hence an alternating p.d is produced.

Question 22

How do microphones work?

Answer 22

• cound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet
• this causes the coil of wire to move in the magnetic field which generates a current
• the microphone converts pressure variations of a sound wave into variation in current in an electric circuit.

Question 23

Suggest and explain what would happen if the student constructed a loudspeaker with the same equipment as the motor.

Answer 23

 both the motor and loudspeaker contain a coil of wire and a magnet
 a simple d.c. motor contains a battery/source of direct current, but a
loudspeaker uses a source of alternating current
 both use the motor effect, but the force is in the same direction in the motor,
but changes direction in the loudspeaker.
 the loudspeaker coil would move out and stay there
 no sound would be produced.

Question 24

Explain how Fleming’s left hand rule can be used to predict the direction in which the
copper rod will move.

Answer 24

• hold thumb first finger and second finger (of left hand) at right angles to each other
• 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

Question 25

Suggest two changes to the equipment that would increase the force on the copper rod.

Answer 25

- decrease the resistance of the variable resistor - use a stronger magnet

Question 26

What is the name of the effect used by the moving-coil loudspeaker to produce sound waves?

Answer 26

motor (effect)

Question 27

Explain how a moving-coil loudspeaker produces a sound wave.

Answer 27

- 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)