Magnetic Fields (Unit 4)

Question 1

Definition of the Tesla

Answer 1

1 Tesla is defined as the magnetic field which, acting on a wire carrying 1 A and placed perpendicular to the magnetic field lines, would exert a magnetic force of 1 N on each 1m length of the wire

Question 2

Use Flemings Left Hand rule

Answer 2

First finger: magnetic Field (points from North to South)
SeCond finger: conventional Current (points from + to -)
ThuMb : Motion (direction of force)

Question 3

Path of charged particle in a uniform magnetic field

Answer 3

Path is circular because,

Magnitude of force is constant but direction always points towards the centre of a circle

Question 4

Magnetic force on a charged particle

Answer 4

There is no magnetic force on a charged particle if it is

1. stationary
2. moving ALONG a magnetic field line

Question 5

Operation of a cyclotron

Answer 5

Two hollow D-shaped electrodes (Dees)
Uniform magnetic field applied perpendicularly to plane of Dees
Charged particles introduced into Dees near centre of cyclotron
Particles move in circular path due to the magnetic field
Alternating voltage applied across gap between Dees
As charged particle emerges from Dee it is accelerated across gap
Voltage reverses while charged particle moves in circular path through the other Dee.
Particle is again accelerated across gap.
The time taken for particle to move along a circular path through one of the Dees does not depend upon its speed (this time remains constant as particle speeds up)

Question 6

Magnetic flux, units

Answer 6

Wb (Webers) OR Tm2 (Tesla metres squared) VECTOR

include symbol

Question 7

Magnetic flux density, B, definition

Answer 7

the number of lines of magnetic flux passing perpendicularly through a cross-sectional area of 1m2.

Question 8

Magnetic flux density units

Answer 8

T (Tesla) or Wbm-2 (Weber’s per metres squared) VECTOR

Question 9

Magnetic flux linkage, definition

Answer 9

the product of the number of turns in a coil and the magnetic flux passing perpendicularly through the coil
(include symbol)

Question 10

Magnetic flux linkage units

Answer 10

Wb turns (Weber turns) VECTOR

Question 11

Faraday’s Law

Answer 11

the magnitude of the induced emf is equal to the rate of change of flux linkage

Question 12

Induced emf units

Answer 12

V (Volts) or Wb s-1 (Weber per second) SCALAR

Question 13

Lenz’s Law

Answer 13

the direction of the induced emf is always such as to oppose the change creating it

Question 14

emf induced in a rotating coil

Answer 14

when coil rotates in a uniform magnetic field, an emf is induced because there is a constantly changing flux linkage through the coil.

Question 15

Magnitude of induced emf

Answer 15

the gradient of a graph of flux linkage against time gives the magnitude of the induced emf

Question 16

Relationship between coil position, flux linkage and induced emf

Answer 16

See sheet

Question 17

Flux linkage and induced emf graph in a rotating coil

Answer 17

See sheet

Question 18

Moving straight conductor in a uniform magnetic field

Answer 18

See sheet

Question 19

Explanation of how a transformer works

Answer 19

• a changing current in primary coil
• produces a changing flux in the transformer core
• producing a changing flux linkage in the secondary coil
• which from Faraday’s Law
• induces an emf in the secondary coil

Question 20

Operation of a transformer

Answer 20

Transformers only work with alternating current. Direct current does not produce a changing magnetic field in primary coil.

Question 21

Step up transformer

Answer 21

Increases voltage (decreases current)
more turns on secondary than on primary

Question 22

Step down transformer

Answer 22

Decreases voltage (increases current)
less turns on secondary than on primary

Question 23

Causes of transformer inefficiencies

Answer 23

See table on sheet

Question 24

Transmission of electrical power at high voltages

Answer 24

Using high voltages reduces current, and hence power loss through heating ( P = I2R). Resistance of wire is unchanged.