Active and Passive Elements: who supplies the energy?
A capacitor stores energy but cannot create it — that's the whole distinction.
A battery pushes energy into a circuit. A resistor turns energy into heat. A capacitor holds energy for a while and gives it back. Only one of those three is genuinely adding energy to the system, and sorting elements on that basis is what "active vs passive" means. The wording trips people up because a charged capacitor certainly can deliver current — but delivering stored energy is not the same as supplying it.
The definitions
ACTIVE element — capable of DELIVERING net energy to the
circuit over time. Can amplify or generate power.
Examples: voltage source, current source, battery,
generator, BJT, FET, op-amp
PASSIVE element — cannot deliver net energy. It either
dissipates it or stores and returns it.
Examples: resistor (dissipates), inductor (stores in a
magnetic field), capacitor (stores in an
electric field), transformer
⚡ What each passive element does with energy
ResistorDissipates — converts electrical energy to heat, irreversibly. Energy is gone from the circuit. P = I²R, always positive.
InductorStores in a magnetic field: E = ½LI². Returns it when the current falls. Net energy over a full cycle is zero.
CapacitorStores in an electric field: E = ½CV². Returns it on discharge. Net energy over a full cycle is zero.
"Net over time" is the phrase that resolves the capacitor confusion. A capacitor can supply current for a moment, but it can never give out more than it was given — its integral of power over a complete cycle is zero or negative. A battery's is positive: it genuinely adds energy. That's the line between passive and active.
Transistors are active — and that's not obvious
A BJT has no battery inside it, so why is it active? Because it can amplify: a small input signal controls a much larger output signal, so the signal power at the output exceeds the signal power at the input. The extra energy comes from the DC supply, and the transistor's ability to convert supply energy into signal energy is what makes it active. A transformer, by contrast, can raise voltage but never power — it's passive.
Transformer: raises V, lowers I, power stays (roughly) equal
→ PASSIVE
Transistor: small signal in, large signal out, drawing on
the DC supply → power gain > 1
→ ACTIVE
Worked example — energy stored
A 100 µF capacitor is charged to 50 V, and a 2 H inductor carries 3 A. Find the energy stored in each.
Capacitor: E = ½CV²
= ½ × 100 × 10⁻⁶ × 50²
= ½ × 100 × 10⁻⁶ × 2500
= 0.125 J
Inductor: E = ½LI²
= ½ × 2 × 3²
= 9 J
Both store — neither generates. When released, each can
deliver at most this amount back to the circuit.
💡 Exam angle: a common 2–4 mark question. Define it using the words "net energy delivered over time", then list three examples on each side. Be ready for the two traps. A transformer is passive — it gives voltage gain, but no power gain. A transistor is active — its power gain comes from the supply. Knowing the two energy-storage formulas, ½LI² and ½CV², is worth having as well.
Syllabus points
Active vs passive elements
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