C23 - Magnetic Fields

Question 1

What’s a magnetic field?

Answer 1

A field surrounding a permanent magnet or current-carrying conductor in which magnetic objects experience a force.

(Field patterns again shown using [magnetic] field lines).

Question 2

What do magnetic field patterns show?

Answer 2

The arrow points from north to south.

Equally spaced and parallel magnetic field lines represent a uniform field (strength of the field doesn’t vary).

Magnetic field is stronger when the magnetic field lines are closer. (For a bar magnetic, the field is strongest at its north and south poles).

Like poles repel, unlike poles attract.

Question 3

How can the direction of magnetic field be determined for a current carrying wire?

Answer 3

Right-hand grip rule.

The thumb points in the direction of conventional current, and the direction of the field is given by the direction in which the fingers curl around the wire.

Question 4

How do magnetic field lines appear for a coiled wire with current passing through?

Answer 4

Same as a bar magnet

Question 5

What is the strength of magnetic fields measured in?

Answer 5

Tesla, T

Question 6

What happens when a current-carrying conductor is placed in a magnetic field?

Answer 6

The external magnetic field and field around the conductor interact (like permanent magnets) and experience equal and opposite forces.

Question 7

What does Flemings left hand rule show?

Answer 7

First finger - (field) Gives the direction of the external magnetic field

Second finger - (current) Gives the direction of the conventional current

Thumb - (motion) Gives the direction of motion/force of the wire

Question 8

What factors affect the size of the force experienced by a wire (current-carrying conductor) in a magnetic field?

Answer 8

Current (I)
Length of wire in the field (L)
Strength of magnetic field (B)
Angle between the magnetic field and current direction (Sin θ)

Therefore:
F = BILsin θ

Question 9

When is the force on a wire (current-carrying conductor) greatest when placed in a magnetic field?

Answer 9

When placed perpendicular (90°) to the direction to magnetic field.
(0 when parallel)

Question 10

What is magnetic flux density?

Answer 10

The strength of the magnetic field.

Question 11

How can magnetic flux density between 2 magnets be determined in a lab?

Answer 11

Equipment: wire, clamp, top-pan balance, crocodile clips, ammeter, variable supply.

Place the magnets on a top-pan balance. The magnetic field between them is almost uniform. A stiff copper wire is held perpendicular to the magnetic field between the 2 poles.
The length of wire in the magnetic field is measured with a ruler.

Using croc clips, a section of wire is connected in series with an ammeter and variable power supply.
The balance is zeroed when there is no current in the wire.
With a current, the wire experiences a force (predicted by Flemings left hand rule.

According to Newton’s third law of motion, the magnets experience an equal and opposite force which can be calculated by the change in mass reading.

B = F / IL

Question 12

What happens when a charged particle moves in a magnetic field? (Circular tracks)

Answer 12

They experience a force.
(E.g. with electrons)
Force on the beam of electrons can be predicted using Flemings left hand rule.

As electrons enter the field, the experience a force (away from the anode).

The electrons change direction but force on each electron always remains perpendicular to its velocity.
Speed of the electrons remains unchanged because the force has no component in the direction of motion.

Once out the field, electrons keep moving in a straight line.

Question 13

Relating BQv = mv^2/2 or r = mv/BQ, how is r related to v, m, B and Q?

Answer 13

Faster moving particles travel in bigger circles

More massive particles move in bigger circles

Stronger magnetic fields make particles move in smaller circles

Particles with greater charge move in smaller circles

Question 14

What’s a velocity selector?

Answer 14

A device that uses both electric and magnetic fields to select charged particles of specific velocity.

For an undeflected particle, electric force = magnetic force
EQ = BQv

Question 15

How is an emf / v induced?

Answer 15

With a coil and magnet.

A sensitive voltmeter is attached to a coil shows no reading when the coil and magnet are stationary.

When the magnet is pushed towards the coil, an emf is induced across the ends of the coil, and when the magnet is pulled away, a reverse emf is induced.
Doing this repeatedly induces an alternating current in the coil.

The faster the movements, the larger the induced emf.

Question 16

What happens when electrons enter a magnetic field?

Answer 16

As electrons enter the field, the experience a force (away from the anode).

The electrons change direction but force on each electron always remains perpendicular to its velocity.
Speed of the electrons remains unchanged because the force has no component in the direction of motion.

Once out the field, electrons keep moving in a straight line.

Question 17

How is force on a conductor calculated with charged particles?

Answer 17

F = BQv (derived from F = BIL)

Question 18

How does a coil and magnet induce a voltage?

Answer 18

When the magnet is pushed towards the coil, an emf is induced across the ends of the coil, and when the magnet is pulled away, a reverse emf is induced.
Doing this repeatedly induces an alternating current in the coil.

The faster the movements, the larger the induced emf.

Question 19

How is electromagnetic induction explained?

Answer 19

Energy is conserved.
Work is done to move the magnet, which is transferred to electrical energy.

Motion of the coil relative to the magnetic field makes electrons move because they experience a force (BQv).

The moving electrons constitute an electrical current within the coil, so the process has produced electrical energy.

Question 20

What’s the symbol for magnetic flux?

Answer 20

ΦB

Question 21

What is magnetic flux, Φ / how is it defined?

Answer 21

It is defined as the product of the component of the magnetic flux density perpendicular to the area, and the cross-sectional area.

Φ = (Bcos θ) * A

Φ = BAcos θ

When the field is normal to the area, Cos 0° = 1 therefore Φ = BA
Changing B, A or the angle can induce an emf.

Question 22

What is magnetic flux linkage?

Answer 22

The product of the number of the turns in the coil, N and magnetic flux.

Magnetic flux linkage = NΦ

Question 23

What is the SI unit of magnetic flux linkage?

Answer 23

A weber

Question 24

What is Faraday’s law?

Answer 24

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

Ɛ ∝ Δ(NΦ) / Δt

Where Ɛ is the induced emf and Δ(NΦ) is the change in magnetic flux linkage in a time interval, Δt.

Question 25

How is Faraday’s law written as an equation?

Answer 25

Ɛ = - Δ(NΦ) / Δt

Question 26

What’s Lenz’s law?

Answer 26

The direction of the induced emf or current is always such as to oppose the change producing it.

Question 27

How does rate of change of flux linkage (of an alternating current) vary over time?

Answer 27

It’s at its maximum as it crosses the x axis, and is zero at max (+/-) flux linkage (peaks).

Question 28

How do emf and flux linkage (of an alternating current) vary over time?

Answer 28

Both fluctuate like an alternating current.

However, emf is at its maximum when flux linkage is 0 (and rate of flux change is maximum).

Question 29

What is maximum induced emf (directly) proportional to?

Answer 29

Magnetic flux density, B

Cross-sectional area, A, of the coil

Number of turns, N

Frequency of the rotating coil, f

Question 30

What is the gradient of a magnetic flux linkage against time graph equal to?

Answer 30

The induced emf

Question 31

How does a simple (step down) transformer work?

Answer 31

It consists of a laminated iron core, a primary coil and secondary coil (input and output).

This produces a varying magnetic flux in the soft iron core.
The secondary coil is linked by this changing flux. The iron core ensures that all the magnetic flux created by the primary coil links to the secondary and that none is lost.

This produces a varying emf across the ends of the secondary coil.

Question 32

How do the number of primary and secondary coils vary on a step up and step down transformer?

Answer 32

Step up: Ns > Np

Step down: Ns < Np

Question 33

How are voltage and current related in a transformer?

Answer 33

VsIs = VpIp

So…
Ip / Is = Vs / Vp