Magnetic Fields (General)

Question 1

Definition of magnetic flux density and Tesla

Exam definition

Answer 1

Measure of strength of the field

• Force on one metre of wire carrying a current of one amp at right angles to a magnetic field
• Unit = Tesla
• Tesla = One newton per amp per metre

Question 2

Equation for finding the force exerted on a wire when inducing a magnetic field

Answer 2

F = BIL

Question 3

Explain the movement of electric current in respect to magnetic fields

Answer 3

When charges move they create both magnetic and electric fields
hence magnetic is due to moving charge and electric is due to static charge

These charged particles experience a force due to the fields == which allows them to move along wire

Question 4

Equation for finding force exerted on charged particles in a magnetic field

Answer 4

F = Bev

Question 5

Describe how the direction of force can contribute to the motion of a charged particle

Answer 5

The force exerted on a moving charged particle is ALWAYS perpendicular to its motion FOR MAGNETIC FIELDS

Hence the force acts a centripetal force thus the particles experience circular motion

Question 6

Explain how you can find an equation to give the radius of the motion of a charged particle

Answer 6

Force for charged particle can be equated to centripetal force equation

Giving:

r = mv/BQ

Question 7

What is a cyclotron

Answer 7

A type of particle accelerator that is used to accelerate particles at high speeds/energies for use in medical applications

Question 8

Describe the structure of a cyclotron (including fields)

Answer 8

Cyclotron consists of two semi circular electrodes (dee’s) (with a magnetic field) separated by a gap

Between this gap a high frequency alternating voltage is applied (creating an electric field between the dee’s)

Question 9

Describe how a charged particle can be accelerated in a cyclotron

Answer 9

A charge particle moves from the centre of the gap to the electrode of which it is attracted due to its charge (hence due to electromagnetic force)

It is accelerated to the electrode since it is experiencing a force in the same direction as its motion when in the electric field (of which is applied due to the voltage)

Once it enters the electrode, it now experiences a magnetic field where the force is perpendicular to its motion (hence it moves in a circular path) and does not increase in speed

The particle then leaves the electrode, at which time the alternating voltage has switched directions, meaning the particle is now attracted to the opposite electrode

Hence the process is repeated

Question 10

Explain how and why the radius of the particles motion in a cyclotron changes

Answer 10

The radius of the circular path increases since the velocity increases every time the particle experiences the electric field

This is due to the force being in the same direction as the motion of said particle

Using the equation r = mv/BQ if all other components are constant then radius is proportional to velocity hence it increases as such

Question 11

What is meant by magnetic flux

Answer 11

A value that describes the magnetic field (or magnetic field lines) passing a through a given area

Represented by phi == Φ

Question 12

Equation for magnetic flux

Answer 12

BA

Magnetic flux density x area

Question 13

What is meant by magnetic flux linkage and how can it be calculated

Answer 13

Denoted as == NΦ
It is a measure of the magnetic flux passing through a coil

Which can be calculated from BAN if perpendicular

Magnetic flux x area x number of turns

Question 14

Equation for finding magnetic flux linkage when magnetic field is not perpendicular to coil

Answer 14

NΦ = BAN cos θ

If in doubt you resolve to find component perp

Question 15

Explain how electromagnetic induction occurs

Answer 15

When a conducting rod moves relative to a magnetic field, the electrons in the rod experience a force (as they are charged particles)

This force causes charge to build up on one side of the rod
Creating an induced emf

(only occurs if you move a bar magnetic relative to a coil wire)

Question 16

What is meant by Faraday’s law

Answer 16

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

Question 17

What is meant by Lenz’s law

Answer 17

States that the direction of induced current is such as to oppose the motion causing it

Essentially the induced current will be in the direction that causes its magnetic poles to repel to magnetic poles of the magnetic object

N to N for example

Question 18

What is meant by motor effect and dynamo effect and when does flemings rules apply?

