Magnetic

Question 1

What is the motor effect?

Answer 1

when you put a current carrying wire in a magnetic field at a non-zero angle, a force acts on the wire

Question 2

Magnetic flux density

Answer 2

the force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines

Question 3

What is a simple electric motor

Answer 3

• it consits of a coil of insulated wire which spins between the poles of a U-shaped magnet

When a current passes through round the coil, the wires at opposite edges of the coil are acted on by forces in opposite directions

the force on each coil makes it spin about its axis

Question 4

How does the split ring commutator work?

Answer 4

The direction of the current round the coil is reversed by the split ring commutator each time the coil rotates through a half turn. This ensures the current along an edge changes direction when it moves from one pole face to the other.

Question 5

Why do electrons travel in curved paths when entering a magentic field?

Answer 5

The direction of force on each electron is perpendicular to the direction of motion

r= mv/BQ

Question 6

At angle to the lines of a field

Answer 6

There is a component of B

therefore F = BQvsinθ

Question 7

Cyclotron how does it work

Answer 7

• Two hollow metal Dee’s with a magnetic field acting perpendicular
• Proton source situated in the middle in an air gap between the Dee’s
• Alternating voltage across the Dee’s
• Electric field between the two Dee’s
• A particle is emitted and is accelerated to one plate
• As it enters the Dee it obeys circular motion as the direction of motion is perpendicular to the magnetic force
• As it leaves the Dee the potential difference reverses and the particle accelerates to the other Dee
• the particle gains speed while accelerating to the other Dee and the radius increases
• The radius of the particle increases until the particle fires out of the cyclotron

Question 8

What is the Time period of a cyclotron?

Answer 8

T = 2mπ/BQ therefore alternating frequency for voltage f = BQ/2mπ

Question 9

What is electromagnetic induction?

Answer 9

the generation of EMF when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines

Question 10

Peak to peak voltage

Answer 10

the difference between peak negative and peak positive voltage

Question 11

Dc voltage graph is a horizontal line at the value of Vrms

Answer 11

it is a horizontal line at the value of Vrms

Question 12

Two examples when a charged particle will not experience a force in a magnetic field

Answer 12

• when a charged particle is at rest it will not experience a force as BQr =mv
• when a charged particle moves parallel to magnetic field lines. Sin(0) is equal to 0 therefore no magnetic force

Question 13

Peak emf

Answer 13

BANω

Question 14

When you place an object in a hollow tube which is a permanent magnet

Answer 14

• the object falls slowly through the tube
• the object experiences a rate of change of flux linkage as it cuts through magnetic flux
• this causes an induced current in the object (as it is a metal)
• the current then produces its own field
• the two fields interfere
• this causes an upwards force on the object which causes it to slow down
• you could also say Ek is lost to the current causing a heating effect on the tube

Question 15

How would the hollow tube experiment change with a magnet of a greater resistivity with the same dimensions?

Answer 15

• resistivity is proportional to resistance
• this means the resistance is greater
• a smaller magnetic field is produced
• therefore smaller induced emf
• the object will fall more quickly
• the opposing magnetic field will be smaller

Question 16

What causes Eddy currents?

Answer 16

Induced current in the iron core itself which produces a heating effect. This reduces the efficiency of the transformer.

Question 17

Soft iron core in transformers

Answer 17

• iron core focuses and directs the magnetic field from the primary to the secondary coil
• soft iron is used as it s easily magnetised and demagnetised

Question 18

How to keep high efficiency of transformers?

Answer 18

• low-resistance windings to reduce power wasted due to the heating effect of the current
• a laminated core which consists of layers of iron separated by insulators help prevent eddy currents from forming
• a core of soft iron which is easily magnetised and demagnetised reduces power wasted
• eddy currents can be reduced by having a core made from a metal of high resistivity

Question 19

Flux density and time graph

Answer 19

EMF is proprtional to delta B divided by delta t

Therefore a straight line on this graph indicates constant induced EMF

Question 20

EMF time graph

Answer 20

• horizontal line with 0 gradient indicates constant EMF
• a straight line with gradient indicates there is a change in EMF

Question 21

What is the phase relationship between EMF and magmetic flux linkage

Answer 21

they are π/2 rad out of phase

This is because when you have max magentic flux, the gradient is 0

This means there is no rate of change of flux linkage therefore there is no EMF

Question 22

What happens when you drop a coil through a magnetic field?

Answer 22

• As it enters the magnetic field, the rate of change of flux linkage increases due to the increase of speed
• When it is halfway through the magnetic field, the EMF and current flips at this point
• This is due to Lenz’s law
• The rate of change of flux linkage at this point is now greater as the coil is moving even faster
• Therefore induced EMF is greater

Question 23

What to do when calculating maximum magnetic flux linkage?

Answer 23

Dont use an angle it should be 90 degrees! therefore x 1

Question 24

Units for Flux linkage

Answer 24

Wb turns

Question 25

Define the tesla

Answer 25

The tesla is the strength of the magnetic field that produces a force of 1 newton in a wire of length 1m with 1 ampere.

Question 26

The time taken by a proton to travel round a semi circle in a cyclotron is independent of the radius of the path

Answer 26

this is because t = πm/BQ

Using circular motion = magnetic force

removing v and r with time