Electromagnetism

Question 1

What does a magnetic field that changes in time create?

Answer 1

An induced electric field (emf)

Question 2

If there are free charge carriers, and induced emf will cause…

Answer 2

an induced current to flow.

Question 3

How does an induced current flow?

Answer 3

Such that it opposes the change in the magnetic flux that created it.

Question 4

A generator converts…

Answer 4

Kinetic energy to electrical potential energy

GKE

Question 5

How does a generator induce emf across the coils to drive a current?

Answer 5

It uses the relative movement of the coils of wire and magnets.

Question 6

What does a transformer do?

Answer 6

Acts to change the magnitude of an AC potential difference.

Question 7

A motor converts…

Answer 7

Electrical potential energy to kinetic energy

MEK

Question 8

How does a motor produces force on the coil?

Answer 8

A current flowing through the coil in a magnetic field.

Question 9

What is the net force on the coil in a motor

Answer 9

Zero

The two sides cancel each other out.

Question 10

What does a DC motor use to change the currents direction every half turn?

Answer 10

A commutator

Question 11

What is a armature?

Answer 11

The frame of the rotating part of the motor or generator, holding one or more coils.

Question 12

What is a diode?

Answer 12

A circuit component that allows current to flow in only one directions.

Question 13

What is an eddy current?

Answer 13

A circular current that is induced in a conductor due to a changing magnetic field.

Question 14

What is electromagnetic induction?

Answer 14

The production of an emf due to a time-varying magnetic field.

Question 15

What is magnetic braking?

Answer 15

Bra King due to the interaction of eddy currents and an external magnetic field.

Question 16

What is magnetic flux?

Answer 16

The magnetic field passing through a given area.

Question 17

What is a rectifier?

Answer 17

A circuit that uses diodes to convert AC to DC.

Question 18

What is rms?

Answer 18

The average AC potential difference or current that produces the same power in a load as a DC potential difference or current of the same magnitude.

Question 19

Explain the difference between AC and DC generators?

Answer 19

AC provides an alternating current

DC provides a smooth output.

Question 20

Why does the current of a DC motor need to be reversed every half turn?

Answer 20

If the direction of current is not reversed, then every half turn the directions of the forces on the sides of the loop are reversed. This would push the loop back in the other direction, preventing it from rotating, and making it oscillate back and forth instead.

Question 21

Why would you not use a battery in a transformer?

Answer 21

Batteries provide a DC current, which won’t work in a transformer.

Question 22

Why does a transformer need an AC current?

Answer 22

To create an induced current in the secondary coil there must be changing flux through the coil. Only produced by an alternating current.

Question 23

What does an electric field that changes in time create?

Answer 23

A magnetic field

Question 24

What will happen if a current carrying coil is placed in a magnetic field?

Answer 24

It will cause motion.

Question 25

What happens when a coil is moved in a magnetic field?

Answer 25

It will produce a force on the charged particles (electrons) within the coil. This causes a current to flow.

Question 26

What does emf mean

Answer 26

Potential difference

Question 27

What is emf measure in?

Answer 27

Volts

Question 28

Where in the conductor is the emf induced?

Answer 28

Between the ends of the conductor.

Question 29

If there is a magnet or coil moving in a magnetic field, what would stop it from producing the emf?

Answer 29

In the conductor did not cut the field, ie it is parallel, it will not produce an emf. The magnetic field must be changing.

Question 30

What is Faraday's law?

Answer 30

Induced EMF is directly proportional to the rate of change of magnetic flux.

Question 31

The faster that the magnetic flux through a circuit changes, the greater the ___________ that is produced.

Answer 31

EMF

Question 32

What are three ways to increase the magnetic flux through a circuit?

Answer 32

Change the velocity with which the magnet or coil moves Change the strength of the magnetic field Change the area of the coil

Question 33

What is Lenz's Law?

Answer 33

An induced current always flows in such a direction as to oppose by its own magnetic field, the motion or change that induced it.

Question 34

What will happen when the magnets North Pole is moved towards the front of the coil?

Answer 34

The coil induces a current flow that opposes this movement. It creates a North Pole to repel the incoming North Pole.

Question 35

What will happen when a magnets North Pole is moved away from the front of a coil?

Answer 35

The coil will induce a current with a flow that will oppose this movement. It will create a South Pole to attract the retracting North Pole.

Question 36

Where is the emf induced when a conducting disk rotates such that it cuts a magnetic field?

Answer 36

Between its centre and its circumference.

Question 37

What is the armature?

Answer 37

A coil of wire that rotates between the poles of a magnet.

Question 38

What are the three factors that can increase the emf of a generator?

Answer 38

Increasing the speed of rotation of the armature Increase the number of turns in the coil of the armature Use a stronger magnetic field

Question 39

What is back emf?

Answer 39

It is the induced emf of the armature rotating in the magnetic field. This emf resists the applied emf that drives the current through the armature (Lenz law)

Question 40

What is the net emf in a motor?

Answer 40

The applied emf- the back emf.

Question 41

What is a transformer?

Answer 41

A device used to increase or decrease AC voltage.

Question 42

How does a transformer work?

Answer 42

A AC current is passed through a primary coil, producing a changing magnetic field that threads through the secondary coil. The changing magnetic field produces an emf in the secondary coil.

Question 43

What are eddy currents?

Answer 43

When a conductor moves in a changing magnetic field, electromagnetic induction can set up circulating currents in the conductor.