Quantity that remains unchanged in a transformer is

Options

A. voltage
B. Current
C. frequency
D. None of these

Answer

To determine the quantity that remains unchanged in a transformer, we can analyze the basic principles of how a transformer operates.

### Step-by-Step Solution:

1. **Understanding the Transformer**: A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It is primarily used to increase (step-up) or decrease (step-down) voltage levels in AC circuits.

2. **Power Conservation Principle**: The fundamental principle of a transformer is that the power input to the transformer is equal to the power output, assuming an ideal transformer with no losses. This can be expressed mathematically as:
\[ P_{in} = P_{out} \]
where \( P_{in} = V_i \cdot I_i \) (input voltage times input current) and \( P_{out} = V_o \cdot I_o \) (output voltage times output current).

3. **Voltage and Current Relationship**: In a transformer, if the voltage is stepped up, the current is stepped down, and vice versa. This means that the input voltage \( V_i \) and input current \( I_i \) will change depending on the output voltage \( V_o \) and output current \( I_o \). Therefore, neither voltage nor current remains unchanged.

4. **Frequency of the AC Signal**: The frequency of the alternating current (AC) signal, however, remains constant throughout the transformer. For example, in most household applications, the frequency is typically 50 Hz or 60 Hz, depending on the region. This frequency does not change as the power is transferred from the primary coil to the secondary coil of the transformer.

5. **Conclusion**: Based on the above analysis, the quantity that remains unchanged in a transformer is the frequency of the AC signal.

### Final Answer:
The quantity that remains unchanged in a transformer is **frequency**.