Which of the following devices is used for storing electric charges?

Step 1: Analyze the question The question asks which device stores electric charge. Think of the function of each device listed.

Step 2: Review each option

• A. Transformer: Changes the voltage of an alternating current (step-up or step-down) using electromagnetic induction. It does not store charge.
• B. Ammeter: Measures current in a circuit. It does not store charge.
• C. Potentiometer: A variable resistor, or a device for comparing and measuring potential differences. It does not store charge.
• D. Capacitor: Two conducting plates separated by an insulator (a dielectric). When connected to a source, charge builds up on the plates, so it stores charge and electrical energy.

Step 3: The key concept The ability of a capacitor to store charge is called its capacitance:

C = Q / V

where C is capacitance in farads (F), Q is charge in coulombs (C), and V is potential difference in volts (V).

The energy stored is:

E = ½ CV²

Step 4: Answer The device used for storing electric charge is the capacitor.

Exam tip: Match each device to its job:

• Transformer → transforms voltage
• Ammeter → measures current
• Potentiometer → compares/divides potential difference
• Capacitor → stores charge (think "capacity" for holding charge)

Whenever you see "storing charge" in a question, think capacitor.