Magnetic Fields

Question 1

What happens a magnetic field is perpendicular to a current-carrying wire

Answer 1

A force is exerted perpendicular to both the direction of current and the magnetic field lines

Question 2

What does each finger represent in Flemings Left hand rule

Answer 2

Question 3

What is magnetic flux density?

Answer 3

The number magnetic field lines passing through a unit area of material

Measured in Teslas or Webers/meters^2

Question 4

What is the equation for the force felt by moving charge in a magnetic field

What assumption has to be made?

Answer 4

F = BQV

. Only applies when the motion to the charged particle is perpendicular to the magnetic field direction

Question 5

Derive the F=BQV equation

Answer 5

Question 6

Which fields do cyclotrons use?

Answer 6

An electric and magnetic field

Question 7

How does a cyclotron work? (7)

Answer 7

A cyclotron is made up of 2 semi circular electrodes called “dees”
with a magnetic field applied perpendicular to the dees

and an alternating pd applied between the Dees

Each Dee is a metal electrode with alternating and opposite charges to oneanother,
this creates a an electric field between the two dees,

Which accelerates each particle every half revolution

The magnetic field causes the particles to move in circular motion, apply a constant centripetal force towards the centre

As the particles speed up their radius of motion increases,

until it breaks free tangential to one of the dees

Question 8

What is magnetic flux?

Answer 8

ф=BA
Flux = Flux density x Area
Where B is normal to A (perpendicular)

Question 9

What is flux linkage?

Answer 9

Nф= Number of turns cutting the magnetic flux at one time

Question 10

What is the flux linkage of a rectangular coil rotating through a magnetic field

Answer 10

Nф= BAN cosx

Question 11

What is Faraday’s Law?

Answer 11

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

Question 12

What is Lenz’ law

Answer 12

The direction of the induced emf is such as to oppopse the change that induces it

to conserve energy

Question 13

What happens when you move a straight conductor through a magentic field

Answer 13

The electrons experience a force pushing them to one end of the conductor creating an EMF across the conductor

Question 14

How do you derive the radius of a charged particle when it is perpendicular to the magnetic field equation

Answer 14

Equate Centripetal force equation MV^2 / r to BQV

Question 15

What is a couple

Answer 15

A pair of equal and opposite forces acting on a body but not at the same point

Question 16

What is the equation for calculating force on a wire

Answer 16

F=BIL sinx

where sinx= 1 when current is perpendicular to magnetic field

Question 17

What is the purpose of a carbon brush in a electrical motor

Answer 17

To make electrical contacts between the circuit and the roatating motor

Question 18

What is the purpose of a split ring commutator in a motor

Answer 18

Changes the direction of the current every half rotation to make sure the coil rotates in the same direction

Question 19

The speed of a motor can be increased by

Answer 19

. Increasing the size of the current
. Number of turns on the coil
. Strength of the magnetic field

Question 20

How does an electron move in a circuit

Answer 20

Current shows directions a positive test charge would take
So electrons flow in the opposite direction to the current.
This is used to make the field line rules universal

Question 21

How is time period of a moving particles affected when accelerated in a cyclotron.

Answer 21

v = 2pi * radius / Time period (circular motion)
Put into MV/BQ = r
T = 2pi*M / BQ
No factors of radius or velocity so it stays constant

Question 22

What is the frequency of changing electric field in a cyclotron

Answer 22

T = 2piM / BQ
f = Bq/ 2pim
have to times frequency by two as has to change polarity every half turn to accelerate the particle.
f = B/pi * specific charge