Magnetism

Question 1

Magnetic flux density (B)

Answer 1

The strength of the magnetic field

B= magnetic flux/ area

Question 2

Soft magnetic material

Answer 2

Easy to magnetise and demagnetise

Question 3

Force

Answer 3

F=BIL
F=BILcos
F=BeV=mv^2/r

Question 4

Magnetic flux density (phi)

Answer 4

Phi=BA

Phi=BAcos

Question 5

Electromagnetic induction

Answer 5

• when conductor moves through a magnetic field a pd is induced across the conductors ends
• As moving through the electrons experience a force and move to the end of the conductor inducing a pd as the electrons move through the field
• if conductor’s connected to a full circuit a current will flow

Question 6

Faradays law

Answer 6

EMF produced proportional to rate of change of flux linkage

EMF=NBA/t

Question 7

Lenz’s law

Answer 7

Any change must oppose the change the made it

EMF=-NBA/t

Question 8

Eddy currents

Answer 8

• when a magnet passes through a conductor the conductor cuts the field lines creating small currents called eddy currents
• these produce a magnetic field which opposes the original field causing the conductor to slow down

Question 9

Magnetic flux linkage

Answer 9

Amount of flux in a coil

Question 10

How a generator works

Answer 10

• cuts field lines so pd and current generated due to faradays law
• coil sounds between opposite facing poles

Question 11

How a microphone works

Answer 11

• causes cone to vibrate that moves coil in magnetic field

* cuts field lines so induces emf in opposite direction

Question 12

Bicycle dynamo

Answer 12

•induced pd doesn’t alternate and charge flows in same direction

Question 13

Induction hob

Answer 13

• coil under plate runs through occulting magnetic field and cuts field lines
• causes eddy currents in the pan that generate heat

Question 14

Electric guitar pick up

Answer 14

• strings made of magnetised steel
• when string plucked vibrate above pick up (bar magnet wrapped in 7000 coils) cutting the field lines
• EMF induces

Question 15

Card readers

Answer 15

Magnetic strip has info on, coil is reader, emf induced due to faradays law

Question 16

EMF

Answer 16

EMF=BLV

EMF=BANwsin(wt)

Question 17

Direct current

Answer 17

All charge carriers flow in 1 direction all the time

Question 18

Alternating current

Answer 18

Current which constantly changes direction

Question 19

I/V rms

Answer 19

I(rms) = I(0)/root2

V(rms)=V(0)/root2

Question 20

How transformers work

Answer 20

• apply alternating pd across primary coil
• current induces a magnetic flux which takes the shape of the soft iron core
• secondary could cuts field lines so emf induced as well as current

Question 21

Transformer equation

Answer 21

Vp/Np=Vs/Ns

Question 22

Step up transformer

Answer 22

High Vs so decreased current

More turns on 2nd coil

Question 23

Step down transformer

Answer 23

Lower Vs so increased current

More turns on 1st coil

Question 24

Efficiency equations

Answer 24

IpVp=IsVs

Ps/Pp=IsVs/IpVp

Question 25

Why are transformer not 100% efficient

Answer 25

• Eddy currents
• Resistance
• Flux leaks
• Hysteresis

Question 26

The National grid

Answer 26

Collection of cables connecting power stations to all the homes and businesses