Magnetic Fields: Transformers

Question 1

Define a transformer.

Answer 1

Device using electromagnetic induction to change AC voltage/current between circuits using primary/secondary coils on iron core.

Question 2

Name the three main components of a transformer.

Answer 2

Primary coil

Question 3

Why must transformers use alternating current (AC)?

Answer 3

AC creates changing magnetic flux linkage inducing e.m.f. in secondary coil (DC gives constant flux

Question 4

Explain step-by-step how transformer induces voltage in secondary coil.

Answer 4

1) AC in primary creates changing magnetic field 2) Iron core conducts changing flux to secondary 3) Changing flux linkage induces e.m.f. in secondary.

Question 5

State transformer equation relating voltages and turns.

Answer 5

Vs/Vp = Ns/Np

Question 6

State transformer equation relating currents and turns.

Answer 6

Ip/Is = Ns/Np

Question 7

Write power conservation form of transformer equation.

Answer 7

VpIp = VsIs (ideal transformer

Question 8

Define step-up transformer.

Answer 8

Ns > Np so Vs > Vp (higher voltage output

Question 9

Define step-down transformer.

Answer 9

Ns < Np so Vs < Vp (lower voltage output

Question 10

Why use soft iron core in transformer?

Answer 10

Easily magnetised/demagnetised

Question 11

Explain National Grid step-up transformer purpose.

Answer 11

Increases transmission voltage (decreases current) to reduce I^2R power losses in cables.

Question 12

Explain National Grid step-down transformer purpose.

Answer 12

Reduces voltage to safe domestic levels (230V) and increases current for household use.

Question 13

Why transmit electricity at high voltage?

Answer 13

Reduces current I

Question 14

Write transformer efficiency equation.

Answer 14

Efficiency = (VsIs)/(VpIp) × 100%

Question 15

State units for transformer quantities.

Answer 15

V = volts (V)

Question 16

If primary has 200 turns and secondary 1000 turns

Answer 16

is it step-up or step-down?

Question 17

For ideal transformer

Answer 17

if Vp = 230V and Ns/Np = 1:100