Electromagnetism

Question 1

What is magnetism? What causes it?

Answer 1

The movement of electric charges.

A moving charge is surrounded by a magnetic field. Current flowing in a wire is a cause of a magnetic field. Electrons orbiting and spinning around nucleus and the medium surrounding the moving charges cause magnetism.

Question 2

Explain how permanent magnets work? Talk about poles.

Answer 2

Non-magnetic material-the magnetism caused by different electrons cancel each other out.

Permanent magnets-have no cancellation as the magnetic fields due to moving electrons align themselves and form magnetic domains in the same direction. Permanent magnets have two poles that exist in pairs, a north and a south. If the magnet is broken in half, each part will have a north and south..

Question 3

Explain Earth’s magnetic field.

Answer 3

Earth’s magnetic field is due to currents in the molten lava within the Earth. It resembles a large bar magnet with the North Pole in the Southern Hemisphere and the South Pole in the Northern Hemisphere.

Question 4

Which type of pole does a current flowing clockwise give?

Answer 4

South Pole is clockwise.
North Pole is anticlockwise

Question 5

What is the direction of magnetic field lines?

Answer 5

Always go from North Pole to South Pole.

Question 6

Explain the right hand thumb rule stating first what it is used for?

Answer 6

Used to the find the direction of magnetic field lines using direction of conventional current.

Thumb of right hand represents direction of convention current and fingers represent direction of magnetic field lines.

Question 7

Define the force in a magnetic field. Give the formula when not perpendicular.

Answer 7

F = ILB (I=current, L=length of conductor in magnetic field, F=Force, B=magnetic flux density)

F=ILB sin (pieta)

Question 8

Define magnetic Flux Density.

Answer 8

B = F / IL (B=magnetic flux density, F=Force on a conductor, I=current, L=Length of conductor in magnetic field.)

Question 9

Define the unit of the Tesla.

Answer 9

1 Tesla = 1 Newton / 1 Ampere x 1 metre

Question 10

Explain Fleming’s Left Hand Rule and first what you use it for.

Answer 10

The direction of the force can be obtained with direction of magnetic field and current.

Thumb represents direction of force, index finger of magnetic field and middle finger represent current.

Question 11

Explain why a charged particle moving in a magnetic field moves in circular motion.

Answer 11

The force is perpendicular to the velocity and the flux density. If the force is centripetal force than this is the same principle as circular motion where objects doesn’t change magnitude of velocity but still accelerates

Question 12

What is the formula for the force on a single charge in a magnetic field?

Answer 12

F = qvB (F=Force, q=charge, v=velocity, B=magnetic flux density)

Question 13

Differentiate between the force between two current carrying conductor when the batteries are in the (a) same direction and (b) opposite direction.

Answer 13

Current flow in the same (opposite) direction in two parallel conductors.
The magnetic fields cancel (add-up) between the two conductors as the directions of the magnetic fields are the opposite (the same)
There is now a weak (strong) magnetic field between the conductors.
Magnetic fields on the outer side of the conductors are stronger (weaker)

The stronger magnetic fields push the conductors into the regions of the weaker magnetic fields. So in (a) the conductors attract each other and in (b) the conductors repel each other.

Question 14

Explain how a motor works.

Answer 14

The left hand side of the coil is forced inwards and the right hand side of the coil is forced outward. These two forces cause the coil to turn in circular motion.
As the magnetic field is uniform these two forces are equal in magnitude, opposite in direction.

Question 15

Give the formula for moment of a force in a motor. Give the formula for moment of a couple in a motor

Answer 15

M = ILB x the perpendicular distance between wire an axis (radius of inner cylinder)
T = ILB x the perpendicular distance between two wires (diameter of inner cylinder)

Question 16

Define electromagnetic induction.

Answer 16

Is when a change in magnetic flux induces an emf.

Question 17

Define magnetic Flux.

Answer 17

Ø (phi) = BA

Magnetic Flux = Magnetic Flux Density x Area

Question 18

Define the unit of electromagnetic induction.

Answer 18

1 Weber = 1 Tesla x 1m2

Question 19

State Faraday’s Law.

Answer 19

If the magnetic flux lines passing through a circuit change than an emf is induced proportional to the rate of change of magnetic flux passing through the circuit.

Question 20

Formula for emf/Faraday’s Law.

Answer 20

emf = -N(dØ/dt)

(Negative sign has no purpose in maths Q’s)

Question 21

State Lenz’s Law.

Answer 21

The induced emf makes a current flow in such a direction as to oppose the change that caused it.

Question 22

What is the frequency of the mains?

Answer 22

50 hertz

Question 23

What does RMS stand for? What is the formula for RMS voltage?

Answer 23

Root mean Squared.

V rms = V0 / root 2
(RMS Voltage = Peak Voltage over root 2)

Question 24

What type of induction do transformers use? Differentiate between step-up and step-down transformer.

Answer 24

Mutual induction.

A large number of turns of wire in the secondary circuit compared to primary will give greater voltage in step-up transformer.
A smaller number of turns of wire in the secondary circuit compared to primary will give a weaker voltage in step-down transformer.

Question 25

Give the formula for transformers,

Answer 25

V(i) / V(o) = N(p) / N(s) = I(o) / I(i)

Question 26

What are eddy currents?

Answer 26

Eddy currents generate heat in iron core. This heat loss is reduced by laminating the iron core. Laminated iron gets looses and release energy in the form of a hum instead.

Question 27

Give two uses of transformers.

Answer 27

Mobile phone rechargers
Old cathode ray tube T.V. sets

Question 28

Explain how self induction works with an a.c. source.

Answer 28

In a.c. current is constantly changing direction which gives changing magnetic flux. This induces a emf in the solenoid that opposes the voltage of the source. Lowering the resultant voltage.

Question 29

Formula for resultant voltage and involving back emf.

Answer 29

Resultant Voltage = source voltage - back emf

Question 30

What effect on self induction of an a.c. source do the following factors have? (a) frequency (b) number of turns (c) iron core.

Answer 30

Self induced emf is greater
Resultant voltage is less
Resultant current is less
The brightness of the lamp decreases

Question 31

Give an application of the principle of induction.

Answer 31

A dimmer switch

Question 32

Explain how self-induction works with a d.c. source.

Answer 32

As switch is initially closed it takes a very small interval of time for the current to reach its constant value. Change in magnetic flux induces emf which due to the iron core is big enough to light bulb briefly. Same occurs when switch is opened.

Question 33

Define magnetic field.

Answer 33

Is the region where a magnetic force acts.

Question 34

What is the effect on sound produced with increased number of turns in coil?

Answer 34

Louder sound with greater amplitude.