Motion and Forces in Electric and Magnetic Fields.

Question 1

Explain Electric Charge.

Answer 1

1. Uncharged objects have an equal number of + and - charges.
2. So if extra electrons are added to an object it becomes negatively charged .
3. A positive charge on an object actually arises, not through addition of + charge, but because electrons are removed because positive charges in a solid cannot move.

Question 2

Where do charge tend to gather?

Answer 2

Charge tends to gather at areas where the radius of a conductor is smallest (sharpest) and hence lightning will always strike to the sharpest object around, such as a tree branch or a church steeple.

• In a storm it is the large electric field between a cloud and the Earth that causes the air to become ionised and makes it able to conduct electricity.

Question 3

Why does a negative charge have a higher potential energy than a positive charge ?

Answer 3

This is because work can be done when the electrons are repelled from an area of negative to positive charge.

-Heat will be generated as the charge moves through a medium.

Question 4

What do we define a “conventional”?

Answer 4

A conventional view of high potential as an area with excess + charge (imagining + charges moving).

-The potential difference between two points A and B is defined as the work done in transferring one coulomb of + charge from A to B.

Question 5

What is potential difference?

Answer 5

Potential Difference is measured in volts and is obviously equal to the energy per coulomb for this transfer.

V = W/q

Question 6

What is a field?

Answer 6

A field is a region where a non-contact force is felt and its direction is shown by the direction of the force on a positive charge.

-The closer the lines of Force the stronger the electric field.

Question 7

What is the field like between parallel plates with opposite charges?

Answer 7

The field in between the plates are uniform (except at the ends).

Formula: E=V/d

Question 8

What is the force on a charge in a field E in other fields?

Answer 8

Electric Force :

Fe=Eq

Question 9

What happens when a charge is placed on a hollow conductor?

Answer 9

1. All the charges repel so as to get as far away as possible from each other and hence they must all reside on the outside of the cylinder; there can be no charge on the inside surface.
2. Since all field lines must come out from charges, it follows that there can be no field inside a hollow conductor and that the electric field E will spread outwards from the outer surface.

Question 10

How can an electron be accelerated?

Answer 10

By allowing it to be attracted by a large positive potential.

-e.g. older, glass-screen, televisions and Cathode Ray Tubes.

Question 11

Explain in steps how electrons are accelerated.

Answer 11

1. The electron is emitted by a hot cathode and moves through a vacuum towards an anode at a high positive potential.
2. The electrons gain kinetic energy from acceleration in the electric field and by the time they reach the anode they are moving at a fraction of the speed of light.
3. However, the anode is hollow, so the electrons can pass straight through it to collide with a phosphor on the TV screen.
4. Here the electron’s KE is converted into a spot of light which goes to make up the TV picture.
5. The electrons can be deflected by current-carrying coils placed either side of the tube which can position the emerging electrons to any point on the screen to produce an image.

Question 12

When a charged particle is fired through a magnetic field it has a magnetic force exerted on it according to the equation:

Answer 12

F=Bqv

-This force will always be at right angles to the motion of the particle so its motion will be circular due to the centripetal acceleration.

1. Bqv = mv^2/r
2. r=mv/Bq

Question 13

How does the Mass Spectrometer work?

Answer 13

1. Charged ions pass through the velocity selector box and are reflected by the magnetic field to the left and to the right by the electric field.
2. Only ions with the correct velocity will be able to emerge through the slit at the end without being deviated.

Question 14

How does the velocity selector work?

Answer 14

-If the electric field (E) and the magnetic field (B) are placed at right angles to each other then B will deflect the + charges up while E deflects them downwards.

2. The upward magnetic force on the ion will be:
   Fm=Bqv
3. The downward electric force:
   Fe=Eq

Question 15

Why don’t we want any deflection in the charged particles?

Answer 15

• So, for there to be no deflection Fm must equal to Fe.
• So that only particles with a velocity of v=E/B will not be deflected and hence pass through the slit at the end.
• Particles with any other velocity will be deflected away from the slit and not pass through.

Question 16

Explain in steps how does the velocity selector work.

Answer 16

1. Ions with the same selected velocity then enter the deflecting magnetic field and move in a circle to strike a detector plate where an electrical signal is transmitted to a computer locating its position from the emission point (giving a value of the circle radius).
2. The computer applies the formula r=mv/Bq to calculate the m/q value and identifies the ion.

Question 17

What is a Cyclotron?

Answer 17

The cyclotron works by accelerating charged particles between successive chambers at higher and higher potentials with magnetic fields at right angles which deflect the particles into a circular path.

Question 18

How does the cyclotron work?

Answer 18

1. The charged particles are accelerated between anodes by gradually increasing the voltage between them.
2. However, the time for the particles to reach the next anode will be shorter each time because the velocity is increasing so the time the next anode will be shorter each time because the velocity is increasing so the time the next anode in the sequence is turned on must get shorter and shorter.
3. This shortening of time switching is controlled by a computer and can be understood better by considering how a linear accelerator works.

Question 19

What is the disadvantage of a Linear Accelerator.

Answer 19

The linear accelerator was built first but had the disadvantage of only being able to accelerate ions in a straight line for a single cycle.

Question 20

How is the cyclotron different from a linear accelerator?

Answer 20

The cyclotron used this idea for phased voltages but bent the accelerating tube into a circle so the cycle of acceleration could occur continuously many times to give the particles energies in the GeV range. (10^9)

Question 21

How did Cyclotrons of the 1960s and 70s worked?

Answer 21

1. They used superconducting magnets which were coils of wire cooled to a very low temperature so their resistance was virtually zero.
2. This meant that extremely high currents in the coils could produce massive fields and result in more energetic particles.