The correct option is **B. To increase or decrease the voltage of alternating current (AC)**. ### Detailed Explanation: 1. **Understanding Transformers**: - A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It primarily operates on alternating current (AC) and is used to change the voltage levels in power systems. 2. **How Transformers Work**: - A transformer consists of two coils of wire, known as the primary coil and the secondary coil, wound around a core (usually made of iron). When an AC voltage is applied to the primary coil, it creates a magnetic field that induces a voltage in the secondary coil. The relationship between the number of turns in the primary and secondary coils determines whether the voltage is increased or decreased. - The formula governing this relationship is: \[ \frac{V_s}{V_p} = \frac{N_s}{N_p} \] where: - \( V_s \) = voltage across the secondary coil - \( V_p \) = voltage across the primary coil - \( N_s \) = number of turns in the secondary coil - \( N_p \) = number of turns in the primary coil - If \( N_s > N_p \), the transformer is a step-up transformer (increases voltage). If \( N_s < N_p \), it is a step-down transformer (decreases voltage). 3. **Why Option B is Correct**: - The primary function of a transformer is to adjust voltage levels for efficient power transmission and distribution. High voltages are used for long-distance transmission to reduce energy loss, while lower voltages are used for safe consumption in homes and businesses. Thus, option B accurately describes the main role of a transformer. ### Why Other Options are Incorrect: - **Option A: To convert direct current (DC) into alternating current (AC)**: - This statement is incorrect because transformers do not work with direct current (DC). They rely on the changing magnetic field produced by alternating current to induce voltage in the secondary coil. Converting DC to AC is done by a different device called an inverter. - **Option C: To store electrical energy for later use**: - This option is misleading. Transformers do not store energy; they merely transfer energy from one circuit to another at different voltage levels. Energy storage is typically done using capacitors or batteries. - **Option D: To measure the resistance of a circuit**: - This option is incorrect as well. Measuring resistance is done using an ohmmeter or multimeter, not a transformer. A transformer does not measure electrical properties; it only changes voltage levels. ### Common Pitfalls: - Confusing transformers with other electrical devices like inverters or capacitors. - Misunderstanding the role of voltage in power transmission and the importance of using AC for transformers. - Overlooking the fact that transformers cannot operate with DC, which is a common misconception. ### Revision Summary: - A transformer changes voltage levels in AC circuits, either stepping up or stepping down the voltage. - It operates based on electromagnetic induction between its primary and secondary coils. - Transformers are essential for efficient power transmission over long distances. - They do not convert DC to AC, store energy, or measure resistance.