Paper 1 Transformers Drills

Q1

A transformer transfers energy between coils by

A. direct charge flow between coils.
B. mutual electromagnetic induction.
C. chemical reaction.
D. static electric field only.

Q2

A transformer requires an alternating primary current because it needs

A. constant flux.
B. changing magnetic flux.
C. zero magnetic field.
D. dc only.

Q3

In an ideal transformer, $V_s/V_p=$

A. $N_s/N_p$.
B. $N_p/N_s$.
C. $I_s/I_p$.
D. $R_s/R_p$.

Q4

For an ideal transformer, current ratio follows from

A. power conservation.
B. charge storage.
C. capacitance.
D. Coulomb force.

Q5

A step-up transformer has

A. $N_s>N_p$.
B. $N_s<N_p$.
C. $N_s=0$.
D. $V_s<V_p$ always.

Q6

An ideal step-up transformer

A. increases power.
B. increases voltage and decreases current.
C. decreases voltage and increases current.
D. works with steady dc.

Q7

Primary and secondary coils are normally

A. electrically connected by one wire.
B. not electrically connected.
C. connected through electrolyte.
D. the same coil only.

Q8

The iron core mainly

A. links changing magnetic flux between coils.
B. stores chemical energy.
C. acts as a resistor only.
D. blocks flux.

Q9

Eddy current loss is reduced by

A. laminating the core.
B. using thicker copper wire only.
C. using dc.
D. removing the core.

Q10

Copper loss occurs because

A. coil wires have resistance.
B. core is laminated.
C. flux is constant.
D. voltage is zero.

Q11

Hysteresis loss is reduced by using

A. soft magnetic core material.
B. plastic core.
C. thin copper only.
D. air gap only.

Q12

Efficiency is

A. output power/input power times $100\%$.
B. input/output times $100\%$.
C. turns ratio only.
D. current ratio only.

Q13

High-voltage transmission reduces cable heating because for fixed power it reduces

A. current.
B. voltage.
C. resistance to zero.
D. frequency.

Q14

Cable loss is calculated by

A. $I^{2}R$.
B. $V/R^2$.
C. $QV$.
D. $N\Phi$.

Q15

An ideal transformer does not operate properly with steady dc because

A. steady dc gives no continuous flux change.
B. dc has too high frequency.
C. dc has rms zero always.
D. dc cannot heat wires.

Q16

Transformer voltages/currents in ac calculations are usually

A. rms values unless stated.
B. peak-to-peak only.
C. mean over cycle only.
D. instantaneous random values.

Q17

Flux leakage means

A. not all primary flux links the secondary.
B. all flux links perfectly.
C. current leaks through air.
D. charge crosses insulation.

Q18

If $N_s<N_p$, the transformer is

A. step-down.
B. step-up.
C. ideal motor.
D. rectifier.

Q19

A real transformer has output power

A. less than input power.
B. greater than input power always.
C. equal to zero always.
D. unrelated to input.

Q20

A transformer core is often laminated with insulation to reduce

A. eddy currents.
B. turns ratio.
C. useful flux.
D. rms voltage.