6 Magnetic fields

Question 1

magnetic field

Answer 1

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

Question 2

how can you detect the presence of a MF?

Answer 2

with a small plotting compass

the needle will deflect in the presence of a MF

Question 3

magnetic field lines

Answer 3

map magnetic field patterns around magnets and current-carrying conductors

• the arrow is the dir in which a free north pole would move- points N to S
• equally spaced and parallel mfl represent a uniform field
• MF is stronger when the mfl are closer.
• like poles repel and unlike poled attract

Question 4

electromagnetism

Answer 4

when a wire carries a current, a MF is created around the wire
field is created by elecs moving within the wire
any charged particle that moves creates a magnetic field in the space around it
for the mf of a bar magnet, the mf is created by the elecs whizzing around the iron nuclei

Question 5

current-carrying conductors

Answer 5

current-carrying wire- mfl are concentric circles centered on the wire and perpendicular to it
the dir of the mf can be determined using the right hand grip rule

Question 6

right hand grip rule

Answer 6

thumb- dir of conventional current

fingers curl- dir of field

Question 7

magnetic field patterns of single coil and a solenoid

Answer 7

both the coil and solenoid produce N and S poles at their opposite faces
magnetic field pattern outside solenoid is similar to that for a bar magnet, and at the centre of the core of the solenoid it is uniform

Question 8

Fleming’s left hand rule

Answer 8

when a current-carrying conductor is placed in an external mf, the two fields interact
the dir of the force experienced by the conductor placed perpendicular to the external mf can be determined using Fleming’s LHR
-first finger gives dir of external mf
-second finger gives the dir of the conventional current
-thumb gives the dir of motion (force) of the wire

Question 9

magnetic flux density

Answer 9

the strength of the field
Tesla T
1T= 1Nm-1A-1
mfd is 1T when a wire carrying a current of 1A placed perpendicular to the mf experiences a force of 1N per metre of its length

Question 10

force on a current-carrying conductor equation

Answer 10

F=BILsinθ
B is mfd. L is length of wire. θ is angle between mf and current direction
when the wire is perpendicular to the mf θ=90 degrees and sinθ=1
therefore F=BIL
B=F/IL

Question 11

determining magnetic flux density in the lab

Answer 11

two magnets placed on top-pan balance
the mf between them is almost uniform
a stiff copper wire is held perpendicular to the mf between the two poles
the length L of the wire in the mf is measured with a ruler
using crocodile clips, a section of the wire is connected in series with an ammeter and a variable power supply
the balance is zeroed when there is no current in the wire
with a current I the wire experiences a vertical upwards force
according to newtons third law of motion, the magnets experience an equal downward force, which can be calculated from the change in the mass reading
the mfd B between the magnets can then be determined from the equation B=F/IL

Question 12

an electron deflection tube

Answer 12

shows that a charged particle moving in a magnetic field experiences a force
the force on a beam of elecs can be predicted using flemings LHR
the beam of elecs is moving from left to right into a region of uniform magnetic field
as the elecs enter the field they experience a downwards force
the elecs change direction but the force F on each elec always remains perpendicular to its velocity
the speed of the elecs remains unchanged because the force has no component in the direction of motion
once out of the field the elecs keep moving in a straight line
a current carrying wire in a uniform mf experiences a force as each elec moving within the wire experiences a tiny force

Question 13

find the force acting on a charged particle of charge Q moving at a speed v at right angles to a uniform mf of flux density B

Answer 13

F=BQv

F=Bev

Question 14

circular orbits of charged particles

Answer 14

charged particle of mass m and charge Q moving at right angles to a uniform mf of flux density B
the particle will describe a circular path because the force acting on it is always perpendicular to its velocity
BQv=mv2/r

Question 15

velocity selector

Answer 15

a device that uses both electric and magnetic fields to select charged particles of specific velocity
consists of two parallel horizontal plates conncted to a power supply
they produce a uniform electric field of field strength E between the plates
a uniform mf of flux density B is also applied perpendicular to the ef
the charged particles travelling at diff speeds to be sorted enter through a narrow slit
the ef and mf deflect them in opposite directions- only for particles with a specific speed v will these deflections cancel so that they travel in a straight line and emerge from the second narrow slit Z
for an undeflected particle:
electric force=magnetic force
EQ=BQv
v=E/B

