Electromagnetism

Question 1

In what direction do magnetic field lines flow around a bar magnet?

Answer 1

From North to South

Question 2

How is a stronger magnetic field drawn?

Answer 2

With the field lines closer together

Question 3

List 3 ways to make a field around a solenoid stronger

Answer 3

• More turns in the coil
• Use a larger current
• Use a different core material

Question 4

Why is an EMF induced when a coil is placed in a magnetic field with AC but not DC?

Answer 4

The back & forth movement of AC produces a changing flux, which induces an EMF (according to Faraday’s Law).
DC only acts in one direction so a changing flux is not induced, so neither is an EMF.

Question 5

What is the symbol and units for flux?

Answer 5

Symbol: capital phi
Units: Wb

Question 6

What is the symbol and unis for flux density?

Answer 6

Symbol: B
Units: Wbm^-2

Question 7

What proportionality does Faraday’s Law state?

Answer 7

d(flux)/dt is proportional to EMF

Question 8

What power ratio would an ideal transformer have?

Answer 8

Input power = output power

Question 9

In a transformer, what are energy losses proportional to?

Answer 9

(I^2)R

Question 10

Equation for permeance

Answer 10

Permeance = flux / current-turns

Question 11

Can an EMF be produced from

a) a changing flux
b) no flux
c) a constant flux

Answer 11

a) yes
b) no
c) no

Question 12

What is permeance?

Answer 12

Permeance is how easily a material allows flux to flow through
(high permeance = large flux)

Question 13

How is the size of the area of a coil related to the size of the induced EMF for

a) a bar magnet?
b) a uniform field?

Answer 13

a) big area = small EMF
small area = big EMF

b) big area = big EMF
small area = small EMF