Magnetic Fields

Question 1

What does a current-carrying conductor produce

Answer 1

a magnetic field

When interacting with an external magnetic field, it will experience a force

Question 2

When will a current-carrying conductor experience the maximum magnetic force

Answer 2

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

It experiences no force if it is parallel to magnetic field lines

Question 3

What is the strength of the magnetic field known as, and what is its unit

Answer 3

the magnetic flux density, B

measured in Tesla (T)

Question 4

Describe the components of the force-flux density equation

Answer 4

Question 5

What does the force-flux density equation show

Answer 5

the greater the current or the magnetic field strength, the greater the force on the conductor

Question 6

What is Flemings’s left hand rule

Answer 6

Question 7

What are the symbols for a magnetic field which goes into or out of the page

Answer 7

Question 8

Describe how moving charges may curve in direction

Answer 8

For example, when a positively charged particle enters a magnetic field into the page from left to right:

-Using Fleming’s left-hand rule, the first finger points into the page and the second finger (current) points to the right
- This means the force is upwards
- The particle is then pulled in the direction of this force (upwards). This means the direction of the current also changes direction slightly (slanting upwards)
- This means the force will also change direction since it still needs to keep perpendicular to the current and the field
- Therefore, the moving charges will follow a circular trajectory

Question 9

Magnetic flux density def

Answer 9

The force acting per unit current per unit length on a current-carrying conductor placed perpendicular to the magnetic field

Measured in Tesla

Question 10

unit of a Tesla def

Answer 10

A wire carrying a current of 1A normal to a magnetic field of flux density of 1 T with force per unit length of the conductor of 1 N m-1

Question 11

how can a particle experience no magnetic force

Answer 11

when it travels parallel to a magnetic field

Question 12

What does the force on a current carrying wire depend on

Answer 12

• Strength of uniform magnetic field
• Size of current
• Length of wire in the field

Question 13

For a particle in a magnetic field, travelling in a circular path, what is the centripetal force

Answer 13

The magnetic force

Question 14

Draw a diagram of the forces acting on a particle which moves in a circular path

Answer 14

Question 15

What are cyclotrons

Answer 15

a type of particle accelerator that accelerates charged particles (eg. protons) from their centre along a spiral path

Question 16

What kind of medical research are cyclotrons used for

Answer 16

Producing medical isotopes (tracers)

Creating high-energy beams of radiation for radiotherapy

Question 17

Draw and label a diagram of a cyclotron

Answer 17

Question 18

Describe the process of accelerating a particle in a cyclotron

Answer 18

• A source of charged particles is placed at the centre of the cyclotron and they are fired into one of the electrodes
• The magnetic field in the electrode makes them follow a semi-circular path, since it is perpendicular to their motion until they eventually leave the electrode
• The potential difference applied between the electrode accelerates the particles across the gap to the next electrode (since there is an electric field in the gap)
• Since the speed of the particles is now higher, they will follow a circular path with a larger radius (since r ∝ v) before leaving the electrode again
• The potential difference is then reversed so the particles accelerate towards the opposite electrode
• This process is repeated as the particles spiral outwards and eventually have a speed large enough to exit the cyclotron
• The alternating potential difference is needed to accelerate the particles across the gap between opposite electrodes
• Otherwise, the particles will only accelerate in one direction

Question 19

Derive the equation for the radius of the path of a particle travelling in a magnetic field

Answer 19

Question 20

What is electromagnetic induction and give examples

Answer 20

• when an e.m.f is induced in a closed circuit conductor due to it moving through a magnetic field
• This happens when a conductor cuts through magnetic field lines
• The amount of e.m.f induced is determined by the magnetic flux

Examples are a flat coil or a solenoid

Question 21

magnetic flux definition

Answer 21

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

(the total magnetic field that passes through a given area)

Question 22

When is magnetic flux at a maximum/minimum

Answer 22

• It is a maximum when the magnetic field lines are perpendicular to the plane of the area
• It is 0 when the magnetic field lines are parallel to the plane of the area

Question 23

Draw a diagram of maximum magnetic flux and minimum magnetic flux

Answer 23

Question 24

What is the unit of magnetic flux

Answer 24

Webers (Wb)

Question 25

What do the components of the magnetic flux equation mean

Answer 25

Question 26

Magnetic flux linkage def

Answer 26

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

Question 27

What do the components of the magnetic flux linkage equation mean

Answer 27

Question 28

What is magnetic flux linkage measured in

Answer 28

Weber turns (Wb turns)

Question 29

What is the symbol for magnetic flux linkage

Answer 29

ΦN

Question 30

What do the components of the magnetic flux linkage equation mean (When the magnet field lines are not completely perpendicular to the area A)

Answer 30

Question 31

When is an emf induced in a circuit

Answer 31

When there is: - A changing magnetic flux density B - A changing cross-sectional area A - A change in angle θ

Question 32

What is electromagnetic induction

Answer 32

when an e.m.f is induced when a conductor moves through a magnetic field

Question 33

What happens when a conductor cuts through magnetic field lines

Answer 33

- This causes a change in magnetic flux (ΔΦ) - Which causes work to be done - This work is then transformed into electrical energy

Question 34

Electromagnetic induction def

Answer 34

The process in which an e.m.f is induced in a closed circuit due to changes in magnetic flux

Question 35

When may electromagnetic induction occur

Answer 35

When: - A conductor cuts through a magnetic field - The direction of a magnetic field through a coil changes

Question 36

What is electromagnetic induction used for

Answer 36

- Electrical generators which convert mechanical energy to electrical energy - Transformers which are used in electrical power transmission

Question 37

What factors increase an induced e.m.f when a bar magnet is moved through a coil connected to a voltmeter to induce an e.m.f

Answer 37

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

Question 38

What factors increase an induced e.m.f when a wire is moved between two magnets connected to a voltmeter to induce an e.m.f

Answer 38

- Increasing the length of the wire - Moving the wire between the magnets faster - Increasing the strength of the magnets

Question 39

What does Faraday's law state

Answer 39

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

Question 40

What does Lenz's law state

Answer 40

The direction of the induced emf is such that it will oppose the change of flux producing it

Question 41

Describe the experimental evidence for Lenz's law

Answer 41

Question 42

What do the components of Faraday's law equation mean

Answer 42

Question 43

What does Faraday's law equation demonstrate

Answer 43

that the gradient of a magnetic flux against time graph is the e.m.f

Question 44

What is the equation that combines Faraday and Lenz's laws AND what does it demonstrate

Answer 44

Question 45

Draw a diagram which shows how a straight conducting rod moving through a magnetic field will have an e.m.f induced in it

Question 46

Derive the equation for the induced e.m.f in a conductor as it moves through the magnetic field

Answer 46

Question 47

When is induced emf at a maximum/ minimum

Answer 47

- Maximum when θ = 90o. The magnetic field lines are parallel to the plane of the area (or the normal to the area is perpendicular to the field lines) - 0 when θ = 0o. The magnetic fields lines are perpendicular to the plane of the area (or the normal to the area is parallel to the field lines)

Question 48

Draw a graph of the magnetic flux linkage and e.m.f induced as a coil rotates in a magnetic field

Answer 48

Question 49

Describe the derivation and components of the 'emf induced in a rotating coil' equation

Answer 49

Question 50

Derive the frequency equation

Answer 50

Question 52

Answer 52