What is Capacitor and capacitance?

Example 30.2 (Orders of magnitude)

One farad is enormous — one coulomb parked per volt. Ceramics of radio circuits: $\mathrm{pF}$ to $\mathrm{nF}$; electrolytics of power supplies and flashes: $\text{µ}\mathrm{F}$ to $\mathrm{mF}$; supercapacitors: thousands of farads.

Example 30.9 (A flash unit’s numbers)

A photo flash charges $C = 800\,\text{µ}\mathrm{F}$ through $R = 1.0\,\mathrm{k}\Omega$: $\tau = 10^3 \times 8.0\times10^{-4} = 0.80\,\mathrm{s}$, so the “ready” light — wired to trip at $95\%$ — waits $3\tau \approx 2.4\,\mathrm{s}$: the whine of the hook. Fired through the flash tube’s mere $0.50\,\Omega$, the same capacitor empties with $\tau' = 0.40\,\mathrm{ms}$: two thousand times faster out than in.

Example 30.16 (Energy orders of magnitude)

The flash capacitor of Example 30.9: $\tfrac12 \times 8.0\times10^{-4} \times 300^2 = 36\,\mathrm{J}$. A $3000\,\mathrm{F}$ supercapacitor at $2.7\,\mathrm{V}$: $1.1 \times 10^{4}\,\mathrm{J}$ — an AA battery’s worth. A $10\,\mathrm{mH}$ coil at $10\,\mathrm{A}$: $0.50\,\mathrm{J}$. Less than batteries hold — but releasable in a millisecond.