4.4 Electromagnetism

Question 1

What is magnetic Field, B (or magnetic flux density)

Answer 1

This is a vector quantity. Its direction is that in which the North pole of a freely-pivoted magnet points. Its magnitude is defined by B = F/IL in which F is the force on a length L of wire carrying a current I, placed perpendicular to the direction of the field.
Unit: T (N/Am)

Question 2

What is Hall Voltage?

Answer 2

When a magnetic field, B, is applied to conductor carrying a current I, at right angles to the field direction, a so-called Hall voltage appears across the specimen, at right angles to the B and I directions.

Question 3

What is an electron volt?

Answer 3

The energy transferred when an electron moves between two points with a potential difference of 1 volt between them. So, for an electron being accelerated in a vacuum it is the KE acquired when accelerated through a PD of 1V
(1ev = 1.6 x 10^-19J)

Question 4

What is the formula for force on a charged particle in an electric field?

Answer 4

F = Eq
E = Electric Field strength
q =charge

Question 5

What is magnetic field strength?

Answer 5

The magnetic field strength (or flux density) is the force per unit amp meter in a magnetic field, in Tesla (T).
T units = 1N/Am

Question 6

What is the formula for magnet field strength of a solenoid?

Answer 6

B = μnI
μ = Permeability of free space (4π x 10^-7)
n = number of turns per unit length in the coil
I = Current through the coil (A)

Question 7

What is the formula for magnet field strength of a wire?

Answer 7

B = μI/2πa
μ = Permeability of free space (4π x 10^-7)
a = distance from the centre of wire
I = Current through the wire (A)

Question 8

What is the formula for force experienced by a wire in a magnetic field?

Answer 8

F = BILsin(θ)
B = Magnetic Field strength, T
I = Current through the wire, A
L = Length of wire in field
θ = Angle between flow of current and field lines

Question 9

What is the formula for force experienced by a charged particle in a magnetic field?

Answer 9

F = Bqvsin(θ)
B = Magnetic Field strength, T
q = charge of particle, C
v = velocity of particle, m/s
θ = Angle between flow of current (line of motion) and field lines

Question 10

How much energy is gained by an electron as it travels between two plates with a potential difference of 1000v?

Answer 10

1000eV
eV = 1.6 x 10^-19 J

Question 11

What are the 3 types of particle accelerators?

Answer 11

Linear - No magnetic field, straight line, increasing length tubes with alternative p.d.

Cyclotron - Constant magnetic field, circular with increasing radius

Synchrotron- Increasing magnetic field and AC, circular path with constant radius

Question 12

How is hall voltage used to find magnetic field strength (Flux density)?

Answer 12

When their is a hall voltage in the hall probe, the electrical force equals the magnetic force:
- qE = Bqv
- E = V/d
- V = Bvd

V = Hall voltage
B = Magnetic flux density (field strength)
d = distance between top and bottom of the hall probe

Question 13

Why do charged particles move in circles in a magnetic field?

Answer 13

Due to Fleming’s lefts hand rule, the force they experience will be at a right angle to their velocity (current) so they move in a circular path

Question 14

What is the radius of a charged particle in a magnetic field?

Answer 14

F = BQv
F = mv²/r

BQv = mv²/r
BQ = mv/r
r = mv/BQ

m = mass of particle
v = velocity of particle
Q = charge of particle
B = magnetic field strength (magnetic flux density)

Question 15

What is the experiment for investigation of the force on a current in a magnetic field?

Answer 15

…

Question 16

.What is the experiment of magnetic flux density using a Hall probe?

Answer 16

…

Question 17

What is a particle accelerator?

Answer 17

It consist of a series of drift tubes of increasing length which are connected alternately to an alternating current power supply.
As a charged particles move through the centre of each tubes, the AC flips meaning the ion becomes attracted to the next tube and repelled from the tube it was just inside.
Because the length of the tubes increase as the velocity increases, the time to pass through each one is constant meaning the AC can also be constant.

Question 18

What is a Cyclotron?

Answer 18

It consists of a two D-shaped electrodes separated by a small gap with an AC supply connected to them. The Dees are sandwiched between two strong magnets which provide a strong uniform magnetic field.

As the ions passes through the gap, the electric field created by the Dees causes it to accelerate. The magnetic field causes it to bend back around to pass through the gap again but because the AC caused the Dees charge to change, the particle accelerates again.
As the velocity of the ion increases so does the radius of it’s circular path until it leaves the cyclotron through an exit on the edge of the Dees.

Note: If the particle moves at relativistic speeds, it goes out of sync with the alternation of the E-field.

Question 19

What is a synchrotron?

Answer 19

It consists of many rounded electrode tubes which are connected to an AC supply and form a circular path. This is all between a variable magnetic field.

Each tube works like a linear accelerator and each attracts then repels the particle one the current direction is flipped. The magnetic field caused the particle to stay in the circular path. At the velocity increases, the frequency of the AC also increases along with the strength of the field to ensure the electrodes are still in sync and the particle stays in the circular path.
This allows the particle to reach very high speeds (even relativistic speeds) after completing as many loops.

These usually have to be very large such as the large hadron collider at