In basic electrical engineering, an ideal transformer can perform several operations on an alternating current (AC) supply. Which of the following is an operation that a transformer cannot perform?

Introduction / Context:
Transformers are widely used devices in electrical power systems. They operate only on alternating current and are used to change voltage and current levels efficiently. The question asks which operation a transformer cannot perform, assuming an ideal transformer. This tests your understanding of energy conservation and the basic relationships between voltage, current, and power in a transformer. Knowing what a transformer can and cannot do is essential for understanding power transmission and distribution networks.

Given Data / Assumptions:

• The transformer considered is ideal, meaning no losses.
• The input is an alternating current supply.
• Possible operations include stepping up voltage, stepping up current, stepping up power, and stepping down voltage.
• We assume standard transformer equations that conserve power in the ideal case.

Concept / Approach:
In an ideal transformer, input power equals output power, ignoring losses. Power is given by P = V * I for AC magnitudes. If the transformer steps up voltage, then current decreases proportionally so that the product V * I remains the same. Similarly, if it steps up current, the voltage decreases. Therefore, a transformer can step up or step down voltage and current, but it cannot increase the total power; it merely transfers power from the primary to the secondary side. Real transformers have small losses, so output power is slightly less than input power, but they still cannot increase power. Thus, the operation of stepping up AC power is not possible for a transformer.

Step-by-Step Solution:
Step 1: Recall the transformer power relation for an ideal transformer: input power P1 = output power P2.Step 2: Express power as P = V * I, where V is voltage and I is current.Step 3: If voltage is stepped up, current must be stepped down so that V1 * I1 = V2 * I2.Step 4: If current is stepped up, voltage must be stepped down to keep total power the same.Step 5: Since a transformer conserves power and cannot create extra energy, it cannot step up AC power; therefore, that operation is impossible.

Verification / Alternative check:
You can verify this by looking at power ratings on actual transformers. A transformer rated at a certain kilovolt ampere value on the primary side has the same or slightly lower kilovolt ampere rating on the secondary side. Engineers design transformers to avoid exceeding this rating. The device can convert high voltage low current to low voltage high current or vice versa, but the overall apparent power remains constant, minus small losses. This observation confirms that a transformer does not have the ability to increase power. It simply transforms voltage and current levels efficiently.

Why Other Options Are Wrong:
Step up AC voltage is a basic and very common function of transformers used in power transmission, especially at generating stations, so option A is clearly a valid transformer operation. Step up AC current is also possible by using a step down transformer, which reduces voltage and increases current, so option B is feasible. Step down AC voltage is the function of distribution transformers that provide safe voltages for domestic and industrial use, so option D is correct as an operation. Only option C, step up AC power, contradicts the principle of energy conservation in an ideal transformer and is therefore impossible.

Common Pitfalls:
Students sometimes confuse increasing voltage or current with increasing power. It is important to remember that power depends on both voltage and current. Increasing one while decreasing the other can keep power constant. Another mistake is to assume that because losses exist, a transformer might increase power on one side, but real devices actually have output power slightly less than input power. Keeping the principle of conservation of energy in mind will guide you to reject any claim that a passive device like a transformer can increase total power on its own.

Final Answer:
Step up AC power