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 magnetic field pattern around a bar magnet

Answer 2

Fields lines are closer at the poles and flow from north to south

Question 3

Describe the field pattern for magnets repelling and attracting

Answer 3

See diagram

Question 4

Describe the field lines around a straight conductor

Answer 4

Circular rings

Question 5

Describe the right hand curl rule

Answer 5

Magnetic fields around a straight conductor are concentric circles.

Curl you right hand into a fist and point your thumb in the direction of the current.

Your fingers will curl in the direction of the magnetic field

Question 6

Describe a solenoid

Answer 6

A wire wrapped into a coil

Question 7

Describe the magnetic field produced by a solenoid

Answer 7

Inside the coil its a strong and uniform field

Outside the field is similar to a bar magnet

Question 8

How can you turn a solenoid into an electromagnet?

Answer 8

Adding an iron nail

Question 9

Describe the motor effect

Answer 9

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 10

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

Answer 10

1. larger current
2. Stronger magnets
3. Longer length of wire in the magnetic field

Question 11

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

Answer 11

1. Reversing the direction of the current
2. Reversing the poles of the magnets

Question 12

Describe Fleming’s left hand rule

Answer 12

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 13

What are the units of magnetic flux density?

Answer 13

Tesla (T)

Question 14

Define magnetic flux density

Answer 14

The force on one meter of wire carrying a current of one amp at right angles to the field

(B = F/IL)

Question 15

Define the tesla

Answer 15

1 tesla = 1 NA^-1m^-1

Question 16

Describe how the motor effect causes a motor to rotate

Answer 16

Current flows into the coil and creates a magentic field.

This magnetic field interacts with the permanent magnetic field creating a force.

The current flows out the coil. As the current has changed directions the force chages direction.

As each side of the motor has a force in opposite directions it rotates.

Question 17

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

Answer 17

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 centre of the circle.

Question 18

Why is the frequency of rotation independent of velocity?

Answer 18

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

Question 19

What is a velocity selector?

Answer 19

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 20

Describe the structure of a cyclotron

Answer 20

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 21

Describe how a cyclotron works

Answer 21

1. Charged particles enter the accelerator with a small velocity at the centre of one of the dees
2. A magnetic field is applied at right angles to the dees, which accelerates particles into a semi circular path until they reach the gap
3. An alternating voltage is applied between the dees to produce an electric field to accelerate the particles
4. The frequency of the electric field must reverse polarity as particles reach the gap each time.
5. 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 22

What is the difference between flux density and flux?

Answer 22

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 23

Describe electromagnetic induction

Answer 23

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 24

What is required for magnetic induction?

Answer 24

1. A conductor
2. Movement
3. A magnetic field
4. A complete circuit

Question 25

How can you calcualte the induced e.m.f. in a straight conductor moving through a magnetic field?

Answer 25

e.m.f. = Blv

Question 26

State Faradays law

Answer 26

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 27

State Lenz's law

Answer 27

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

Question 28

Sketch a graph of flux linkage against time for a coil rotating in a magnetic field.

Answer 28

Maximum flux linkage when the coil is perpendicular to the magnetic field Zero flux linkage when the coil is parallel to the magnetic field see diagram

Question 29

Sketch a graph of induced emf against time for a coil rotating in a magnetic field.

Answer 29

Zero emf when the coil is perpendicualr to the magnetic field. Maximum emf when the coil is paralle to the magnetic field

Question 30

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

Answer 30

Gradient

Question 31

If a coil that was rotating in a magentic field had its speed doubled how would a) flux linkage against time graph change b) emf against time graph change

Answer 31

a) flux linkage against time graph - flux linkage stays the same frequency doubles b) emf against time graph - emf doubles frequency doubles

Question 32

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

Answer 32

1. Pendulum bob cuts through magnetic field which induces a voltage across metal bob 2. Currents circulate in bob (eddy currents) 3. Current produced a magnetic field 4. Current will always produce a magnetic field that apposes the motion that created it `This is known as Lenz’s law 5. Magnetic fields (from current and permanent magnets) interact and dampen motion

Question 33

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

Answer 33

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

Question 34

Describe the structure of a step up transformer

Answer 34

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

Question 35

Describe the structure of a step down transformer

Answer 35

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

Question 36

Describe the role of a step up transformer

Answer 36

Increases the potential difference and decreases the current

Question 37

Describe the role of a step down transformer

Answer 37

Deceases the potential difference and increase the current

Question 38

Explain how a transformer works?

Answer 38

1. a.c. flows in primary coil this creates an alternating magnetic field in primary coil 2. Magnetic field from primary coil cuts through secondary coil via the iron core 3. This induces a voltage and current in the secondary coil 4. Current in secondary coil is also a.c.

Question 39

Do transformers work with ac or dc? Why

Answer 39

only ac Need a changing magnetic field

Question 40

Explain why transformers are not 100 % efficient.

Answer 40

1. 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. 2. 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. 3. Heat is also produced by the resistance in the coils. To reduce this use low resistance wires with a large diameter. 4. Energy is needed to magnetize and demagnetize the core, to minimize this use a magnetically soft material (Iron) for the core 5. Not all the flux from the primary coil will pass through the secondary coil. To reduce this wrap the coils on top of each other on the same part of the core