23 - Magnetic Fields

Question 1

ways to draw magnet’s magnetic field

Answer 1

dump iron filings or use a plotting compass with a bar magnet

Question 2

direction of magnetic field lines are always

Answer 2

north to south

Question 3

direction of earth’s magnetic field

Answer 3

south pole towards north pole (geographically) therefore bar magnet would have north pole and geographic south pole

Question 4

define magnetic field

Answer 4

the region around a permanent or magnet or a current carrying conductor in which other magnetic objects will experience a force

Question 5

what is the current carrying conductor responsible for the earth’s magnetic field

Answer 5

the earth’s molten core

Question 6

where is magnetic field strongest around a magnet and why

Answer 6

at poles due to field lines being closest

Question 7

what is responsible for creating a magnetic field around a bar magnet

Answer 7

the flowing of free electrons

Question 8

electromagnetism

Answer 8

when a charges particle moves, it creates a magnetic field so when a current passes through a wire, a magnetic field is produced.

Question 9

how to find direction of magnetic field around current carrying wire

Answer 9

using right hand grip rule (thumb is current, rest is field direction)

when representing in exam, make sure each field line gets further away from each other from the wire

Question 10

magnetic field of solenoid is

Answer 10

uniform in the middle

Question 11

how to find direction of field for solenoid

Answer 11

use right hand grip rule but thumb is field direction and rest are current direction

Question 12

north and south poles for solenoid

Answer 12

the solenoid produce north and south poles at their opposite faces

Question 13

how to determine direction of force on current carrying conductor in external magnetic field

Answer 13

Fleming’s left hand rule

Question 14

force on current carrying wire

Answer 14

F=BILsinϑ
force = magfluxdensity x current x length x sinϑ
ϑ is the angle betwee FIELD and CURRENT

if perpendicular F=BIL
if parallel F=0

Question 15

what is magnetic flux density

Answer 15

the strength of the magnetic field measured in Tesla T

Question 16

what is one tesla equal to

Answer 16

the magnetic flux density when a wire of length 1m carrying a current of 1A is perpendicular to the field, experiences a force of 1N

Question 17

other equations relating to magnetic fields that can be used in qs where factors increase and decrease

Answer 17

R=pL/A and V=IR - V is constant

Question 18

how are northern lights created

Answer 18

charged particles from the sun spiral down the Earth’s magnetic field to the poles (strongest), colliding with atoms in the atmosphere to emit light

Question 19

electron flow and flemings left hand rule

Answer 19

electron flow is opposite to conventional current - important in flemings left hand rule

Question 20

force on electric charge in magnetic field

Answer 20

F=Bqvsinϑ
force = magfluxdensity = charge x velocity x sinϑ
F=Bqv if perpendicular

Question 21

motion of elec charge in magnetic field

Answer 21

circular motion

Question 22

radius of circular motion equation and derived

Answer 22

F=mv2/r and F=BQv
cancel out v so
r=mv/BQ

Question 23

electron motion around field lines - radius of circular motion

Answer 23

electrons spiral around field lines due to constant component velocity in direction of field lines

Question 24

velocity selector

Answer 24

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

Forcemagnetic = Forceelectric

BQV=EQ
so V=E/B

Question 25

when to use FLHR

Answer 25

motors

Question 26

when to use FRHR

Answer 26

generators
when wanting to find direction of current, opposite to LHR due to lenz’s law.

Question 27

how is current induced in a wire

Answer 27

when a wire moves relative to a magnetic field a current is induced due to the cutting of magnetic field lines. the amount of cutting is quantised as magnetic flux

Question 28

other way to induce emf

Answer 28

moving magnet through coil of wire

Question 29

define magnetic flux

Answer 29

the product of the component of the magnetic flux density perpendicular to the cross-sectional area

Question 30

mag flux equation

Answer 30

ɸ=BAcosϑ
flux = flux density x cross sectional area
ϑ - angle measured to normal of surface

ɸ=BA if perpendicular

Question 31

unit of magnetic flux

Answer 31

Weber (Wb)

Question 32

define one weber

Answer 32

the magnetic flux when a field of magnetic flux density 1T passes at right angles through a coil with area of one metre squared

Question 33

things to remember about ɸ=BA

Answer 33

ɸ unchanged if loop raised as field is uniform
A is the area only in contact with the field

Question 34

magnetic flux linkage

Answer 34

when there are several coils (motors and generators)

flux linkage = Nɸ
number of turns x magnetic flux

UNITS - Wb turns

Question 35

moving bar magnet through coil of wire (N pole to ground)

Answer 35

second peak greater and shorter time interval due to acceleration due to gravity
peaks opposite direction due to Lenz’s law
induced emf is 0 when in magnet due to field coil being parallel to flux density

number of coils are direc proportional to induced voltage

Question 35

define magnetic flux linkage

Answer 35

the product of the number of turns in the coil and the magnetic flux

Question 36

Faraday’s law

Answer 36

induced emf is directly proportional to the rate of change of magnetic flux linkage

ε∝ ∆(Nɸ) / ∆t

Question 37

direction of induced emf when magnet moves through coil

Answer 37

direction of induced emf and hence the current changed direction when magnet is pulled away and pushed towards the coil due to lenz’s law

Question 38

Lenz’s law

Answer 38

the direction of any induced emf or current is always in a direction that opposes the change producing it
conservation of ENERGY

Question 39

combining Faraday’s and Lenz’s laws

Answer 39

ε = -∆(Nɸ) / ∆t

Question 40

what is constant for AC generator

Answer 40

ε = -∆(Nɸ) / ∆t
ε = -∆(BANcosϑ) / ∆t

BAN is constant

Question 41

how is AC current generated

Answer 41

there is magnetic field surrounding the magnet and when the magnet spins, the field lines are cut, causing a change in magnetic flux inducing an emf and current. because the field is rotating, AC is produced, emf induced causes electrons to flow one way then the other

Question 42

emf max occurs when

Answer 42

Nɸ is 0

Question 43

emf is 0 when

Answer 43

Nɸ is at maximum

Question 44

emf relation to flux linkage and time graph

Answer 44

-ve gradient

Question 45

how transformers work

Answer 45

an AC current causes the core to be magnetised and demagnetised. this rapid change in flux means equally fast change in flux in secondary coil as iron core links flux to secondary coil, inducing AC and emf in secondary coil opposite to direction of flux change causing it

Question 46

the faster the change in flux…

Answer 46

higher the induced emf

Question 47

step up and down transformers

Answer 47

up - Vs>Vp
down - Vs<Vp

Question 48

transformers coil equation

Answer 48

Ns/Np = Vs/Vp = Ip/Is

Question 49

why is core soft iron

Answer 49

easy to magnetise and demagnetise quickly, also improves efficiency

Question 50

100% efficiency for a transformer =

Answer 50

output power from Vp = Vs

Question 51

why is a transformer not 100% effiecient?

Answer 51

heating of P and S coil
eddy currents induced in core
- heating due to current

Question 52

how to improve efficiency of transformers

Answer 52

using low resistance to make more efficient and reduce heating effect of current
laminate core to minimise eddy currents

Question 53

voltage for transmission lines - national grid

Answer 53

high to minimise heat loss and reduce heating effect of larger current