Question 16

electromagnetic induction with coil and magnet

Answer 16

a sensitive voltmeter attached to the coil shows no reading when the coil and the 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
repeatedly pushing and pulling the magnet will induce an alternating current in the coil
the faster the magnet is moved the larger is the induced emf

Question 17

electromagnetic induction with dc electric motor

Answer 17

a simple dc electric motor in reverse induces an emf
e.g. using a falling mass to rotate the coil between two poles of the stationary magnet
the induced emf can be large enough to operate a lamp
an emf is induced in a loop of copper wire when it is moved perpendicular to the mf lines of a magnet
the magnitude of the emf is bigger when the wire is pulled away faster from the mf

Question 18

emf induction explanation

Answer 18

energy is always conserved
so where does the electrical energy produced in the coil come from?
some of the work done to move the magnet is transferred into electrical energy
the motion of the coil (and the elecs in it) relative to the mf makes the electrons move because they experience a magnetic force given by Bev
the moving elecs constitute an electrical current within the coil, so the process has produced electrical energy

Question 19

magnetic flux

Answer 19

the product of the component of the magnetic flux density perpendicular to the area and the cross sectional area
magnetic flux Φ = (Bcosθ) x A
when the field is normal to the area cos0°=1 and Φ =BA
SI unit for Φ is the weber Wb

Question 20

magnetic flux linkage

Answer 20

the product of the number of turns in the coil N and the magnetic flux
magnetic flux linkage= NΦ
SI unit is also the weber Wb

Question 21

an emf is induced when…

Answer 21

there is a change in the magnetic flux linking the circuit

since Φ =BAcosθ, you can induce an emf by changing B, A or θ

Question 22

Faraday’s law of electromagnetic induction

Answer 22

the magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage
ℰ∝Δ(NΦ)/Δt
this relationship can be written as an equation where the constant of proportionality is equal to -1:
ℰ= -Δ(NΦ)/Δt

Question 23

Lenz’s law

Answer 23

the direction of the induced emf or current is always such as to oppose the change producing it
an expression of conservation of energy

Question 24

Lenz’s law demonstration

Answer 24

coil and magnet arrangement with the wires connected tog (no voltmeter) so that any induced currents in the coils are large enough to create their own strong mf
the direction of the induced emf and hence the current, changes direction when the magnet is pulled away from coil instead of being pushed into the coil.
-when the N pole of a magnet is brought close to end X of a coil, you have to do work to push the magnet towards the coil. the work done on the magnet is equal to the electrical energy produced in the coil. X must have a north polarity.
-when the magnet is pulled away from the coil, the motion of the magnet must once again be opposed so that you must do work. the end X therefore has a south polarity and the induced emf and current are reversed

Question 25

negative sign in Faraday’s law

Answer 25

a mathematical way of expressing Lenz’s law

Question 26

alternating current generator

Answer 26

simple ac generator consists of a rectangular coil of cross sectional area A and N turns of coil rotating in a uniform magnetic field of flux density B
flux linkage= NΦ = N(BAcosθ) = BANcosθ
as the coil rotates at a steady frequency, the flux linkage changes with time t. this variation is referred to as sinusoidal and is caused by the changing cosθ factor

Question 27

simple transformer

Answer 27

consists of a laminated iron core, a primary coil and a secondary coil
an ac is supplied to the primary coil
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 the secondary coil and none is lost
according to Faradays law, a varying emf is produced across the ends of the secondary coil

Question 28

turn-ratio equation

Answer 28

ns/np=Vs/Vp

Question 29

step up transformer

Answer 29

has more turns on the secondary coil and Vs>Vp

Question 30

step down transformer

Answer 30

has fewer turns on the secondary coil and Vp>Vs

Question 31

efficient transformers

Answer 31

for a 100% efficient transformer the output power from the secondary coil is equal to the input power into its primary coil
VsIs=VpIp or Ip/Is=Vs/Vp
so in a step up transformer the voltage is stepped up but the current is stepped down

Question 32

how can you make transformers efficient?

Answer 32

by using low-resistance windings to reduce power losses due to the heating effect of the current
making a laminated core with layers of iron separated by an insulator helps to minimise currents induced in the core itself, so this too minimises losses due to heating
the core is made up of soft iron which is very easy to magnetise and demagnetise and this also helps to improve overall efficiency

Question 33

national grid

Answer 33

transports electrical power across the country
consists of transformers and cables on pylons and underground
electrical power is transmitted at high voltage so as to minimise heat losses in the transmission cables