Ch23: Electromagnetism

Question 1

Which direction do magnetic field lines go?

Answer 1

North to South

Question 2

How is a uniform field represented in a diagram?

Answer 2

Equally spaced lines in the same direction

Question 3

Which hand rule is used for a magnetic field around a current carrying wire?

Answer 3

Right hand rule, to determine direction of field

Question 4

What is fleming’s left hand rule for?

Answer 4

A current perpendicular to a magnetic field, inducing a force.

Question 5

What is magnetic flux density used as?

Answer 5

A measure of the strength of a magnetic field

Question 6

What is the definition of Magnetic flux density? What is the symbol used to represent it?

Answer 6

• The force on one metre of wire carrying a current of 1 amp at right angles to the magnetic field
• B

Question 7

What equation is used to find the force on a wire in a magnetic field?

Answer 7

F = BIL
or
F = BILsinθ

Question 8

What experiment can be used to find magnetic flux density in the lab?

Answer 8

A wire between two magnets over a scale, the wire experiences a downwards force when current flows and the force can be read on the balance

Question 9

What is a velocity selector used for?

Answer 9

To separate particles of a certain velocity from a stream of accelerated charged particles moving at a range of speeds.

Question 10

How does a velocity selector work?

Answer 10

• A charged particle is fired at a collimator.
• It is subject to an electric field and a magnetic field which are applying opposite forces on the particle.
• Unless the two forces cancel out, the particle is deflected.
• By rearranging F=BQv and F=EQ, any particles not deflected have v=E/B

Question 11

How can a magnetic field induce an E.M.F?

Answer 11

If a conducting rod is moving through a magnetic field, or stationary in a changing magnetic field, the electrons in the rod experience a force and an E.M.F is induced.

Question 12

What is the symbol for magnetic flux? What is an equation for it?

Answer 12

Φ

- Φ=BA

Question 13

What is magnetic flux density measured in? What is that unit equal to?

Answer 13

Teslas, T

T=Wb/m^2

Question 14

What is the difference between magnetic flux density, magnetic flux, and magnetic flux linkage?

Answer 14

Flux: ‘number of field lines’
Flux density: ‘number of field lines per unit area’
Flux linkage: ‘number of field lines for a coil with N turns’

Question 15

How is flux linkage calculated?

Answer 15

flux linkage= NΦ

Question 16

What is magnetic flux measured in?

Answer 16

webers, Wb

Question 17

What is flux linkage measured in?

Answer 17

webers, Wb

Question 18

How is magnetic flux calculated if the field is not perpendicular to the coil?

Answer 18

Φ=BAcosθ

Question 19

What is faraday’s law?

Answer 19

The induced e.m.f is directly proportional to the rate of change of flux linkage (flux linkage = BAN(cosθ)

Question 20

Define a weber (Wb)?

Answer 20

A change in flux linkage of 1 weber per second will induce an emf of 1 volt in a loop of wire

Question 21

How is faraday’s law represented on graphs?

Answer 21

1. On a graph of flux linkage (BAN) against time, the gradient is the e.m.f
2. On a graph of e.m.f against time, the area under the graph gives the change in flux linkage (-ΔBAN)

Question 22

What is Lenz’s law? (How does it affect Faraday’s law?)

Answer 22

The induced e.m.f is always in a direction such as to oppose the change that caused it.
- It is the reason for the minus sign in Faraday’s law

Question 23

When a changing magnetic field induces an e.m.f in a coil, it produces a current which in turn produces its own magnetic field.
What does Lenz’s law say about this?

Answer 23

• If the original magnetic field is getting stronger, the induced field will oppose it.
• If the original magnetic field is collapsing, the induced field will be in the same direction.

Question 24

How can Faraday’s law be used in an experiment to find magnetic flux?

Answer 24

By using a search coil in a magnetic field connected to a data logger recording e.m.f.

• On a graph of e.m.f against time, the area under gives change in flux linkage.
• Flux linkage = BAN
• Repeat and take a mean of B

Question 25

How does a generator use magnetic fields? What does it produce?

Answer 25

Generators convert kinetic energy to electric, by spinning a coil in a magnetic field and producing an alternating current.

Question 26

What is a transformer?

Answer 26

Devices that change the size of a voltage for alternating current.

Question 27

In a transformer, what happens when there are more turns on the secondary coil?

Answer 27

It is a step up transformer, and and voltage is increased.

Question 28

What can be done to a transformer to increase it’s efficiency?

Answer 28

Using a laminated core.

Question 29

Why is electricity in the national grid transported at a low current?

Answer 29

To reduce power loss because P=I²R

Question 30

What is a standard mains voltage?

Answer 30

230V

Question 31

What is a standard voltage for national grid transmittion?

Answer 31

400,000V

Question 32

Why are transformers important for the national grid?

Answer 32

To reduce current and increase voltage for transmission around the national grid, this low current reduces power loss.