Magnetic Fields

Question 1

How is a magnetic field created

Answer 1

By moving electric charge
Permanent magnets

Question 2

Define magnetic flux density, B, of a field

Answer 2

The number of magnetic flux lines passing through a region of space per unit area

Question 3

What is the unit of magnetic flux density

Answer 3

Tesla

Question 4

Define 1 tesla

Answer 4

The flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux

Question 5

What happens to the strength of the magnetic field if flux density is higher

Answer 5

Stronger magnetic field

Question 6

What do flux line represent

Answer 6

Direction and magnitude of a magnetic field

Question 7

What direction are flux lines in a magnetic field

Answer 7

North pole to south pole

Question 8

Equation for the force on a current-carrying conductor in a B field

Answer 8

F = BIL x sin theta

where F is the force
B = magnetic flux density of applied B field
I = current in conductor
L = length of conductor
theta = angle between conductor and B field

Question 9

When will a current carrying conductor such as a wire experience max magnetic force

Answer 9

If the current through it is perpendicular to the direction of the magnetic field lines

Question 10

When will a current carrying conductor such as a wire experience no magnetic force

Answer 10

When current is parallel to the magnetic field lines

Question 11

At what angle is the force on a current carrying wire in a B field greatest

Answer 11

90 degrees as sin theta = 1

Question 12

What is the direction of current flow

Answer 12

The flow of positive charge so from positive to negative.
This is in the opposite direction to the flow of electrons

Question 13

Describe Fleming’s left hand rule

Answer 13

Point the thumb, first finger and second finger at right angles to each other
Thumb points in direction of force
First finger points in direction of B field
Second finger points in direction of current flow (+ to -)

Question 14

What does a dot represent in magnetic fields

Answer 14

Magnetic field is directed out of the plane of the page

Question 15

What does a cross represent in magnetic fields

Answer 15

The field is directed into the plane of the page

Question 16

What does the direction of the magnetic force on a charged particle depend on

Answer 16

Direction of flow of current
Direction of the magnetic field

Question 17

Describe and explain the path of a charged particle when it enters a uniform magnetic field

Answer 17

It travels in a circular path as the direction of the force is perpendicular to the velocity and directed towards centre of the path

Question 18

What does a cyclotron do

Answer 18

Makes used of the circular trajectory of charged particles in a magnetic field to create a spiral path.
It accelerates charged particles such as protons to very high speeds

Question 19

Medical applications of cyclotrons

Answer 19

Produce tracers for imaging
Create high energy beams of radiation for radiotherapy

Question 20

What does a cyclotron consist of

Answer 20

2 hollow semicircular electrodes/ dees
Strong uniform magnetic field
Alternating electric field

Question 21

Describe how particles moves in a circular path inside the dee

Answer 21

1. There is a source of charged particles at the centre of the cyclotron.
2. A B field is applied perpendicular to the path of particles.
3. Magnetic force acts perpendicular to the field and path of particles.
4. This causes particles to move in circular path

Question 22

Describe how particles cross the gap between dees

Answer 22

There is a gap between each semi-circular dee.
An electric field is applied in the gaps between the dees.
An electric force acts on the charged particle, causing it to accelerate.
This causes them to enter next dee at higher speed and follow a path with a larger radius

Question 23

Describe how particles leave the dee

Answer 23

The charged particles travel with constant speed in the dees.
Time spent in one dee is constant.
Direction of e field alternates each time particles reach a gap.
This means they always accelerate to the opposite dee.
Process repeats as particles spiral outwards till they leave

Question 24

What is the role of the uniform magnetic field in a cyclotron

Answer 24

To supply the centripetal force needed to keep the particles moving in a circular motion

Question 25

What is the role of the alternating electric field in a cyclotron

Answer 25

To accelerate the particles between the dees

Question 26

What is electromagnetic induction

Answer 26

Process in which an emf is induced in a closed circuit due to changes in magnetic flux

Question 27

When does electromagnetic induction occur

Answer 27

When a conductor cuts through magnetic field lines

Question 28

Define magnetic flux

Answer 28

The product of the magnetic flux density and the cross sectional area perpendicular to the direction of the magnetic flux density

Question 29

Units for magnetic flux

Answer 29

Webers (Wb)

Question 30

Equation for magnetic flux

Answer 30

Φ = BA where Φ is magnetic flux B is magnetic flux density A is cross sectional area

Question 31

When is magnetic flux maximum

Answer 31

When magnetic field lines and the area they are travelling through are perpendicular

