A camera flash needs a burst of current far bigger than its own battery can supply in an instant. A defibrillator needs to deliver a jolt strong enough to reset a heart's rhythm, from a device small enough to carry in an ambulance. Both rely on the same quiet piece of physics happening in the second or two before either device is actually used: a capacitor, charging up.
What's actually happening on the plates
A capacitor is two conductive plates separated by an insulating gap. Connect it to a battery through a resistor, and current flows — not through the gap itself, which nothing can cross, but onto one plate and off the other, as electrons pile up on one side and are pulled away from the other. As charge builds up, it creates its own electric field opposing the battery's, so the current needed to add more charge keeps shrinking the fuller the capacitor gets. That self-limiting behaviour is the entire reason the charging process has a curve at all, rather than switching on and off instantly.
Current flows onto the plates through the resistor — fast at first, tailing off as the plates fill up and start pushing back against the battery's own voltage.
Why the curve looks the way it does
Plot voltage across the capacitor against time, and you get a curve that rises steeply at first, then flattens out as it approaches (but never quite reaches) the supply voltage — an exponential charging curve. The resistor and capacitor together set a fixed timescale for this, called the time constant, τ = RC: after one time constant, the capacitor has reached roughly 63% of the supply voltage, regardless of what that supply voltage actually is. A bigger resistor or a bigger capacitor both slow the whole process down — the resistor by limiting current directly, the capacitor by simply needing more charge to reach the same voltage.
The charging curve: steep at first, flattening as it approaches the supply voltage. One time constant (RC) always corresponds to about 63% charged, whatever the actual voltage and component values are.
Same curve, two very different real devices
A camera flash is designed around a short time constant on purpose — you don't want to wait long between shots, so a small resistance charges the capacitor in around a second. A defibrillator is engineered the same way in principle, but scaled up: it charges a much larger capacitor to a much higher voltage over several seconds, which is exactly why there's an audible whine and a short wait before a defibrillator is ready to deliver a shock — that pause is the RC charging curve, not the machine being slow. Same equation, same shape of curve, two completely different stakes. If capacitors, exponential curves or any other A-Level physics or maths topic need explaining with the actual mechanism rather than a formula to memorise, that's exactly what our A-Level physics and maths tutoring is for — see the full learning pathway here.
Frequently asked questions
Why does a camera flash need a capacitor at all — why not just use the battery directly?
A camera's flash tube needs a huge burst of current for a fraction of a second — far more than a small battery can deliver directly without its voltage collapsing. The capacitor solves this by charging up slowly, over a second or two, storing that energy, then discharging almost all of it in one instant — the battery never has to supply the flash's peak current itself, only the much gentler current needed to charge the capacitor.
Does a capacitor ever finish charging completely?
Mathematically, no — the exponential charging curve only ever approaches the supply voltage, getting closer and closer without ever quite touching it. In practice, after about 5 time constants (5RC) the capacitor is charged to over 99% of the supply voltage, which is indistinguishable from "fully charged" for any real purpose.
Is a defibrillator's capacitor charged and discharged the same way every time?
The charging phase (from the internal battery, up to a high voltage) works on exactly this RC charging curve. The discharge into the patient is deliberately shaped differently by the device's circuitry — a raw, uncontrolled capacitor discharge is far too fast and spiky to be safely used on a person, so defibrillators use additional circuitry to shape that discharge into a safer waveform.
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Sudershan Soni
Founder & Lead Tutor at Mostak Services — an MSc-qualified Mathematics, Science, Computer Science & STEM tutor with 20+ years of professional experience, teaching students from 11+ and GCSE to A-Level and beyond, online worldwide.
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