7. Magnetic fields

Question 1

Define a magnetic field:

Question 2

How is a magnetic field created?

Answer 2

A magnetic field is created by moving electric charge in a conductor or by permanent magnets which inherently have a magnetic field

Question 3

What are the key aspects of magnetic field lines?

Answer 3

1. The arrows always point out of a North Pole and into a South Pole
2. The field lines are stronger the closer the lines are together
3. The field lines are weaker the further apart the lines are
4. The magnetic field lines never cross

Question 4

Draw the magnetic field lines around a bar magnet:

Question 5

What do magnetic field lines represent?

Answer 5

The direction and magnitude of a magnetic field

Question 6

How do the poles of a magnetic field interact with each other?

Answer 6

Two like poles will repel each other and two opposite poles will attract each other

Question 7

Draw the two types of magnetic fields between two bar magnets:

Question 8

What is a uniform magnetic field?

Answer 8

A magnetic field in which the strength of the magnetic field is the same at all points - this is seen as equally spaced parallel lines

Question 9

How does a compass work?

Answer 9

A compass that is not in the presence of any magnets will always point towards North (which is the geographic South Pole) - because the North Pole of the compass is attracted to the South Pole of the Earth

Question 10

Define magnetic flux density:

Answer 10

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

Question 11

Define the tesla (T):

Answer 11

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

Question 12

What is the equation for the force on a current-carrying conductor in an external magnetic field?

Answer 12

F = BILsin(θ)

B = magnetic flux density of the external magnetic field in T
I = current in the conductor in A
L = length of the conductor in the external magnetic field in m
θ = angle between the conductor and the external magnetic field

Question 13

When is the maximum force on a current-carrying conductor experienced?

Answer 13

The maximum force occurs when the conductor is perpendicular to the B field or when sin(θ) = 1

Question 14

When is the minimum force on a current-carrying conductor experienced?

Answer 14

The minimum force occurs when the conductor is parallel to the B field or when sin(θ) = 0

Question 15

What is Fleming’s left hand rule?

Answer 15

Fleming’s left hand rule can be used to determine the directions of the force on a current-carrying conductor, the B-field and current - they are always all mutually perpendicular to each other

Question 16

How do you use Fleming’s left hand rule?

Answer 16

1. Point your thumb, first finger and second finger at right angles to each other
2. Your thumb points in the direction of the force on the conductor
3. Your first finger points in the direction of the B-field
4. Your second finger points in the direction of the flow of conventional current in the conductor

Question 17

How do you know if a magnetic field points into or out of a page?

Answer 17

1. Dots represent the magnetic field directed out of a page
2. Crosses represent the magnetic field directed into a page

Question 18

How do you calculate the force on a moving charge placed in an external magnetic field?

Answer 18

F = BQvsin(θ)

The direction of the force can be found using Fleming’s left hand rule - the second finger represents the current flow or the flow of positive charge:

1. For a positive charge, the current points in the same direction as its velocity
2. For a negative charge, the current points in the opposite direction to its velocity
3. When a charged particle moves in a uniform magnetic field, the force acts perpendicular to the field and the particle’s velocity - as a result, it follows a circular path

Question 19

Define hall voltage:

Answer 19

The potential difference produced across an electrical conductor when an external magnetic field is applied perpendicular to the current through the current

Question 20

Explain how hall voltage is achieved:

Answer 20

1. When an external magnetic field is applied perpendicular to the direction of current through a conductor, the electrons experience a magnetic force
2. As a result, the electrons drift to one side of the conductor, causing it to become more negatively charged and the opposite side to become more positively charged
3. As a result of this separation of charge, a potential difference is set up across the conductor

Question 21

What is the hall voltage equation?

Answer 21

VH = BI / ntq

B = magnetic flux density (T)
q = charge of the electron (C)
I = current (A)
n = number density of electrons (m-3)
t = thickness of the conductor (m)

Question 22

What is a hall probe?

Answer 22

A hall probe is used to measure the hall voltage and magnetic flux density between two magnets based on the Hall effect - the instrument consists of a cylinder with a flat surface at the end

Question 23

How does a hall probe work?

Answer 23

To measure the magnetic flux density between two magnets, the flat surface of the prove must be directed between the magnets; this is to ensure the magnetic field lines pass completely perpendicular to this surface; the prove is connected to a voltmeter to measure the hall voltage and since the hall voltage is directly proportional to the magnetic flux density, the flux density of the magnets can be obtained

Question 24

What happens when a charged particle enters a uniform magnetic field?

Answer 24

The charged particle travels in a circular path because the direction of the magnetic force will always be perpendicular to the particle’s velocity and is directed towards the centre of the path, resulting in circular motion - The centripetal acceleration is in the same direction as the magnetic (centripetal) force

Question 25

What is the equation for the radius of the path of a charged particle in a perpendicular magnetic field?

