Magnetism

Question 1

What are the rules for drawing magnetic field lines?

Answer 1

The lines should never cross
The spacing of the lines represents the strength of the field
An arrow should be drawn to show the direction of force that a North Pole would feel

Question 2

Describe the field lines around a straight conductor

Answer 2

Circular rings

Question 3

Describe a solenoid

Answer 3

A wire wrapped into a coil

Question 4

Describe the magnetic field produced by a solenoid

Answer 4

Inside the coil its a strong and uniform field

Outside the field is similar to a bar magnet

Question 5

How can you turn a solenoid into an electromagnet?

Answer 5

Adding an iron nail

Question 6

Describe the motor effect

Answer 6

A current carrying wire produces a magnetic field field. If this wire is placed near a permanent magnet, then the two fields will either attract or repel each other, exerting a force on each other.

Question 7

Describe three ways to increase the size of the force experienced with the motor effect

Answer 7

larger current
Stronger magnets
Longer length of wire in the magnetic field

Question 8

Describe two ways the direction of the force in the motor effect can be reversed?

Answer 8

Reversing the direction of the current

Reversing the poles of the magnets

Question 9

Describe Fleming’s left hand rule

Answer 9

Thumb, first finger and second finger are placed at right angles to each other.
ThuMb - Motion
First Finger - direction of magnetic field (North to south)
seCond finger - direction of current (positive to negative)

Question 10

What are the units of magnetic flux density?

Answer 10

Tesla (T)

Question 11

Define magnetic flux density

Answer 11

The force on one meter of wire carrying a current of one amp at right angles to the field (B = F/IL)

Question 12

Define the tesla

Answer 12

1 tesla = 1 NA-1m-1

Question 13

Describe a charged particles motion when travel through a magnetic field thats at a right angle to its motion?

Answer 13

Flemings LHR shows that a particles velocity and force are always perpendicular (90°) to each other. This means that a charged particles will always move in a circular path.

The particle will have a constant speed in a magnetic field but will always be accelerating towards the center of the circle.

Question 14

Why is the frequency of rotation independent of velocity?

Answer 14

If a particles speed increases it will follow a circular path with a larger radius so the timer period and frequency remain constant

Question 15

What is a velocity selector?

Answer 15

A velocity selector uses both electric and magnetic fields to select particles at a particular velocity.
It contains two parallel plates connected to a power supply with a uniform magnetic field applied perpendicular to the magnetic field.
Particles with a range of speeds enter and are deflected in opposite ways by the electric and magnetic fields.
Only particles with a specific speed will these deflections cancel so they travel in a straight line and emerge without being deflected.

Question 16

Describe the structure of a cyclotron

Answer 16

A cyclotron is made up of two hollow semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes.
An alternating p.d. is applied between the electrodes.

Question 17

Describe how a cyclotron works

Answer 17

Charged particles enter the accelerator with a small velocity at the centre of one of the dees
A magnetic field is applied at right angles to the dees, which accelerates particles into a semi circular path until they reach the gap
An alternating voltage is applied between the dees to produce a electric field to accelerate the particles
The frequency of the electric field must reverse polarity as particles reach the gap each time.
The particles gain energy so travel faster and move in a larger semi circular path meaning they take the same time to travel each semi circle.

Question 18

What is the difference between flux density and flux?

Answer 18

Magnetic flux density, B is a measure of the strength of a magnetic field. (number of field lines per unit area)

The magnetic flux, Φ passing through an area, perpendicular to the magnetic field is define as (the total number of field lines):
Φ = BA

Question 19

Describe electromagnetic induction

Answer 19

If there is relative motion between a conductor and a magnetic field, the free electrons will experience a force (F = BQv), which causes them to move to one end of the conductor.

This induces an electromotive force (e.m.f.) across the ends of the conductor.

If the coil is part of a complete circuit then an induced current will flow through it.

Question 20

State Faradays law

Answer 20

The induced e.m.f. is directly proportional to the rate of the change of flux linkage.
Magnitude of the induced e.m.f. = rate of change of flux linkage

Question 21

State Lenz’s law

Answer 21

the direction of the induced e.m.f. or current is always such as to oppose the change producing it

Question 22

How can emf induced be found from a graph of flux linkage against time?

Answer 22

Gradient

Question 23

Why would a pendulum bob that swings through a magnetic field be heavily damped?

Answer 23

Pendulum bob cuts through magnetic field
Induced a voltage across metal bob
Currents circulate in bob (eddy currents)
Current produced a magnetic field
Current will always produce a magnetic field that apposes the motion that created it
`This is known as Lenz’s law
Magnetic fields (from current and permanent magnets) interact and dampen motion

Question 24

How could you compare a 2V dc battery and a 2V ac supply?

Answer 24

To compare them properly you need to average out the ac voltage. We can find the root mean square (rms) voltage

Question 25

Describe the structure of a step up transformer

Answer 25

An iron core with an insulated primary coil and a secondary primary coil. the secondary coil must have more turns.

Question 26

Describe the structure of a step down transformer

Answer 26

An iron core with an insulated primary coil and a secondary primary coil. the secondary coil must have less turns.

Question 27

Describe the role of a step up transformer

Answer 27

Increases the potential difference and decreases the current

Question 28

Describe the role of a step down transformer

Answer 28

Deceases the potential difference and increase the current

Question 29

Explain how a transformer works?

Answer 29

a.c. flows in primary coil
this creates an alternating magnetic field in primary coil
Magnetic field from primary coil cuts through secondary coil via the iron core
This induces a voltage and current in the secondary coil
Current in secondary coil is also a.c.

Question 30

Do transformers work with ac or dc? Why

Answer 30

only ac

Need a changing magnetic field

Question 31

Explain why transformers are not 100 % efficient.

Answer 31

The iron core is cut by magnetic flux, this induces an emf and allows eddy currents to flow. Current creates heat energy (P = I2R) and therefore energy is lost.
The eddy currents produced their own magnetic field that opposes the magnetic field that created it (Lenz’s law). This can be reduced by laminating the core.
Heat is also produced by the resistance in the coils. To reduce this use low resistance wires with a large diameter.
Energy ins needed to magnetize and demagnetize the core, to minimize this use a magnetically soft material (Iron) for the core
Not all the flux from the primary coil will pass through the secondary coil. To reduce this wind the coils on top of each other on the same part of the core