Topic 9

Question 1

What are the rules for drawing magnetic field lines?

Answer 1

1) Lines start at the North Pole and end at the South Pole (or form closed loops for point fields).
2) Lines cannot cross.
3) The closer together the lines, the stronger the field.

Question 2

Definition: Magnetic flux density

Answer 2

The strength of magnetic field at a point in space, which is represented by the closeness of field lines.

(T)

Question 3

Definition: Magnetic flux

Answer 3

The amount of magnetic field passing perpendicularly through a defined area.

(Wb)

Question 4

Equation: Magnetic flux

Answer 4

Φ = BAsinθ

Φ = magnetic flux (Wb)

B = magnetic flux density (T)

A = area (m²)

sinθ = angle between magnetic field and area

NOTE: if the field is perpendicular to the area,

θ = 90°

⇒ sinθ = 1

so Φ = BA

Question 5

Definition: Magnetic flux linkage

Answer 5

The total magnetic flux inside a coil of N turns.

Question 6

Equation: Magnetic flux linkage

Answer 6

MFL = NΦ

MFL = magnetic flux linkage (Wb turns)

N = number of turns

Φ = magnetic flux (Wb)

Question 7

Fleming’s left-hand rule

Answer 7

Used on positive charges to predict the direction of magnetic force acting upon it.

Thumb = direction of force

First finger = direction of field (N -> S)

Second finger = direction of motion of particle

Question 8

Fleming’s right-hand rule

Answer 8

Used on negative charges to predict the direction of magnetic force acting upon it.

Thumb = direction of force

First finger = direction of field (N -> S)

Second finger = direction of motion of particle (or current)

Question 9

Equation: Magnetic force (on charged particle) *

Answer 9

F = Bqvsinθ

F = magnetic force (N)

B = strength of magnetic field (T)

q = charge (C)

v = velocity (ms⁻¹)

θ = angle between direction of movement and field

Question 10

Equation: Magnetic force (on current-carrying conductor) *

Answer 10

F = BIlsinθ

F = magnetic force (N)

B = strength of magnetic field (T)

I = current (A)

l = length of current-carrying conductor inside the field (m)

θ = angle between flow of current and field

Question 11

What is the motion of a charged particle in a field?

Answer 11

1) The magnetic force is always perpendicular to the direction of motion, so it acts as a centripetal force on the charged particle.
2) Therefore, the charged particle accelerates centripetally, so moves in a circular path.

Question 12

What are the features of induced emf?

Answer 12

• If there is no motion, there is no induced emf.
• When the cool and magnet move in relation to each other, an emf is induced.
• If the direction of motion changes, so does the sign of emf.
• The faster the motion, the greater the induced emf.
• The effect is the same whether the magnet or coil are moved.

Question 13

Definition: Electromagnetic induction

Answer 13

1) Relative motion between a coil and a magnet occurs (or) the current through a wire periodicaly changes direction due to an A.C. power supply (or) current is frequently switched on and off.
2) This causes the flux linkage to change.
3) Therefore an emf is induced onto the coil.
4) The size of induced emf is directly proportional to the rate of change of flux linkage.

Question 14

How does a spinning coil generator work?

Answer 14

1) As the coil rotates, the flux through the coil changes continuously, inducing an emf at the same frequency as the rotation.
2) Increasing the frequency of the rotation increases the amplitude and frequency of the induced alternating emf.

Question 15

How does a rotating magnet generator work?

Answer 15

1) The induced emf alternates with the same frequency as the rotating magnet.
2) Increasing the rate of rotation increases the frequency and amplitude of alternating emf.
3) Therefore, the alternating current is increased.

Question 16

How does a transformer work?

Answer 16

1) An alternating current is applied to the primary coil, causing the magnetic flux to continuously change.
2) This induces an emf in the secondary coil.

Question 17

Why do transformers not work with a D.C. power supply?

Answer 17

Because there is no change in magnetic flux linkage and so an emf isn’t induced.

Question 18

How does flux linkage affect emf?

Answer 18

• The faster the rate of change of magnetic flux, the greater the induced emf. Therefore the larger the gradient, the greater the induced emf.
• The positive peak of induced emf relates to the negative gradient of magnetic flux.

Question 19

Definition: Faraday’s Law

Answer 19

The magnitude of induced emf is equal to the rate of change of magnetic flux linkage.

Question 20

Definition: Lenz’s Law

Answer 20

The direction of an induced emf is such as to oppose the change creating it.

Question 21

Equation: Faraday’s and Lenz’s Law *

Answer 21

ε = -d(NΦ) / dt

ε = emf (V)

• = emf opposes change

NΦ = magnetic flux linkage (Wb turns)

d / dt = rate of change

Question 22

What is the difference between D.C. and A.C.?

Answer 22

For D.C. supplies, current only flows in one direction, of equal magnitude at all times.

For A.C supplies, the direction of current flow changes periodically, with differing magnitudes.

Question 23

What is a root mean square value?

Answer 23

The D.C. equivalent of an A.C. Power supply.

Question 24

Equation: Root-mean-square values *

Answer 24

V_rms = Vo / √2

I_rms = I_o / √2

V_rms = pd of D.C. supply

V_o = peak pd of A.C. supply

Question 25

How do you work out the direction of magnetic field around a point charge?

Answer 25

Use the right hand fist rule with the thumb showing the direction of the flow of current.

Question 26

How does mass affect radius of path of particle in magnetic field?

Answer 26

The larger the mass, the larger the radius.

Question 27

How are fields indicated when drawn on a page?

Answer 27

X = into the page

O = out of the page

Question 28

How can you work out the radius of a charged particle in a magnetic field?

Answer 28

F = Bqv

F = mv² / r

⇒ Bqv = mv² / r

⇒ r = mv² / Bqv

⇒ r = mv / Bq

Question 29

What is the relationship between centripetal and magnetic force?

Answer 29

Bqv = mv² / r

Question 30

What is an assumption made when using F = Bqv?

Answer 30

That current is perpendicular to the magnetic field so that sinθ = 1

Question 31

What are the three ways in which emf can be enduced?

Answer 31

• Alternating current.
• Flick the current on and off.
• Move the wire and coil relative to each other.

These all cause a change in the magnetic flux linkage.