Which of the following statements are correct about transformers?
A. Transformers can not be used to step up DC.
B. The frequency of output across the secondary is always equal to the frequency of input across the primary.
C. The output power of a transformer may be increased to a value which is more than the input power.
D. Transformation ratio is equal to turns ratio.
E. In a step up transformer, secondary current is less than primary current.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, B, D, E
A transformer works by mutual induction: a changing flux in the core induces an emf in the secondary. For an ideal transformer
\( \displaystyle \frac{V_s}{V_p} = \frac{N_s}{N_p} = \frac{I_p}{I_s} \)
A. Transformers can not be used to step up DC. — Correct. A steady current gives a steady flux, and an unchanging flux induces no emf in the secondary.
B. The frequency of output across the secondary is always equal to the frequency of input across the primary. — Correct. The secondary emf is produced by the same alternating flux, so it changes at the same rate as the primary and has the same frequency.
C. The output power of a transformer may be increased to a value which is more than the input power. — Incorrect. A transformer only changes voltage and current; by conservation of energy the output power cannot exceed the input power.
D. Transformation ratio is equal to turns ratio. — Correct. The same flux links every turn, so the emf per turn is the same in both coils, which gives \( V_s/V_p = N_s/N_p \).
E. In a step up transformer, secondary current is less than primary current. — Correct. With \( V_s I_s = V_p I_p \), a rise in voltage means the current falls in the same ratio, so \( I_s \lt I_p \).
NCERT (2026-27):
Transformers – “… If the transformer is assumed to be 100% efficient (no energy losses), the power input is equal to the power output, and since p = i v,
\( \displaystyle i_p v_p = i_s v_s \qquad (7.34) \)”
Transformers – “… Since i and v both oscillate with the same frequency as the ac source, Eq. (7.35) also gives the ratio of the amplitudes or rms values of corresponding quantities.”
Transformers – “… We have:
\( \displaystyle V_s = \left(\frac{N_s}{N_p}\right)V_p \quad \text{and} \quad I_s = \left(\frac{N_p}{N_s}\right)I_p \qquad (7.36) \)
That is, if the secondary coil has a greater number of turns than the primary (Ns > Np), the voltage is stepped up (Vs > Vp). This type of arrangement is called a step-up transformer. However, in this arrangement, there is less current in the secondary than in the primary (Np/Ns < 1 and Is < Ip).”