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Chapter 4. Capacitors


A capacitor consists of two metal plates with a thin insulator in between, as its symbol shows:

What will happen if we connect a DC voltage source (battery) to a capacitor?

The positive side of the battery attracts the electrons in the top plate of the capacitor. This plate will become positively charged. Because the insulator is very thin, the top plate will attract the electrons in the bottom plate. The gaps these electrons leave behind, will be filled up with the electrons from the negative end of the battery. So it seems if the current flows right through the capacitor, as if there were no insulator at all. But of course, this can't continue for ever. Eventually, there will be no electrons left on the top plate, and no room for more electrons on the bottom plate. The capacitor is now completely charged, and the current flow will stop.

Now let's swap the terminals of the battery. The positive terminal of the battery will attract the electrons on the bottom plate of the capacitor and the negative end of the battery will emit electrons to fill in the gaps on the top plate. This process will continue until the capacitor is charged again.

If we continually swap the terminals of the battery, there will be a continuous current flow. In other words: a capacitor conducts AC currents, but blocks DC currents.

The capacitance depends on the size of the plates and the matial between them. This material is called the dielectric and reduces the electric field between the plates. This will increase the capacitance.

The capacitance can be calculated with: C = εA/d, where ε is the dielectric constant, A the area of one plate and d the distance between the plates. Since we can buy capacitors in any electronics show, we'll seldomly need this equation.

The unit of capacity is Farad, symbol F. This unit is usually far too large; uF (micro Farad), nF (nano Farad), and pF (pico Farad) are more common. 1F = 1000000uF, 1uF = 1000nF, 1nF = 1000pF.