Electromagnetism

Question 1

What is magnetic flux density?

Answer 1

• A measure of magnetic field strength

- Magnetic flux per m^2

Question 2

What is the unit of magnetic flux density?

Answer 2

Tesla or Weber per metre

Question 3

What is Fleming’s Left Hand Rule?

Answer 3

• First Finger Field
• Second finger current
• Thumb gives motion

Question 4

How is magnetic flux density calculated?

Answer 4

B = F / Il when the wire is perpendicular to the field lines.

Question 5

How is the force on a wire calculated?

Answer 5

F = BIlsinθ
F = Bqv

Question 6

When a metal frame is balanced, how can magnetic flux density be calculated?

Answer 6

B = mg/Il

Question 7

When is there no force acting on a particle moving through a magnetic field?

Answer 7

• When the particle is moving parallel to the field lines.

- When the particle is stationery

Question 8

How does an electron gun work?

Answer 8

• The heater produces electrons through thermionic emission.
• The electrons are repelled from the heater and attracted towards the anode.
• The electrons are accelerated as they pass through the gun.

Question 9

What is the electron gun equation?

Answer 9

v = (2eV/m)^(1/2)

Question 10

How is the radius of the circular path of a particle calculated?

Answer 10

r = mv/Bq

Question 11

How is specific charge calculated?

Answer 11

e/m = v / Br
e/m = 2V/(Br)^2

Question 12

How does an electron move in a magnetic field?

Answer 12

In a circular path

Question 13

How does an electron move in an electric field?

Answer 13

In a parabolic curve towards the positive charge since y ∝ x^2

Question 14

What is a cyclotron?

Answer 14

A particle accelerator

Question 15

How does a cyclotron work?

Answer 15

• Two dees are placed in a vacuum chamber between the poles of a powerful magnet so the dees are subjected to a perpendicular uniform magnetic field.
• Protons move in a circular path within the dees due to the magnetic field.
• A high frequency alternating voltage is applied between the dees creating an electric field causing the proton to accelerate.
• Once the proton has reached the maximum radius, the proton is deflected out by a charged plate.

Question 16

How is the time taken to move through a dee calculated?

Answer 16

t = πm/Bq

Question 17

How is the required frequency of a cyclotron calculated?

Answer 17

f = Bq/2πm

Question 18

What is a galvanometer?

Answer 18

An ammeter that is designed to measure small currents.

Question 19

What is the unit of magnetic flux?

Answer 19

Weber (Wb) or Tm^2

Question 20

How is magnetic flux calculated?

Answer 20

Φ = BA

Question 21

Why do the electrons in a wire become displaced when moved through a magnetic field?

Answer 21

The electrons experience a magnetic force.

Question 22

How is the force on an electron in a wire calculated?

Answer 22

F = Bev

Question 23

How is the electric force on an electron in a wire calculated?

Answer 23

F = eE = e(ε/l)

Question 24

How is the emf induced in a conductor moving through a magnetic field calculated?

Answer 24

ε = Blv

Question 25

When is an emf induced but no current?

Answer 25

When there is no external load across the wire.

Question 26

What is Lenz's Law?

Answer 26

When an emf is generated by a change in magnetic flux in accordance with Faraday's Law, the polarity of the induced emf is such that it produces a current to generate a magnetic force that opposes the change which produced the emf.

Question 27

Give four examples of Lenz's Law.

Answer 27

- Dynamo - Spinning copper disc - Aluminium plate pendulum - Magnet falling down a copper tube

Question 28

What happens to a magnetic brake if slits are cut in the disc?

Answer 28

The brake has no effect because no current can flow in the disc.

Question 29

What is Faraday's Law of electromagnetic induction?

Answer 29

ε = -N(dΦ/dt)

Question 30

A wire carrying a current is held mid-way between two poles of a magnet on a top pan balance. If the balance shows X what does the balance show if the current is doubled and reversed?

Answer 30

-2X

Question 31

Give three causes of energy losses in a transformer.

Answer 31

- Induced currents in the soft iron core - Reversal of magnetism in the soft iron core - Resistances in the coils

Question 32

For a rotating coil, at what value of magnetic flux is the emf greatest?

Answer 32

0

Question 33

For a rotating coil, when is the rate of change of magnetic flux greatest?

Answer 33

Parallel to the field

Question 34

For a rotating coil, when does the coil cut flux lines?

Answer 34

Perpendicular to the field