Magnetic Fields

Question 1

What direction does a magnetic field travel ?

Answer 1

North to South

Question 2

What happens when 2 like poles face each other ?

Answer 2

They repel

Question 3

What happens when 2 opposite poles face each other ?

Answer 3

They attract

Question 4

What is the right hand grip rule ?

Answer 4

When there is a coil have fingers in direction of the current
Thumb will be north

Question 5

How to do the right hand grip roil without coil ?

Answer 5

Thumb in direction of current
Direction of fingers in direction of field

Question 6

What is Flemings left hand rule ?

Answer 6

Thumb, Index and Middle finger at 90° to each other (F, B, I)
Thumb is force
Index is direction Magnetic field
Middle is the direction of the current (direction of a moving proton / opposite to e-)

Question 7

How to derive force equations to relate velocity to magnetic field

Answer 7

F = BQV = mv^2 / r —> r = vm / BQ

Question 8

Why does a charged particle’s acceleration change in a magnetic field ?

Answer 8

• Acceleration and Velocity are vectors
• Change in direction -> change in v and a
• No work done so no change in Ek

Question 9

What are a the differences between Electric and Gravity potential ?

Answer 9

E : attracts and repels // G : only attracts
E : electromagnetic force, varies with charge magnitude // G : gravity force, varies with mass
E : potential always positive // G : potential always negative

Question 10

What are the similarities between Electric and Gravity fields ?

Answer 10

• both potentials dependant on 1 / r (in radial)
• both potentials are work done per unit charge / mass
• field strengths are proportional to 1 / r^2

Question 11

What angle does there have to be between the magnetic field and the coils to calculate flux linkage ?

Answer 11

90°
(cosθ = 1)

Question 12

When is the induced emf in a magnetic field greatest ?
(ε = BANωsinωt)

Answer 12

When the coil is parallel to the field (θ = 0)

Question 13

How is peak emf calculated from a flux linkage against time graph

Answer 13

ω = 2π / T x y intercept
BANω(1)

Question 14

What is Fleming’s right hand rule ?

Answer 14

Thumb, index and middle fingers all at 90° to each other
Thumb : direction if motion
Index : direction of field
Middle : direction of current

Question 15

What is Lenz’s law ?

Answer 15

Induced emf is in such a direction to oppose the change that caused it

Question 16

What are transformers ?

Answer 16

2 coils wrapped around an iron core
- AC in primary
- changing magnetic field
- transfers changing field to secondary coil
- induces and emf