Magnetic Fields

Question 1

Flemming’s left hand rule

Answer 1

For current in magnetic field
Thumb= force
First Finger= Field (north to south)
Second Finger= conventional current

Question 2

Right hand grip rule

Answer 2

Finding the direction of magnetic field from a current
Thumb= direction of current

Question 3

Define Tesla

Answer 3

1 Tesla is the magnetic flux density that causes a 1 newton force to act on 1 metre of wire carrying 1 amp of current

Question 4

When can the equation F=Bqv be used?

Answer 4

When the field is perpendicular to velocity

Question 5

How is the force different on positive and negative charged particles.

Answer 5

Opposite direction on negative particles (as second finger will be in the opposite direction)

Question 6

Explain how a cyclotron works

Answer 6

Consists of 2 dess which are attached to an alternating supply
The dees are placed in a vacuum chamber and a magnetic field which acts at right angles to them
A particle will move in a circle because of the magnetic field. When it reaches the gap between the dees the alternating supply makes the opposite charge to the particle. This causes the particle to accelerate across the gap and enter the second dee. Its radius will increase as its velocity increases. This continues to happen, with the velocity of the particle increasing until it is at the required speed, where It will leave the cyclotron

Question 7

Explain how a mass spectrometer works

Answer 7

Particles to be identified (such as nuclei) are accelerated using an electric field. Then, a velocity selector is used to ensure all the nuclei are at a known velocity - all the rest are discarded. These nuclei enter a uniform magnetic field where they move in a circle. However, they are only allowed to move half a circle, since they are collected at this point, and the number of particles arriving at each point is measured.

Question 8

How does electromagnetic induction occur?

Answer 8

When a conducting rod is moved in a magnetic field the electrons in the rod will experience a force and build up on one side of the rod. This will cause an EMF to be induced on the rod as a current is induced. Similarly if a bar magnet is moved relative to a coil of wire, if the coil forms a complete circuit, then a current will be induced.

Question 9

What is Faraday’s law?

Answer 9

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

Question 10

What is Lenz’s law?

Answer 10

The direction of the induced EMF is such that it opposes the change that induces it.

Question 11

What happens when you move a straight line conductor through a magnetic field?

Answer 11

The electrons feel a force, pushing them to one side of the conductor, creating a emf across the conductor. The rod obeys Faraday’s law as it is the changing flux that causes the emf to be induced.

Question 12

Fleming’s right hand rule

Answer 12

First finger= field
Second= direction of conventional current
Thumb= direction wire is moved

Question 13

What is the root mean square voltage?

Answer 13

The rms voltage represents the dc voltage that dissipates the same power as the average power dissipated by the a.c. voltage. Or it is the square root of the mean of the squares of all the values of the voltage in one cycle

Question 14

How to read an oscilloscope

Answer 14

X-axis= time, with scale for time base eg 0.2s per cm
y-axis= voltage with scale eg. 10v per cm

Question 15

Explain the purpose of the core in a transformer

Answer 15

The core directs the the magnetic field round to the secondary coil

Question 16

State the main causes of inefficiency in a transformer

Answer 16

Eddy currents
Energy lost due to resistance in the coils
Energy can be lost if the core is not easily magnetised

Question 17

What is an Eddy current?

Answer 17

As the primary coil induces an EMF in the secondary coil, it also induces an EMF and hence mini currents in the iron core. These currents are know as eddy currents.Due to Lenz’s law they oppose the field that produced them, reducing the field’s flux density. They generate heat which causes energy to be lost.

Question 18

How to reduce inefficiencies in a transformer

Answer 18

To reduce eddy currents:
Use a laminated iron core, as currents cannot pass through the insulator so their amplitude is reduced
Make core out of high resistivity material
To reduce resistance in coils, use a lower resistivity material, or use a thicker wire
Use a soft iron core for easy magnetisation.