Magnetism

Question 1

Is magnetism a contact force?

Answer 1

No, non-contact.

Question 2

What direction do magnetic field lines always go in?

Answer 2

North to south.

Question 3

There’s a higher magnetic flux density of there are more or less lines in a region of space?

Answer 3

More.

Question 4

Define magnetic flux, Φ. What is it measured in?

Answer 4

Net number of magnetic lines passing through a region of space, Webers.

Question 5

Define magnetic flux density, B. What is it measured in?

Answer 5

It is the amount of magnetic flux per unit area, Teslas.

Question 6

What does everything carrying an electric current cause?

Answer 6

Magnetic field.

Question 7

What rule do we use to find the direction of the magnetic field in a wire?

Answer 7

Right hand thumb rule. Thumb points in direction of the current. Magnetic field circles the wire in the direction of your fingers.

Question 8

What do circles and crosses mean in regards to the way the current is flowing?

Answer 8

Circle = current flowing towards you
Cross = current flowing away from you

Question 9

What is a solenoid?

Answer 9

A current carrying wire wrapped up into a coil.

Question 10

How do the magnetic field lines look for a solenoid?

Answer 10

Like a bar magnet around the solenoid, inside there’s a uniform field everywhere.

Question 11

What is the motor effect?

Answer 11

Put a current carrying wire in an external magnetic field and it experiences a force.

Question 12

How do we increase the motor effect?

Answer 12

External magnetic field gets bigger, current gets bigger.

Question 13

Where does a wire in a magnetic field feel the most force?

Answer 13

90 degrees.

Question 14

Where does a wire in a magnetic field feel no force?

Answer 14

Parallel.

Question 15

How does Fleming’s left hand rule work?

Answer 15

First finger along magnetic field line
Second finger at a right angle to it – point that along current
Thumb points in the direction of the force

Question 16

What is the equation for radius of curvature, how do we derive it?

Answer 16

r = mv / BQ
By combining F = mv^2 / r and F = BQv

Question 17

What is the equation for the frequency of rotation, how do we derive it?

Answer 17

f = BQ / 2πm
By combining f = v / 2πr and r = mv / BQ

Question 18

What are the six steps to how a cyclotron works?

Answer 18

1. Charged particles fired into electrode
2. Magnetic field makes them follow a semi-circular path
3. Applied p.d. between electrodes accelerates particles across the gap into the
   next semi-circle
4. Speed of particle now higher – so follows a path with a slightly bigger radius
5. When it gets to next gap, p.d. has reversed so it is accelerated again –
   speed goes up – radius gets even bigger
6. Particle spirals outwards getting faster

Question 19

What is the generator effect (Faraday’s Law)?

Answer 19

The induction of a potential difference in a conductor placed in a changing magnetic field.

Question 20

What do you have to do in order to have a changing magnetic field?

Answer 20

• Actually change the B field
• Move a conductor in a constant B field

Question 21

How does Fleming’s right hand rule work?

Answer 21

Thumb is the direction of motion
First finger is the direction of the B field
Second finger is the direction the current will move it

Question 22

What is the equation for magnetic flux linkage?

Answer 22

NΦ = BAN

Question 23

What is the equation for magnetic flux linkage at an angle?

Answer 23

NΦ = BANcosθ

Question 24

What is the equation for Faraday’s law?

Answer 24

ε = NΔΦ / Δt

Question 25

What is Lenz’s law?

Answer 25

The B field produced by an wire due to an induced current that acts opposite to the external B field that made it.

Question 26

What is flux linkage in a rotating coil?

Answer 26

NΦ = BAN cos ωt

Question 27

What is the emf in a rotating coil?

Answer 27

ε = BANω sin ωt

Question 28

What are the two different types of current?

Answer 28

Direct current – current flows in same direction all the time
Alternating current – current constantly switching direction back and forth

Question 29

What does the graph of V over t look like for an AC current?

Answer 29

Sine curve. Varies between Vp and -Vp (peak voltage)

Question 30

What is Vpp?

Answer 30

2Vp

Question 31

What is Vrms and what is the equation for it?

Answer 31

The root mean square voltage
Vrms = Vp / root2

Question 32

For AC what is the average power equation?

Answer 32

Average power = Irms x Vrms

Question 33

What is the UK mains Vrms value?

Answer 33

230V

Question 34

Explain how transformers change the p.d using the genrator effect in five steps.

Answer 34

1. a.c. Current passes into primary coil – makes a magnetic field
2. Magnetic field of coil magnetises iron core (induced magnetism)
3. Magnetic field in iron core is alternating – because current in coil is a.c.
4. Alternating B field in the iron core makes an alternating p.d. in second coil (generator
   effect)
5. Get a.c. out of the second coil

Question 35

What is the transformer equation?

Answer 35

Ns / Np = Vs / Vp

Question 36

What is the difference between a step up and step down transformer?

Answer 36

Step-up = more turns in the 2nd coil than 1st coil - so get bigger p.d. in second coil than in first coil
Step-down = less turns in the 2nd coil than first, so get a p.d. out that is smaller than the p.d. you put in

Question 37

How do transformers lose energy?

Answer 37

• The iron core being continuously cut by flux causes currents in the core, called eddy currents
• Eddy currents dissipate energy as heat
• Heat generated from resistance in the coils
• Energy is also needed to magnetise and demagnetise the core which is wasted
• Not all the flux from the first coil cuts through the second coil in practice

Question 38

What is the transformer efficiency equation?

Answer 38

η (efficiency) = IsVs / IpVp

Question 39

How do we reduce power loss in transmission lines?

Answer 39

Use a small current (high p.d.) to reduced energy lost to heating.