Magnetic Fields

Question 1

In the Formula F=BIL, what does each letter stand for

Answer 1

F=Force (N)
B=Flux Density (Tesla)
I=Current (A)
L=Length (M)

Question 2

Define Magnetic Flux Density

Answer 2

The force per unit current per unit length on a wire placed at 90 degrees to the direction of the magnetic field

Question 3

In Flemming Left Hand Rule, what does each finger represent

Answer 3

Thumb= Force
First= Direction of Magnetic Field
Second=Conventional Current

Question 4

When would you use Flemmings Left Hand Rule

Answer 4

Motors. If a wire is carrying an electric current, electrons are moving along the wire. If the wire is placed in a magnetic field, the force exerted by the field on the moving electrons can move the wire.

Question 5

Define the Tesla

Answer 5

A unit of magnetic flux density, 1T is the magnetic flux density when 1m of wire carrying 1A of current at right angles to a magnetic field experiences a force of 1N

Question 6

In the formula F=BQv what does each letter stand for

Answer 6

F=Force
B= Magnetic Flux Density
Q=Charge of Particle
V=Speed

Question 7

Name the 3 particle accelerators that use magnetic fields

Answer 7

Cyclotron, Line Arc and Synchrotron

Question 8

How does a Cyclotron Work

Answer 8

• They use circular deflection between 2 hollow dees
• An alternating electric field is maintained across the gap between the 2 dees by an alternating applied pd
• The direction of the electric field between the dees needs to be in the direction that the charged particle is moving in order to accelerate it.
• When it returns to the dee it will be travelling in the opposite direction and therefore the electric field needs to be in the opposite direction. This is achieved by reversing the polarity of the applied pd
• A constant uniform magnetic field acts throughout the whole of the accelerator and provides the centripetal force

Question 9

What is the unit for Magnetic Flux and Magnetic Flux Density

Answer 9

Magnetic Flux= Weber (Wb)

Magnetic Flux Density= Tesla (T)

Question 10

Difference between magnetic flux and magnetic flux density

Answer 10

The amount of magnetic flux a magnet displays is a direct result of the material that makes up the magnet. The magnetic flux density is a product of the magnetic flux and the area that this flux is present within.

Question 11

Define Magnetic Flux Linkage

Answer 11

The product of the magnetic flux passing through a conducting coil and the number of turns of wire on the coil

Question 12

State Faraday’s Law

Answer 12

The induced emf is directly proportional to the rate of change of flux linkage

Question 13

State Lenz’s Law

Answer 13

The direction of an induced current opposes the change of magnetic flux that produces it

Question 14

What is the magnitude of the induced EMF equal to

Answer 14

The rate of change of flux linkage

Question 15

What is the frequency and rms voltage of mains electricity

Answer 15

Frequency=50Hz and V=230

Question 16

Define root mean squared

Answer 16

equivalent DC voltage that produces the same heating effect

Question 17

Whats the difference between Peak voltage and peak-to-peak voltage

Answer 17

Peak=Distance from equilibrium to Peak

Peak-to-Peak=Distance between peak and trough

Question 18

If the line on an oscilloscope is horizontal what are the 3 potential reasons

Answer 18

1. It is a direct voltage
2. The voltage gain is zero
3. No signal

Question 19

If the line on an oscilloscope is vertical what is the potential reason

Answer 19

The time base is very small

Question 20

On an oscilloscope what does the horizontal and vertical height represent

Answer 20

Vertical Height= the input voltage

Width=Time base

Question 21

What does a transformer do

Answer 21

They change the size of the voltage for an alternating current

Question 22

How does a transformer work

Answer 22

1. An alternating voltage is applied to the primary coil, causing an alternating current to flow
2. The current creates an alternating magnetic field
3. This creates a changing magnetic flux in the secondary coil
4. The changing magnetic flux induces a change in the rate of change of flux linkage
5. This results in an emf being produced

Question 23

What are eddy currents

Answer 23

They are looping currents induced by the changing magnetic flux in the core, they create a magnetic field that acts against the field that induced them

Question 24

What are the 3 causes of inefficiencies in transformers

Answer 24

1. Power is wasted due to heating effect of the current in the coils due to a resistance
2. Eddy currents which dissipate energy due to heating, they also produce magnetic flux which acts to reduce the overall flux in the core
3. Energy lost due to work done repeatedly magnetising and demagnetising the core

Question 25

How do you reduce the power wasted due to resistance in the wires in transformers

Answer 25

Use thick copper wires as they have less resistance

Question 26

How do you reduce the energy lost due to eddy currents in transformers

Answer 26

Laminate the core with layers of insulation

Question 27

How do you reduce the energy lost due to work done magnetising and demagnetising the core in a transformer

Answer 27

Use a soft iron core as it is easy to change the magnetic polarity of it

Question 28

Why do the National Grid use transformers ( why do they transmit at high voltages)

Answer 28

They use they to decrease and increase voltages, because transmission over long distances is more efficient at high voltages, because higher voltages mean lower currents, which means less heating. However it has to be stepped down for recreational consumption