Magnetic Fields

Question 1

Which fingers represent which quantities in Fleming’s left hand rule?

Answer 1

• remember FBI
• thumb - force
• first finger - B (magnetic field / flux density)
• second finger - I (conventional current)

Question 2

What symbol represents flux or current into the page

Answer 2

x

Question 3

What symbol represents flux or current out of the page?

Answer 3

.

Question 4

F = BIL when…?

Answer 4

I is perpendicular to B

Question 5

For a charge travelling perpendicular to B in a magnetic field, what type of motion occurs?

Answer 5

circular motion occurs

Question 6

Is work done by a centripetal force?

Answer 6

no

Question 7

For a charge travelling perpendicular to B in a magnetic field, why is it that speed is constant?

Answer 7

• work done = force x distance
• d=0 and r = constant
• therefore speed is constant

Question 8

For a mass spectrometer, why is it that only ions with the same velocity pass through into the mass analyser? (hint: use formula sheet)

Answer 8

• force due to B-field = Bqv
• force due to E-field = Eq
• pass through if F due to B-field = F due to E-field, thus E = Bv, v = E/B, so must all have the same velocity in order to pass through

Question 9

What is meant by flux?

Answer 9

total magnetic field that passes through a given area (think of the number of field lines)

Question 10

What is the unit for flux?

Answer 10

Weber [Wb]

Question 11

What is meant by flux density?

Answer 11

the amount of magnetic flux per unit area

Question 12

What is the unit for flux density?

Answer 12

Tesla [T]

Question 13

What is meant by flux linkage?

Answer 13

the flux in all the loops added together (flux x number of turns)

Question 14

What is the unit for flux linkage?

Answer 14

Weber turns [Wb]

Question 15

In the equation: NΦ = BANcosθ, what is θ?

Answer 15

• angle between the axis of the coil and the flux
• or
• angle between flux and NORMAL of the coil

Question 16

What is Faraday’s law?

Answer 16

Faraday’s law states that the induced emf is directly proportional to the rate of change of flux linkage

Question 17

Derive that ε = BLv

Answer 17

• F = BIL
• W = FΔs
• W = BILΔs
• Q = IΔt
• ε = W/Q
• ε = BLΔs / Δt
• ε = BLv

Question 18

What is Lenz’s law?

Answer 18

the induced emf will always attempt to oppose the change in flux linkage making it

Question 19

What is the ‘recipe’ for explaining electromagnetic induction?

Answer 19

• relative motion between B-field and conductor
• causes ΔNΦ
• causes an emf
• causes an induced I (if there’s a complete circuit)
• causes an induced B-field
• two B-fields interact
• to oppose the ΔNΦ

Question 20

What is the role of the soft iron core of a transformer?

Answer 20

to guide all the flux from the primary coil through the secondary coil

Question 21

Explain how eddy currents can form in the soft iron core?

Answer 21

• the core is a conductor which contains ΔNΦ

- therefore an emf and (eddy) current could be induced

Question 22

Give 2 reasons why eddy currents in the soft-iron core can be a cause of inefficiency for the transformer?

Answer 22

• will waste energy as heat in the core

- will have an associated B-field that opposes the ΔNΦ in the core

Question 23

How can we limit the inefficiencies caused by eddy currents in the soft-iron core?

Answer 23

laminate the core with insulating glue

Question 24

What is meant by hysteresis?

Answer 24

where iron atoms (in the soft iron core) rotate with the alternating B-field, friction causes them to heat up

Question 25

Give 3 causes of inefficiencies in a tranformer

Answer 25

- eddy currents in the core - hysteresis - resistive heating (in coils and power lines)

Question 26

What equation should you use to think of power losses in the national grid?

Answer 26

P = (I^2)(R)

Question 27

Why does having a high V and a low I reduce heating effects in power cables?

Answer 27

as low I means less frequent and less 'hard' collisions between electrons and positive ions

Question 28

For a transformer which is 100% efficient, show that Vp/Vs = Is/Ip

Answer 28

- if 100% efficient - Pp = Ps - IpVp = IsVs - Vp/Vs = Is/Ip