Question 32

What is the relationship between coils in a wire and emf induced

Answer 32

More coils in a wire = larger emf induced

Question 33

Define flux linkage

Answer 33

The product of the magnetic flux and the number of turns on the coil

Question 34

Equation for magnetic flux linkage

Answer 34

magnetic flux linkage = ΦN = BANcos theta

Question 35

Units for magnetic flux linkage

Answer 35

Wb turns

Question 36

What is the voltmeter reading when the bar magnet is not moving through a coil

Answer 36

Voltmeter shows a zero reading. When bar magnet held still, the rate of change of flux is zero so no emf induced

Question 37

What is the voltmeter reading when a bar magnet moves inside a coil connected to a voltmeter

Answer 37

As the bar magnet moves, its magnetic field lines cut through the coil, generating a change in magnetic flux. This induces a emf within the coil and so there is a reading on the voltmeter

Question 38

What happens to the voltmeter reading when a bar magnet is taken back out of a coil connected to a voltmeter

Answer 38

Emf induced in opposite direction. As the bar magnet changes direction the direction of current chages. Voltmeter shows reading with the opposite sign.

Question 39

What if the effect of increasing the speed of the magnet through a coil

Answer 39

Emf induced with a greater magnitude. As the speed of the magnetic increases, the rate of change of flux increases

Question 40

What is the direction of the electric current and emf induced in the conductor

Answer 40

Such that it opposes the change that produces it

Question 41

What factors increase induced emf when moving a bar magnet through a coil

Answer 41

Moving the magnet faster through the coil Adding more turns to the coil Increasing the strength of the bar magnet

Question 42

Faradays Law

Answer 42

The magnitude of the induced emf is directly proportional to the rate of change in magnetic flux linkage.

Question 43

Lenz's Law

Answer 43

The induced emf acts in such a direction to produce effects that oppose the change causing it.

Question 44

Equation for magnitude of induced emf

Answer 44

𝜀 = 𝑁 x (ΔФ) / Δt where e is induced emf N is the number of turns of coil ΔФ is the change in magnetic flux t is time interval

Question 45

What does the gradient of a graph of magnetic flux against time represent

Answer 45

EMF

Question 46

What does this equation : 𝜀 = -𝑁 x (ΔФ) / Δt show

Answer 46

When a bar magnet goes through a coil, an emf is induced within the coil due to a change in magnetic flux. A current is also induced which means the coil now has its own magnetic field. The coil's magnetic field acts in the opposite direction to the magnetic field of bar magnet which is shown by the negative sign

Question 47

Equation for the induced emf in a conductor as it moves through the magnetic field involving velocity

Answer 47

𝜀 =BLv

Question 48

What is the emf and flux linkage when the plane of the coil is perpendicular to the field lines

Answer 48

Flux linkage = MAX EMF = 0

Question 49

What is the emf and flux linkage when the plane of the coil is parallel to the field lines

Answer 49

Flux linkage = 0 EMF = MAX

Question 50

Define alternating current

Answer 50

A current which periodically varies between positive to negative value with time

Question 51

How can the variation of current/pd with time be describe

Answer 51

Sinusoidal

Question 52

Define peak current (Io) or peak voltage (V0)

Answer 52

Max value of the alternating current or voltage

Question 53

What is seen on an oscilloscope when the time base is turned off

Answer 53

A vertical line on the voltage-gain axis

Question 54

Uses of an oscilloscope

Answer 54

DC and AC voltmeter To measure time intervals and frequencies To display AC waveforms

Question 55

What is a transformer

Answer 55

A device that changes high alternating voltage at low current to low alternating voltage at high current and vice versa

Question 56

Why are transformers used

Answer 56

To reduce heat energy lost whilst electricity is transmitted down electrical power lines

Question 57

What is a transformer made up of

Answer 57

A primary coil, secondary coil and a soft iron core

Question 58

Why is the soft iron core necessary in a transformer

Answer 58

It focuses and directs the magnetic field from the primary to secondary coil

Question 59

Why is the core made up of soft iron

Answer 59

It can easily be magnetised and demagnetised

Question 60

How do eddy currents arise

Answer 60

A changing magnetic field and flux from the alternating current creates a changing magnetic field in the core that acts against the field that induced them. An emf is therefore induced. A current flows, as the core is made from a conducting material

Question 61

How do eddy current cause energy loss in a transformer

Answer 61

They generate heat in the wires

Question 62

How are eddy currents reduced

Answer 62

Laminating the iron core with layers of insulation so current does not flow between them Having a core made from a high resistivity metal

Question 63

Ways to reduce energy loss in a transformer

Answer 63

Make the core from soft iron to allow easy magnetisation and demagnetisation Laminating the core Using thick wires

Question 64

Where do inefficiencies in the core appear from

Answer 64

Induced eddy currents. Reversal of magnetism Poor insulation between primary and secondary coil

Question 65

Why are step up transformers used duirng transmission

Answer 65

To increase the voltage which decreases the current through transmission lines. This reduces overall heat energy lost in wires during transmission