Which quantity does a transformer primarily change in an electrical circuit?

• A. $\mathrm{Voltage}$
• B. $\mathrm{Total energy}$
• C. $\mathrm{Frequency}$
• D. $\mathrm{Current direction}$

Answer: (A)

Verified step by step guidance

1. Understand the basic function of a transformer: it is a device used in electrical circuits to modify the voltage level of alternating current (AC).
2. Recall that a transformer consists of two coils, the primary and secondary coils, wound around a magnetic core. The alternating current in the primary coil creates a changing magnetic field, which induces a voltage in the secondary coil.
3. Recognize that the transformer changes the voltage magnitude between the primary and secondary coils according to the turns ratio, given by the formula: \( V_s = V_p \times \frac{N_s}{N_p} \), where \(V_s\) and \(V_p\) are the secondary and primary voltages, and \(N_s\) and \(N_p\) are the number of turns in the secondary and primary coils respectively.
4. Note that while the voltage changes, the total energy transferred (ignoring losses) remains the same, and the frequency of the AC current remains constant because the transformer does not alter the source frequency.
5. Conclude that the primary quantity a transformer changes in an electrical circuit is the voltage.