Answer 25

r = mv / BQ r = radius of the path (m) m = mass of the particle (kg) v = linear velocity of the particle (m s−1) B = magnetic field strength (T) or magnetic flux density Q = charge of the particle (C)

Question 26

Define a velocity selector:

Answer 26

A device consisting of perpendicular electric and magnetic fields where charged particles with a specific velocity can be filtered

Question 27

What does a velocity selector consist of?

Answer 27

A velocity selector consists of two oppositely charged parallel plates situated in a vacuum chamber The plates provide a uniform electric field with strength E between them There is also a uniform magnetic field with flux density B applied perpendicular to the electric field If a beam of charged particles enters between the plates, they may all have the same charge Q but travel at different speeds

Question 28

What is the right hand grip rule?

Answer 28

The direction of the magnetic field can be determined using the right-hand grip rule This is determined by pointing the right-hand thumb in the direction of the current in the wire and curling the fingers onto the palm The direction of the curled fingers represents the direction of the magnetic field around the wire

Question 29

What does a magnetic field around a current carrying wire look like?

Answer 29

Magnetic field lines in a current-carrying wire are circular rings, centred on the wire The field lines are closer together near the wire, where the field is strongest The field lines become further apart with distance from the wire as the field becomes weaker

Question 30

Draw the magnetic field lines around a solenoid:

Question 31

Draw the magnetic field lines around a flat circular coil:

Question 32

How do you increase the magnetic field strength around a solenoid?

Answer 32

adding a core made from a ferrous (iron-rich) material e.g. an iron rod adding more turns in the coil

Question 33

What happens when two parallel current carrying conductors are placed next to each other?

Answer 33

If the currents are in the same direction in both conductors, the magnetic field lines between the conductors cancel out – the conductors will attract each other If the currents are in the opposite direction in both conductors, the magnetic field lines between the conductors push each other apart – the conductors will repel each other When the conductors attract, the direction of the magnetic forces will be towards each other When the conductors repel, the direction of the magnetic forces will be away from each other The magnitude of each force depend on the amount of current and length of the wire

Question 34

Define magnetic flux:

Answer 34

The product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density or in other words, it is the measure of number of magnetic field lines passing through a given area

Question 35

When is magnetic flux at a maximum and a minimum?

Answer 35

Magnetic flux is at a maximum when the magnetic field lines are perpendicular to the plane of the area and is at a minimum when the magnetic field lines are parallel to the plane of the area

Question 36

How do you calculate magnetic flux?

Answer 36

Φ = BAcos(θ) Φ = magnetic flux (Wb) B = magnetic flux density (T) A = cross-sectional area (m2) θ = angle between magnetic field lines and the line perpendicular to the plane of the area (often called the normal line) (°)

Question 37

When is an EMF induced in a circuit?

Answer 37

An e.m.f is induced in a circuit when the magnetic flux linkage changes with respect to time This means an e.m.f is induced when there is: A changing magnetic flux density B A changing cross-sectional area A A change in angle θ

Question 38

Define magnetic flux linkage:

Answer 38

The product of the magnetic flux and the number of turns

Question 39

How do you calculate magnetic flux linkage?

Answer 39

NΦ = BANcos(θ) Φ = magnetic flux (Wb) N = number of turns of the coil B = magnetic flux density (T) A = cross-sectional area (m2) θ = angle between magnetic field lines and the line perpendicular to the plane of the area (degrees) The flux linkage NΦ has the units of Weber turns (Wb turns)

Question 40

What is electromagnetic induction?

Answer 40

The phenomenon which occurs when an EMF is induced due to relative movement between a conductor and a magnetic field

Question 41

When does electromagnetic induction occur?

Answer 41

This could occur when: a conductor moves relative to a magnetic field a magnetic field varies relative to a conductor

Question 42

Explain how electromagnetic induction occurs:

Answer 42

When a conductor cuts through magnetic field lines: the free electrons in the conductor experience a magnetic force this causes work to be done as charges in the conductor become separated mechanical work is transferred to the charges as electric potential energy a potential difference is created between the ends of the conductor, or in other words, an e.m.f. is induced

Question 43

Define Faraday's law:

Answer 43

Faraday's law tells us the magnitude of the induced EMF in electromagnetic induction and is defined as the magnitude of the induced EMF is directly proportional to the rate of change in magnetic flux linkage

Question 44

What is Faraday's law equation?

Answer 44

ε = NΔΦ / Δt ε = induced e.m.f (V) N = number of turns of coil ΔΦ = change in magnetic flux (Wb) Δt = time interval (s)

Question 45

Define Lenz's law:

Answer 45

Lenz's law gives the direction of the induced EMF as defined by Faraday's law and is defined as the induced EMF acts in such a direction to produce effects which oppose the change causing it

Question 46

What is the Lenz's law equation?

Answer 46

ε = -NΔΦ / Δt

Question 47

What does the Lenz's law equation show?

Answer 47

This equation shows: When a bar magnet goes through a coil, an e.m.f 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 the bar magnet If a direct current (d.c) power supply is replaced with an alternating current (a.c) supply, the e.m.f induced will also be alternating with the same frequency as the supply