Which device converts electrical energy into heat?

• A. Relay
• B. Microphone
• C. On-Off Switch
• D. Heater

Answer: (D)

Electrical energy becomes heat when it flows through a resistive element and is dissipated as thermal energy—this is resistive heating. A heater is built to use that principle, with a high-resistance element that heats up as current passes through it, releasing heat to warm air or objects around it. The amount of heat follows the power produced by the current and resistance (P = I^2R or P = V^2/R), so more current or higher resistance means more heat.

The other devices aren’t meant to turn electrical energy into heat as their main function: a relay acts as a switch to open or close a circuit, a microphone converts sound into an electrical signal, and an on-off switch simply controls whether a circuit is connected. That’s why the heater is the best answer here.