Magnetic Fields

Question 1

What is a magnetic field?

Answer 1

A region in which a force will act on magnetic materials or induced magnets

Question 2

What are field lines?

Answer 2

• A way of representing a force field
• Called flux lines in a magnetic field

Question 3

Which way do field lines travel for bar magnets?

Answer 3

From north to south

Question 4

Describe the diagram for two north magnets facing each other

Answer 4

• Lines curving away from each other
• Arrows pointing outward

Question 5

What happens when a current flows through a wire?

Answer 5

• A magnetic field is induced around the wire
• The field lines are concentric circles centred around the wire

Question 6

How is the direction of the magnetic field induced by a wire identified?

Answer 6

By using the right hand rule

Question 7

What happens when a current carrying wire is put into a external magnetic field?

Answer 7

• The field from the wire and magnets combine, causing a resultant field.
• Where the field is stronger, the lines are closer together
• These bunched up lines cause a resulting force on the wire, perpendicular to the direction of the magnetic field

Question 8

How is the direction of the force identified?

Answer 8

• Fleming’s left hand rule
• Upward: Force
• Forward: External magnetic field
• Right: Current (Positive to negative)

Question 9

Define magnetic flux density

Answer 9

The force on one metre of wire carrying a current of one amp, perpendicular to the magnetic field

Question 10

Derive the equation for the circular path of particles

Answer 10

• F = mv²/r = BQv
• mv²/r = BQv
• mv = BQr
• r = mv/BQ

Question 11

State the equation, which includes v, for the frequency of an object in circular motion

Answer 11

f = v/2πr

Question 12

Derive State the equation for frequency of rotation in terms of B, Q and m.

Answer 12

• f = v/2πr and r = mv/BQ
• f = v/2π(mv/BQ)
• f = 1/2π(m/BQ)
• f = BQ/2πm

Question 13

State one application of circular motion of particles

Answer 13

Particle accelerators, i.e cyclotrons

Question 14

Describe how a cyclotron works

Answer 14

• Charged particles are produced and fired into one of the two hollow semicircle electrodes with a uniform magnetic field
• The uniform magnetic field is applied perpendicular to the plane of the electrodes which have an alternating p.d between them
• Charged particles are accelerated in a circular path by an applied p.d in the electrodes
• As particles speeds up, path radius is bigger until it exits the cyclotron at high speed

Question 15

Define magnetic flux

Answer 15

The number of field lines passing through an area, in Webers

Question 16

How is an electromotive force induced?

Answer 16

When there’s relative motion between a conducting rod and a magnetic field

Question 17

Define electromotive force

Answer 17

• Energy transferred from chemical energy to electrical energy in a power supply
• Electrical energy per unit charge

Question 18

State Faraday’s Law

Answer 18

Induced e.m.f is directly proportional to rate of change of flux linkage

Question 19

State Lenz’s Law

Answer 19

The induced e.m.f is always in such a direction as to oppose the change that caused it

Question 20

What do generators do?

Answer 20

• Convert kinetic energy into electric energy
• They induce an electric current by rotating a coil in a magnetic field

Question 21

State the equation for average power

Answer 21

Average power = I(rms) x V(rms)

Question 22

What is the value for UK mains electric supply?

Answer 22

230V(rms)

Question 23

What is a transformer?

Answer 23

Devices which make use of electromagnetic induction to change the size of the voltage for an alternating current

Question 24

Describe how a transformer works

Answer 24

• An alternating current flowing in the primary coil causes the core to magnetise, demagnetise and re-magnetise continuously in opposite directions
• This produces a rapidly changing magnetic flux across the core which is why a magnetically soft material is needed(iron or special alloy)
• Rapidly changing flux in iron core passes through secondary coil
• This induces an alternating voltage of the same frequency

Question 25

State the 2 types of transformers and their roles

Answer 25

- Step up transformers: Increase the voltage by having more turns on the secondary coil than primary coil - Step down transformers: Decrease the voltage by having more turns on the primary coil than secondary coil

Question 26

State the factors which cause a transformer to have a decrease in efficiency

Answer 26

- Eddy currents - Heat by resistance in the coils - Heat by magnetising and demagnetising the core - Magnetic flux loss over distance

Question 27

What are eddy currents?

Answer 27

Looping currents induced by the changing magnetic flux in the core

Question 28

How do eddy currents decrease efficiency in a transformer?

Answer 28

- The rapidly changing flux cuts the metallic core, inducing an e.m.f - This e.m.f induces looping currents in the core - The currents create a magnetic field which opposes the field which induced them, reducing field strength - They also dissipate energy by generating heat

Question 29

How can a transformer be adapted to reduce the effect of eddy currents?

Answer 29

By having layers of the core separated out by a thin insulator, so a current can't flow (laminating the core)

Question 30

How can a transformer be adapted to reduce energy lost by heat as resistance in the coils?

Answer 30

Use wires with low resistance, such as thick copper wire which has a low resistivity and a larger diameter meaning smaller resistance

Question 31

How can a transformer be adapted to reduce the energy lost as heat by magnetising and demagnetise the core?

Answer 31

By using a magnetically soft material which magnetises and demagnetises easily, such as iron

Question 32

How can a transformer be adapted to reduce the magnetic loss from the primary coil to the secondary coil?

Answer 32

By designing the core so that the coils are as close as possible

Question 33

For the national grid, state the equation for the power losses due to resistance of the cables

Answer 33

P = I²R

Question 34

Define Terminal Potential Difference

Answer 34

- Energy transferred from electrical energy to another type - Work done per unit charge