Electrical- Magnetism

Question 1

What are the full names of the North and South pôles on a magnet?

Answer 1

North seeking and South seeking

Question 2

Filed lines for a bar magnet

Answer 2

Come out of North Pole and go to the South Pole in a curve. Curve increases in radius as you start further towards the centre of the North Pole.

Question 3

What does closeness of field lines indicate?

Answer 3

Filed strength. Stronger when lines closer together

Question 4

Symbols for conductors going into and out of the page

Answer 4

Into: circle with cross in it
Out of: circle with dot in it
Remember using a dart with a tail

Question 5

Field lines around a current-carrying wire going into and out of page

Answer 5

Into: clockwise circles

Out of: anti-clockwise circles

Question 6

Field lines for a solenoid with current going through it

Answer 6

If drawn with current entering at the bottom. Filed lines loop up through the coil and back down outside the coil in a continuous loop. Radius of curves increases as you go closer to centre of coil. Either side of centre is mirror image of field lines.

Question 7

What happens to flux when a conductor is wrapped around a looped iron core and current is in the conductor from the bottom?

Answer 7

The flux follows the iron core out of the top of the coil and loops round the iron core until it rejoins the bottom of the coil.

Question 8

Formula for flux around an iron core (analog of current)

Answer 8

φ=NI/S
Where NI is magnetomotive force (turns x current) units AT
S is reluctance units H^-1
Units weber for flux
Analog of I=V/R where V is electromotive force

Question 9

Formula for reluctance (analog of resistance)

Answer 9

S=l/μA
μ is permeability units H/m
l is length of conductor
A is CSA
Analog of R=l/σA where σ is conductivity

Question 10

Formula for flux density

Answer 10

B=φ/A
Units Tesla for flux density
φ is flux units weber 
A is area
Analog of current density, J

Question 11

Permeability of free space

Answer 11

μ0=4πx10^-7H/m

Question 12

What is permeability?

Answer 12

The ease of creating a magnetic field (and therefore a magnetic flux)

Question 13

Formula for permeability of a material

Answer 13

μ=μrμ0

Where μr is relative permeability with no units

Question 14

What are the two flux paths for a simple magnetic circuit?

Answer 14

One goes through and around the core (most flux does this). The other goes through the air because it’s a shorter route with greater area. There are other leakage paths as well.

Question 15

What is a toroid?

Answer 15

A conductor wrapped many times around a continuous doughnut shaped core.

Question 16

How do different reluctances combine in series?

Answer 16

They add together like resistors

Question 17

How do reluctances combine in parallel?

Answer 17

Like resistors do in parallel.

Question 18

What happens when a bar magnet moves through a coil.

Answer 18

A voltage is produced in the coil unless the magnet is stationary of far away. This is because the flux passing over the wire changes.

Question 19

What is flux linkage?

Answer 19

The amount of flux that passes through the middle of a coil or the total flux through which a moving conductor passes.

Question 20

What happens when a conductor moves at right angles through a magnetic field?

Answer 20

An EMF (voltage) is produced in the conductor equal to
Blv
Where B is field strength, l is length, v is velocity

Question 21

What is Fleming’s right hand rub and when is it used?

Answer 21

Thumb is direction of motion
1st finger is direction of field lines
2nd finger is direction of generated emf
Used for conductor moving at right angles to a magnetic field.

Question 22

Formula for induced emf in a coil

Answer 22

|E|=Ndφ/dt
Where N is number of turns on coil
φ is flux

Question 23

What happens when a conductor is moved parallel to a magnetic field?

Answer 23

No emf is produced

Question 24

What is self-induction?

Answer 24

When the current through a coil induced a voltage on itself

Question 25

What is the inductance of a coil wrapped around a core with a current going through it?

Answer 25

N^2/S
Where N is number of turns on coil
S is reluctance of core

Question 26

Formula for back emf of an inductor

Answer 26

E=Ldi/dt

Where L is inductance

Question 27

Two sources of voltage drop across an inductor

Answer 27

Back emf, potential difference across the resistance of the wire.