EM2

Question 1

what happens when current passes through a wire?

Answer 1

• a magnetic field is induced
• true for any straight current carrying conductor
• forming concentric rings around the wire

Question 2

what is magnetic flux density

Answer 2

• measure of the strength of the field

Question 3

what is one tesla defined as?

Answer 3

• a force of 1N on 1 metre of wire carrying 1A of current perpendicular to a magnetic field

Question 4

what happens when a current carrying wire is placed in a magnetic field?

Answer 4

• a force is exerted on the wire
• if the current is parallel to the magnetic field, the force is 0N

Question 5

what equation can be used to find the force exerted on a wire

Answer 5

F = BIL

Question 6

how can we use FLHR to find the direction of the force exerted

Answer 6

• ThuMb represents the direction of the force
• First finger represents the direction of the field
• SeCond finger represents the direction of conventional current

Question 7

what is meant by conventional current?

Answer 7

• current that flows in the opposite direction to electron flow

Question 8

why is there a force exerted on a current - carrying wire?

Answer 8

• a force acts on charged particles moving in a magnetic field

Question 9

how can we calculate the magnitude of the force exerted on a particle with a charge Q moving at a velocity V perpendicular to a field with flux density B

Answer 9

F = BQv

Question 10

how can we use FLHR for F=BQV

Answer 10

same principle as F = BIL but use the second finger as direction of travel ( v )

Question 11

why do the particles follow a circular path inside the magnetic field?

Answer 11

• the force exerted is always perpendicular to the motion of travel

Question 12

how can we calculate the radius of a particles circular path?

Answer 12

r = mv / BQ

Question 13

what are cyclotrons used for

Answer 13

• producing ion beams
• radiotherapy
• radioactive tracers

Question 14

what is a cyclotron made up of

Answer 14

• two semicircular electrodes called dees
• uniform magnetic field acting perpendicular to the plane of the electrodes
• high frequency alternating voltage applied between the electrodes

Question 15

why does the particles speed not increases inside a cyclotron?

Answer 15

• because of the magnetic field, which is why theres an alternating electric field between the electrodes

Question 16

how do the particles exit the cyclotron

Answer 16

• particles are accelerated by the electric field
• radius of their circular paths will increase
• alternating electric field changes direction allowing the particles to be accelerated again
• process repeats several times

Question 17

what is magnetic flux?

Answer 17

• value which describes the magnetic field passing through a given area
• product of magnetic flux density and given area

ø = BA

Question 18

what is magnetic flux linkage

Answer 18

• magnetic flux multiplied by number of turns N of a coil

Nø = BAN

Question 19

what happens if the magnetic field is not perpendicular to a coil of wire

Answer 19

• resolve the magnetic field vector into components

BAcosTheta
BANcosTheta

Question 20

what is meant by electromagnetic induction

Answer 20

• when a conducting rod moves relative to a magnetic field, electrons will experience a force and build up on one side of the rod
• causing an emf to be induced

Question 21

what is Faraday’s Law

Answer 21

the magnitude of the emf induced is proportional to the rate of change of flux linkage

Question 22

what is Lenz’s Law

Answer 22

the direction of the induced current is such as to oppos the motion causing it to

Question 23

how can we demonstrate Lenz’s Law

Answer 23

• measure the speed of a magnet falling through a coil of wire
• measure the speed of the magnet falling through the same height without the coil

Question 24

why does it take longer for the magnet to reach the ground when it moves through the coil

Answer 24

• as magnet approaches coil there is a change of flux through the coil so an emf is induced
• due to lenz’s law, the direction of the current is such to oppose the motion of the magnet, so the poles match causing repulsion so magnet slows down
• as the magnet passes through the centre there is no change in flux so no emf induced
• as the magnet leavs the coil, there is a change in flux, so a current is induced that opposes the motion of the magnet , therefore an opposite pole, causing attraction so the magnet slows down

Question 25

give the equation for Faradays law

Answer 25

emf = - N delta flux / delta t

Question 26

what is the equation for magnitude of emf induced by a straight conductor

Answer 26

emf = B L V

Question 27

how can we calculate the emf with frequency

Answer 27

emf = BANwsin(wt)

Question 28

what is peak voltage

Answer 28

• distance from equilibrium to the highest or lowest point

Question 29

what is peak to peak voltage

Answer 29

• distance from minimum point to maximum point

Question 30

what is root mean square voltage

Answer 30

• average of all the squares of the possible voltages

Question 31

what is time period

Answer 31

• distance from peak to peak

Question 32

why are transformer cores laminated?

Answer 32

• to reduce energy losses due to eddy currents which generate heat

Question 33

what is the purpose of a cyclotron

Answer 33

a cyclotron accelerates charged particles using a combination of a magnetic field to bend their path and an altenating electric field to increase their speed

Question 34

how does a velocity selector work

Answer 34

• ensures all ions entering the sprectrometer have the same
  speed

Question 35

link the equation Bqv = Eq

Answer 35

Bqv = Eq
Bv = E
v = B / E

Question 36

what is flux

Answer 36

a measure of how much magnetism passes through an area

Question 37

what is the role of the secondary coil ?

Answer 37

• changes the voltage level of the energy supplied

Question 38

what is the role of the secondary coil ?

Answer 38

• changes the voltage level of the energy supplied