Which component is commonly tested for voltage when assessing its functionality in a circuit?

Testing voltage in a circuit is crucial for determining the functionality of various components. The capacitor is commonly tested for voltage as it stores electrical energy in an electric field, and checking its voltage helps to assess whether it is charged to the correct level or if it is functioning properly.

Capacitors are used in circuits for a variety of functions, including filtering, timing, and coupling. When testing, the voltage across a capacitor indicates if it has been charged and whether it can hold that charge. A capacitor that is unable to hold its rated voltage may be faulty and could affect the entire circuit's performance.

In contrast, while batteries, transistors, and relays have their own specific checks, their functionality isn't solely assessed by voltage testing. Batteries are typically checked for overall charge or capacity, transistors require current and voltage testing under specific conditions to determine their operational status, and relays are tested more for their ability to open or close a circuit rather than just voltage. Thus, the capacitor's ability to hold and store voltage makes it a prime candidate for such assessments in a circuit.