Magnetic Fields:

Question 1

Magnetic field:

Answer 1

Force field around a magnet or current wire which acts on any other magnet or current wire in the field. Goes from north to south by aligning itself with earths magnetic field.

Question 2

Motor effect:

Answer 2

Current wire placed at a non zero angle to magnetic field experiences a force.

Question 3

When motor effect is greatest:

Answer 3

At 90 degrees to the magnetic field.

Question 4

Magnetic flux density:

Answer 4

Force per unit length per unit current on a current carrying conductor at a right angle to magnetic field. In N per metre per amp.

Question 5

F=BILsin theta:

Answer 5

Component of magnetic field is perp. to current so this equation exists.

Question 6

Couple on coil in a mag. field:

Answer 6

Each side experiences a force in opposite directions due to the mag field so moments applied where distance is the per distance between the line of action of the forces.

Question 7

Circular motion of electron beam in mag. field:

Answer 7

Experiences force perpendicular to direction of motion due to mag. field so circular motion happens.

Question 8

Prove F = BQv

Answer 8

I =Q/t and L =vt so F =BIL becomes F=BQv or F=Bev (also becomes F=BQvsin theta if it is not perp to the field).

Question 9

Why magnetic field doesn’t do work on a moving particle:

Answer 9

Force is perpendicular to the motion so it only changes the direction and not the speed and Ek stays the same.

Question 10

Why must equipment for moving charge particles be evacuated first:

Answer 10

To prevent loss of speed of charged particles by collisions with air molecules / particles.

Question 11

What cyclotron is used for:

Answer 11

Produce high energy beams for radiation therapy.

Question 12

How cyclotron works:

Answer 12

2 Dees with magnetic field running through them ( perp to the planes).
High energy alternating voltage across them in vacuum chamber.

1)Charged particles directed into a dee near the centre of the CT. They are forced on circular path by the magnetic field.
2)When it crosses into another dee the voltage reverses and they accelerate into the other dee.
3) This process is repeated.

They accelerate because the time taken for the particle to move around does not depend on its speed.

4) Particles radius increases each rotation and when it is equal to the radius of the dee then it is ejected.

Question 13

Electromagnetic induction and how increase it:

Answer 13

EMF induced in a wire when it cuts across the lines of a magnetic field (if it part of a complete circuit then the induced EMF forces electrons around the circuit.

Increase : move wire faster, use stronger magnet make wire into a coil and pushing the magnet in and out of the coil.

Question 14

Other ways to induce an EMF:

Answer 14

Use electric motor in reverse - falling weight makes coil turn and then an emf is induced and forces a current around the circuit.

Using a cycle dynamo - EMF is induced in the coil as the dynamo spins (magnet on a stick that spins???)

Question 15

Work done in an induced emf:

Answer 15

E.g. in a cycle dynamo, the work done to keep it spinning is equal to the energy of induced EMF. Rate of energy transfer from the source of induced EMF is equal to EMF supplied to components

Question 16

Energy transferred per second in induced EMF equation:

Answer 16

E * I = energy transferred per unit charge. Therefore: E * I * Q = Energy transferred from the source per second.

Question 17

Explain why EMF is induced:

Answer 17

When a metal rod or wire etc is moved across a magnetic field or vice versa then all the free electrons are forced to one end of the rod. Hence one end is negative and the other positive so emf is induced.

If the relative motion between rod and field stops then the EMF stops as the field stops exerting a force on the free electrons

Question 18

Which way current travels in north pole of a solenoid:

Answer 18

North pole - aNticlockwise

Question 19

Lenz’s Law:

Answer 19

Direction of induced emf always is such that it poososes the change that caused the current.

Question 20

Explanation of Lenz’s Law:

Answer 20

Energy must be conserved so the induced current can’t be in a direction to help the change that caused it as this would create electrical energy out of thin air.

Question 21

Prove faradays law:

Answer 21

Work done by applied force to conductor is W = Fs = BIls
Charge transferred along the conductor in the same time is Q = It

Therefore induced E = W/Q = BILs/It

I cancels out and L change in s is the area swept out by the conductor.

Therefore E = BA/change in time

Question 22

Weber:

Answer 22

Unit of magnetic flux = BA

1 Wb = 1Tm^2

it only acts at right angles.

Question 23

Faradays law:

Answer 23

Induced EMF in a circuit is equal to the rate of change of flux linkage through the circuit.

Question 24

Why does faradays law include a minus sign:

Answer 24

Indicates that the induced emf acts in a direction as to oppose the change that caused it.

Question 25

Alternate EMF equation:

Answer 25

E = Blv as A = l*change in s

Question 26

How does fixed coil in changing magnetic field create induced emf:

Answer 26

Current in the solenoid changes so magnetic field changes so an emf is induced in the coil

Question 27

Peak value of AC current:

Answer 27

Peak pd or current in either direction.

Question 28

Peak to peak value:

Answer 28

Difference between the peak value one way and the peak value in the opposite direction.

Question 29

Waveform:

Answer 29

Variation with time on an oscilloscope.

Question 30

How lamp varies connected to oscilloscope of ac source:

Answer 30

Lamp lights up at peak value in either direction - fades between these peak values.

Question 31

Sinusoidal current:

Answer 31

Mean power over a full cycle is half the peak power - 1/2I0^2R where I0 is peak power.

Question 32

Root mean square of AC:

Answer 32

Direct current that would give the same power and heating effect as AC in the same resistor - root mean square.

Therefore, (Irms)^2R = 0.5I0^2R

Question 33

Describe transformer:

Answer 33

Primary coil and secondary coil with he same iron core.

When primary is connected to ac pd an alternating magnetic field is produced in the core which passes through the secondary coil so an alternating emf is induced in the 2nd coil.

Question 34

Transformer rule:

Answer 34

Vs/Vp = Ns/Np

Question 35

Difference between step up and step down transformer:

Answer 35

Step up - more turns on secondary coil.
Step down - Fewer turns on secondary coil so voltage is decreased.

Question 36

How efficiency of transformers is increased:

Answer 36

• Low resistance windings to reduce power wasted through current heating’s.
• Laminated core (layers of iron separated by insulator) which reduced induced currents in the actual core / eddy currents so magnetic flux is high as possible.
  -core is soft iron so is easily magnetised and demagnetised which reduced wasted power through repeated magnetisation and demagnetisation.

Question 37

Efficiency of transformer equation:

Answer 37

Power to secondary coil/power ot primary coil

Question 38

Back emf in the transformer:

Answer 38

Changing flux creates back emf in both coils. Acts against primary voltage which makes the primary current very small when the secondary current is ‘off’.

When secondary is on, it opposes primary coils magnetic field so back emf is reduced in primary coil.

Primary current is larger than when when the secondary current is off.

Question 39

Why is transmission of power over long distance more efficient at high volts than lower volts:

Answer 39

Current needed to deliver a certain amount of power is reduced if voltage is increased so power wasted due to heating is reduced.