What is a Capacitor?
A capacitor is a passive electronic component that stores electrical energy in an electric field between two conductive plates separated by an insulating material (dielectric). Capacitors are everywhere — from camera flashes to power-supply filters.
Capacitance measures a capacitor's ability to store charge per unit of voltage: C = Q/V, measured in farads (F). Larger plate area, smaller plate separation, and a stronger dielectric all increase capacitance.
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Step-by-step worked examples
A capacitor stores 200 µC of charge at 20V. Find its capacitance.
C = Q/V C = 200 µC / 20 V C = 10 µF
A 5 µF capacitor is charged to 12V. Find the stored charge.
Q = C × V Q = 5 µF × 12 V Q = 60 µC
Find the energy stored in a 4 µF capacitor charged to 10V.
E = 0.5 × C × V² E = 0.5 × 4×10⁻⁶ × 10² E = 0.5 × 4×10⁻⁶ × 100 = 2×10⁻⁴ J = 200 µJ
Flashcards
Quick quiz
Q1.A capacitor stores 50 µC at 5V. What is its capacitance?
Q2.What is the unit of capacitance?
Q3.What is the formula for energy stored in a capacitor?
Q4.Doubling the plate separation does what to capacitance (all else equal)?
The full card deck, worked steps and AI-tutor support for “What is a Capacitor?” are in Notek — study by hand before your exam.
Common mistakes
Thinking capacitors store current. — Correct: Capacitors store charge (and energy) in an electric field, not current.
Rearranging C = Q/V incorrectly to solve for an unknown. — Correct: They're algebraically equivalent — Q = CV and V = Q/C — just isolate whichever variable is unknown.
Ignoring unit prefixes (F vs µF vs pF). — Correct: Always convert to consistent units (usually farads) before calculating.
Believing capacitance depends on charge or voltage. — Correct: Capacitance is a fixed property of the capacitor's geometry and dielectric — Q and V vary together to keep C constant.
FAQ
What is a capacitor?
A component that stores electrical energy in an electric field between two conductive plates separated by an insulator.
What is the capacitor formula?
C = Q/V — capacitance equals the stored charge divided by the voltage across the plates.
What are examples of capacitors?
Camera flash energy storage, power-supply smoothing filters, timing circuits, and touchscreen sensors.
How do you calculate capacitance?
Divide the stored charge (Q) by the voltage across the plates (V): C = Q/V.