Magnetic Fields

Question 1

The shape of a magnetic field produce by a solenoid is similar in shape to

Answer 1

The field of a permanent bar magnet

Question 2

Fleming’s right-hand rule

Answer 2

Predicts the direction in which an induced current flows. For example when a wire is moved through a the magnetic field between two magnets

Question 3

Fleming’s left-hand rule

Answer 3

Predicts the direction of the force in a current-carrying conductor in a magnetic field. For example the direction of rotation of an electric motor. This is for CONVENTIONAL CURRENT

Question 4

Field

Answer 4

A region of space where force is exerted on an object by a non-contact force/by a virtue of the object’s mass, charge or magnetic properties

Question 5

Magnetic flux density

Answer 5

A quantity which represents the strength of a magnetic field, symbol is B. Unit is Tesla, T, flux density of 1 T causes a force of 1 N to be exerted on every 1 m of wire carrying a current if 1 A in a direction perpendicular to the field

Question 6

Flux linkage

Answer 6

It is the product of the magnetic flux and the number of turns in a wire, for a conducting coil. Unit is the Wb turn. ΝΦ = BAN cos θ

Question 7

The magnetic force F on a wire of length l carrying current I in a field of flux density B is given by

Answer 7

F = BIl

Question 8

Magnetic flux density defined as

Answer 8

The force per unit length per unit current on a wire placed at 90° to the direction of the magnetic field

Question 9

Derive the equation for the radius of the circular path of charged particles in a magnetic field

Answer 9

t = l/v, l = vt, I = Q/t, F = BIl = B x Q/t x vt, F = BQv

BQv = mv²/r, r = mv/BQ

r inversely proportional to specific charge, r proportional to momentum, mv

Question 10

Derive equation for time of a charged particle to complete one full circle in a cyclotron, t is time to complete semicircle, v is speed of particle, r is radius of semicircle

Answer 10

t = πr/v, BQv = mv²/r, v/r = BQ/m
→ t = πr/v = mπ/BQ
→ time for complete circle = 2t = 2mπ/BQ

Question 11

What is the frequency of the alternating pd required for the acceleration of the particles in a cyclotron given by?

Answer 11

f = 1/T = BQ/2mπ

Question 12

What are the limitations of a cyclotron?

Answer 12

• the spiral path of the particles being accelerated needs to be synchronised with the frequency of the alternating pd. Mass of a particle increases as it gets closer to the speed of light. This means the particle stops accelerating as its approach becomes out of phase at each dee.

Question 13

Cyclotron

Answer 13

A circular particle accelerator in which the accelerated charged particles follow an outward spiralling path

Question 14

Synchrotron

Answer 14

A particle accelerator in which particles are accelerated as they travel in a circular path of constant radius. This is achieved by varying the the frequency of the accelerating voltage and the strength of the magnetic field so they can be synchronised with the increasing mass of the particles

Question 15

What path does a charged particle moving at right angles to a uniform magnetic field follow?

Answer 15

A circular path

Question 16

Magnetic flux

Answer 16

The number of magnetic field lines passing through a closed surface, such as an area. Calculated from the product of the magnetic flux density and the area. Unit is the Weber (Wb)

Question 17

Magnetic flux is given by

Answer 17

Φ = BA, where B is normal to the area.
If B not normal to the area, Φ = BA cos θ, where θ is the angle between the normal to the plane of the coil and the direction of the magnetic field

Question 18

What is the unit of magnetic flux?

Answer 18

The weber (Wb) where 1 Wb = 1 Tm²