(7.5) - Magnetic Fields

Question 1

1. What is Magnetic Flux Density
2. What is a Tesla?

Answer 1

1. Magnetic Field Strength
2. A tesla is the strength of a magnetic field that pushes with 1 newton on 1 meter of wire carrying 1 ampere of current

Question 2

When do you use Fleming’s left hand rule vs right hand rule on a current

Answer 2

• Left hand when current is movoing according to convention
• Right hand when current is moving according to electron flow

Question 3

When do you use Fleming’s left hand rule vs right hand rule on a moving charge

Answer 3

• Left hand when particle is positively charged
• Right hand when particle is positively charged

Question 4

Explain the circular path of particles using magnetic fields and give an example

Answer 4

1. Magnetic force is perpendicular to velocity
2. It causes it to act as a centripetal force
   * An example is a type of particle accelerator called a cyclotron, used to make ion beams for radiotherapy and radioactive tracers

Question 5

Magnetic Flux definition

Answer 5

A measure of the magnetic field lines passing through a given area (Unit : 1 Weber)

Question 6

Faraday’s Law

Answer 6

The magnitude of induced emf is equal to the rate of change of flux linkage

Question 7

Lenz’s Law

Answer 7

Induced emf opposes the change in magnetic flux that produces it

Question 8

Explain why a magnet falls more slowly through a copper tube than through air

Answer 8

1. Magnet falls, its magnetic field moves with it
2. As the magnet moves, it’s field which is pushing through the copper is changing
3. The changing magnetic field induces eddy currents
4. The eddy currents induces its own magnetic field which is in the opposite direction of motion, slows down motion of magnet

Question 9

Alternating Current

Answer 9

Current that changes direction periodically, typically following a sin pattern

Question 10

Direct current

Answer 10

Voltage is constant no matter the current

Question 11

Root Mean Square

Answer 11

The square root of the mean of the squares of all values in one cycle

Question 12

Peak voltage

Answer 12

Distance from the equilibrium to the highest (or lowest) point.

Question 13

Peak-to-peak voltage

Answer 13

Distance from the minimum point to the maximum point.

Question 14

How are eddy currents produced in a tranformer

Answer 14

1. Alternating current enters the coil
2. Magnetic field constantly changing as current direction keeps changing
3. The changing magentic field induces and emf
4. This induced emf drive circulating currents to flow within it (eddy currents)

Question 15

How do eddy currents cause inefficiencies in transformers?

Answer 15

They dissipate energy as heat (via I²R heating) due to the electrical resistance of the core

Question 16

How can eddy current losses in a transformer be minimised?

Answer 16

• Using a laminated core
• They are insulated, increasing the resistance to the path of eddy currents

Question 17

What are the main causes of inefficiency in a transformer?

Answer 17

• Eddy currents
• Heat due to I²R
• Energy lost magnetising/demagnetising
• Magnetic flux from the primary coil not linking to the secondary coil.

Question 18

How do we reduce energy loss

Answer 18

• Laminated core
• Low resistance wire
• Soft iron core (allows for easy magnetise)
• Keep coil close together

Question 19

Why is electrical power transmitted at high voltages?

Answer 19

• Higher voltage = lower current
• Since power loss is proportional to current^2, lower current means dramatically lowers energy lost as heat