Magnetic Fields (Transformers)

Question 1

Explain the main use of a transformer

Answer 1

To increase/decrease the current in a wire to reduce the energy loss during transmission over long distances

Question 2

Describe the general structure of a transformer

Answer 2

A iron core in which two coils are parallel to one another
The left being the primary coil
The right being the secondary coil

Question 3

Explain how a step up transformer works (long write it down)

Answer 3

An alternating current flows through the primary coil
This produces a magnetic field
Due to the alternating current == The magnetic field direction will be alternating
Therefore there is a change in magnetic flux
Due to Faradays law this must induce an emf

The same magnetic field will be felt in the secondary coil due to the iron core and close distance
Therefore again an emf is induced, and this induces a current in the secondary coil
Due to a greater number of turns in the secondary coil the voltage will be higher

Question 4

Explain how energy conservation (or power conservation) for a step up transformer works (assuming 100% efficient)

Answer 4

Since P = IV
An increase in voltage means that there must be a lower current

Since
Power (primary) = Power (secondary) == Due to energy conservation

Question 5

How does the step up transformer reduce the power lost to heat when transporting electrical energy over long distances

Answer 5

Since increase in V means decrease in I for constant power
P = IV

And using equation for power lost to heat as === P = I^2R
A smaller current means less power lost

Question 6

Equation for total voltage of one coil

Answer 6

V = N x (Flux / Time)

Number of turns x (Flux / Time)

(Note N x Flux = flux linkage)

Question 7

Relationship between emf and number of turns

Answer 7

Proportional

Question 8

How can you identify if a transformer is step up or step down from the number of turns it has

Answer 8

Secondary coil has MORE turns than primary == Step up

Secondary coil has LESS turns than primary == Step down

Question 9

Equation to find efficiency of transformer

Answer 9

Power of secondary / Power of primary

IV (s) / IV (p)

Question 10

How can an eddy current be reduced in a transformer?

Answer 10

Laminate core

Magnetic lines do not naturally cut coil at right angles
Therefore laminating current makes it so they do
(Essentially add material in certain pattern) So only iron propagates field

If not laminated can cause eddy currents

Question 11

Name 3 inefficiencies in core

Answer 11

• Induced eddy currents
• Reversal of magnetism
• Poor insulation between coils

Question 12

Name 3 ways to reduce inefficiency in core

Answer 12

• Laminating core
• Soft iron core
• Low resistance wiring