Fundamental function of a transformer: identify the correct statement about what a transformer does.

Introduction / Context:
Understanding the core purpose of a transformer is essential for any study of power systems and electronics. Transformers rely on electromagnetic induction, which inherently requires alternating magnetic fields, and therefore alternating currents and voltages.

Given Data / Assumptions:

• Standard iron-core or air-core transformer behavior.
• No active circuitry inside the transformer; it is a passive device.

Concept / Approach:
Transformers work by mutual induction: a changing current in the primary creates a changing magnetic flux that induces a voltage in the secondary. A changing flux requires AC excitation. Thus, transformers cannot convert AC to DC or DC to AC by themselves, nor do they affect DC voltages except transiently at switching moments.

Step-by-Step Reasoning:
AC in primary ⇒ changing flux ⇒ induced AC in secondaryVoltage ratio equals turns ratioTherefore a transformer steps AC voltage up or down while ideally conserving apparent power

Verification / Alternative check:
Apply steady DC to a transformer: after a brief transient, there is no changing flux and thus no sustained induced secondary voltage. This confirms that only AC is effectively transformed.

Why Other Options Are Wrong:

• changes ac to dc / changes dc to ac: These are rectification or inversion functions performed by active devices, not a passive transformer alone.
• steps up or down dc voltages: Not possible in steady state without switching.

Common Pitfalls:
Confusing transformer operation with that of rectifiers or inverters. A transformer can be part of those systems but is not sufficient to perform conversion by itself.

Final Answer:
steps up or down ac voltages