Magnetic fields

Question 1

In F=BIL, what is B and what is its unit

Answer 1

Magnetic flux DENSITY

T.

Question 2

Units of Magnetic Flux

Answer 2

Wb

Question 3

How many/which longitudinal waves are there (with rarefactions and compressions):

Answer 3

Sound waves and seismic waves

Question 4

Rules for conventional current with a wire passing perpendicular through a magnetic field, and a free electron passing through a magnetic field.

Answer 4

Wire:
-Conventional current, so don’t need to flip your hand for LHR
Free electron:
-The motion of the free electron directed on a question is the OPPOSITE to conventional current, therefore you must flip the current to the opposite direction

Question 5

Circular motion with a free electron in a magnetic field:

Answer 5

-F perpendicular to V
(Motion of electron charge referred to as its velocity)
-Speed is constant
-Therefore KE is constant
ONLY GOES IN A PERFECT CIRCLE IF THE ELECTRONS VELOCITY IS ON THE PLANE THAT IS PERPENDICULAR TO THE MAGNETIC FIELD
(unlike where we fire an electron into an electric field where we see acceleration (its speed changing))

Question 6

When do free electrons spiral:

Answer 6

When the motion of the electron isnt exactly perpendicular to the magnetic field so only part of the velocity is perpendicular so it spirals

Question 7

What happens if a free moving positive charge is fired into an electric field?

Answer 7

You dont flip LHR for current as it already is conventional current as it means THE FLOW OF POSITIVE CHARGES

Question 8

Motor effect

Answer 8

-LHR Applies (think cars drive on the left) and is the effect described when a force is applied on an object that is interracting with a magnetic field

Question 9

F=BQV

Answer 9

For a free moving particle
v in this has to be perpendicular to field for this to be true.
F=BeV (for an electron)

Question 10

Linking motion of a free electron in a magnetic field force wise

Answer 10

When a moving charge is moving in a circle of radius r. the force perpendicular to the v F=BQV then we know the force is equal to CENTRIPETAL FORCE needed to make it go round in a circe.
BQV=mv^2/r (can cancel v’s)
r proportional to mv, so lighter particles results in a smaller radius (spirals when loses energy from collisions)

Question 11

SPINNING/CIRCULAR MOTION OF POS IONS AND ELECTRONS

Answer 11

Spin/make a circle in opposite directions due to the flow of charge motion (velocity)

Question 12

Mass spectroscopy

Answer 12

Heavier ions have a bigger radius therefore sensors are set up, lighter ions curve towards the film
ALL NEED TO HAVE SAME V.
-Achieved by a velocity selector, where the force has to be equal to an opposing force, created by using positive and negative plates (forming electric field)
Therefore BQV=EQ
BV=E V=E/B therefore velocity selected is constant as it is relative to the values of the field (not the particles)

Question 13

Cyclotrons

Answer 13

-Used to create a single stream of high energy protons
(can be used in nuclear physics or medical situations)
-Need radius of protons to increase, achieved by accelerating when they cross the gap, through positive and negative charges. (Oppositely charged dees) charges must be flipped at either end. (every half a turn)
Time of one complete loop
T=2pi/T T=d/s T=2piR/s Time=2piR/(BQR/m) Try rearrangements. 1/t gives frequency
TIME INDEPENDENT OF RADIUS DUE TO REARRANGEMENT, THEREFORE ALL PROTONS CROSS GAP AT THE SAME TIME REGARDLESS OF POS. (as t not proportional to r in eq)

Question 14

Dynamo effect

Answer 14

Dynamo effect tries to stop motor effect (if you try motor effect on opposite hand, direction of current tries to oppose, and same with force)

Question 15

Lenz Law:

Answer 15

The direction of an induced current is such that it will oppose the change that caused it

Question 16

Solanoid:

Answer 16

Magnet S
[ ] drops into
N
-When magnet is about to come in a north pole is induced at the top to try resist this (therefore south must be at bottom)
-Magnet comes in, its magnetic flux cuts the wire so a current is induced in solanoid wire
-That wire produces its own magnetic field which actually opposes the magnet coming in
-As it falls through, halfway throw magnet is classed as being on its way out (solanoid doesnt want it to leave) so current in solanoid and circuit flips
-And attracts it at the end to try stop it from leaving (therefore North is now at the bottom end to try attract the south end back)
ALL DUE TO LENZ LAW
*Magnet reaches a terminal velocity as a force is produced that is equal and opposite to the MAGNETS weight (due to lenz law) so falls at TV. (constant v all the way down)
(magnets dont like change)

Question 17

In the presence of a suitably directed uniform magnetic field, charged particles move at constant speed in a circular path of constant radius. By reference to the force acting on the particles, explain how this is achieved and why it happens.(4 marks)

Answer 17

• Magnetic field acts perpendicular to path of particle
• Force acts perpendicular to path of particle
• Force depends on speed of particle
• Force provides centripetal acceleration towards centre of circle

Question 18

The speed of the protons gradually increases as their energy is increased by the LHC.
State and explain how the magnetic field in the LHC must change as the speed of the protons is increased. (2 marks)

Answer 18

-Magnetic field must be increased
-To increase centripetal force/ keep r constant
(otherwise protons would attempt to travel in a path of longer radius)

Question 19

B particle moving throufh field compared to alpha particle. what is m of alpha particle compared to a b particle

Answer 19

mass is 2m

as we are only counting masses of electrons and 2 electrons in alpha particle compared 1 electron for B

Question 20

The balance, which was zeroed before the switch was closed, read 161 g after the switch was closed. When the current is reversed and doubled, what would be the new reading on the balance?
A - -322g
B- -161g
C- zero
D- 322g

Answer 20

A

Question 21

Back EMF

Answer 21

Coil spinning in a field
When parallel to field lines:
Flux(work function sign) = 0
Change of emf here: max therefore emf max
When perpendicular to field lines:
Flux = Max
Emf= 0
THINK FISHING NET

for emf induced in a rotating coil, use cos if cos graph (if emf started at max, parallel to lines)
Use sin normally

When a motor turns, dynamo effect is also produced (due to lenz’s law)

• Opposing emf is high when spinning fast (no load)
• Opposing emf low when spinning slower (heavy load)

Question 22

State two situations in which a charged particle will experience no magnetic
force when placed in a magnetic field. (2 marks)

Answer 22

• When charged particle is at rest

- When charged particle moves parallel to magnetic field

Question 23

A charged particle moves in a circular path when travelling perpendicular to a
uniform magnetic field. By considering the force acting on the charged particle,
show that the radius of the path is proportional to the momentum of the
particle. (2 marks)

Answer 23

-BQV=mv2/r
Therefore BQ=mv/r
r=mv/BQ
-FOR SECOND MARK, HAVE TO SAY:
B and Q are CONSTANT so r is proportional to momentum(mv)

Question 24

In a second experiment, an alternating current is passed through the wire. Explain
why the wire will vibrate vertically (3 marks)

Answer 24

• force F is perpendicular to both B and I
• magnitude of F changes as size of current changes
• force acts in opposite direction when current reverses
• continual reversal of ac means process is repeated