Answer 18

Motor effect == Current creates a force
(Use fleming left hand rule)

Dynamo effect == Force creates a current (emf induction)
(Use fleming left hand rule BUT WITH RIGHT HAND)

Question 19

Explain how a change in flux linkage induces an emf

Answer 19

A change in magnetic flux induces an electromotive force (EMF) according to Faraday’s law
ε=−dΦ / dt

When there is a change in either area or field strength, the magnetic flux through the conductor changes (hence moving positions will change field strength)

When a change occurs, an electric field is induced which drives free electrons hence inducing an emf

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Question 20

For a magnet falling down a hollow conductor explain why its acceleration is zero

(may be slightly more advance than spec but helps understanding)

Answer 20

When a magnet interacts with a wire (hollow conductor) and emf is induced (due to a change in magnetic flux linkage following Faradays law)

At points where the magnetic field of the magnet and the motion of the magnet are perpendicular == There will be a circular electric field with a current (hence eddy currents) (this is the emf induced btw)

These circular currents act like solenoids and produce their own magnetic field

Due to Lenz’s law the magnetic field (poles) they produce must oppose the motion of the magnet (North pole near a north pole ect)

Due to the repulsion felt by this, the magnet reaches terminal velocity hence has zero acceleration

Question 21

How can you work out the direction of the current (electric field) produced when a magnet falls through hollow conductor

Answer 21

Use the North South rule

If the magnetic field of the current induced is North at top then anticlockwise

If the magnetic field at the current induced is South on top then clockwise

(To know if its S or N just do the pole which will cause repulsion to the falling magnet)

Question 22

Explain what happens when a magnet falling through a hollow conductor exits

Answer 22

Technique due to the poles the magnet should repel at the bottom and shoot out

However == it always wants what it cant have like a shower

However we need attraction due to Lenz’s law (the poles must be opposite essentially)

Therefore the current and poles flip half way

Question 23

Describe the graph of Flux linkage against time

Answer 23

Straight line through origin (Proportional)

Emf is the gradient

Question 24

Describe graph of emf against time

Answer 24

Constant horizontal line

Area underneath = Change in flux linkage

Question 25

The equation for emf that is induced when a conductor moves through a magnetic field

Answer 25

e = Blv

B = magnetic field strength
L = length of conductor
V = speed of conductor

Question 26

Describe how the induced emf varies with a rotating coil (hence why alternating voltage emf?)

Answer 26

When the conductor is perpendicular to the magnetic field lines == Minimum emf
(When normal is parallel hence theta is zero)

When the conductor is parallel to the magnetic field lines == Maximum emf
(When normal is perpendicular hence theta is 90)

Question 27

Explain how the magnetic flux linkage varies with a rotating coil

Answer 27

Opposite to the emf

Perpendicular = max, parallel = min

Question 28

Equation for theta (for rotating coil calculations)

Answer 28

wt

Angular speed x time

Question 29

Equation for magnetic flux linkage (for rotating coil calculations)

Answer 29

NI = BAN cos (wt)

Question 30

Equation for emf (for rotating coil calculations)

Answer 30

e = BAN w sin (wt)

Question 31

Describe graph of emf against time for rotating coil

Answer 31

Sinusoidal starting at origin

Question 32

Describe graph of magnetic flux linkage against time for rotating coil

Answer 32

Sinusoidal starting at 1

Question 33

What type of wire always induced a magnetic field when current flows

Answer 33

Coil / solenoid ALWAYS generates magnetic field

Question 34

What is meant by an alternating current in terms of electrons

Answer 34

AC current periodically varies between positive to negative value with time

Therefore the electrons in an AC current are moving back and forth with simple harmonic motion

Question 35

How do you find the peak voltage/current

Answer 35

Take a vertical peak to peak reading

Divide by 2

Question 36

How do you convert AC current and voltage values into their DC equivalent

Answer 36

Use root mean square

Peak / Root 2 = root mean square value

Question 37

How do diodes work in terms of electrons

Answer 37

The diode is split into a negative and positive side

The negative side contains electrons and the positive side contains holes
The electrons can jump into the wholes
But they cannot jump back

therefore current flows in on direction