Which device is used to increase or decrease AC voltages by coupling two coils around an iron core?

Which device is used to increase or decrease AC voltages by coupling two coils around an iron core?

• A. Resistor
• B. Open Switch
• C. Thermistor
• D. Transformer

Answer: (D)

The key idea is transforming voltage levels through magnetic coupling in a device with two coils around an iron core. When AC flows in the primary coil, the magnetic field in the core changes and links to the secondary coil. The induced voltage on the secondary is determined by the turns ratio between the coils. More turns on the secondary give a higher voltage (step-up), fewer turns give a lower voltage (step-down). The iron core provides a low reluctance path, making the coupling efficient and the transformation possible. Other components don’t change AC voltage in this way: a resistor drops voltage and dissipates power, a switch simply opens or closes the circuit, and a thermistor changes resistance with temperature. Therefore, the device described is the transformer.

The key idea is transforming voltage levels through magnetic coupling in a device with two coils around an iron core. When AC flows in the primary coil, the magnetic field in the core changes and links to the secondary coil. The induced voltage on the secondary is determined by the turns ratio between the coils. More turns on the secondary give a higher voltage (step-up), fewer turns give a lower voltage (step-down). The iron core provides a low reluctance path, making the coupling efficient and the transformation possible. Other components don’t change AC voltage in this way: a resistor drops voltage and dissipates power, a switch simply opens or closes the circuit, and a thermistor changes resistance with temperature. Therefore, the device described is the transformer.