DC Electrics - Circuit Protection and Capacitors — Page 49, Lesson 45

I want to walk you through what happens inside a capacitor once it's connected to a DC supply, and then we'll see how it behaves in an AC circuit and how we combine capacitors. Let's start with Figure 3.6. We have a 12-volt battery connected across a capacitor. Initially, electrons flow onto one plate and away from the other, building up opposite charges. After a short time, the difference in charge between the plates results in a potential difference existing between the plates. That potential difference grows until it equals the supply voltage. At that point, the flow of electrons will reduce and stop. The capacitor is now fully charged, current has stopped flowing, the plates are said to be charged, and there exists an electric field between the plates. The key takeaway here is that the capacitor is now blocking DC flow — it acts as an open circuit to direct current once it's fully charged. If you then open the switch and disconnect the capacitor from the battery, it holds its charge. A capacitor stores electrical energy by the formation of an electric field between the plates. It will only discharge if it is now connected to an external circuit — that's how it releases the stored energy. Now let's move to a capacitor in an AC circuit. Figure 3.7 shows the battery replaced with an alternating current supply, and a light bulb is placed in series with the supply and the capacitor. The terminals X and Y are now changing from positive to negative at a rate depending on the frequency of the supply. So current is first flowing in one direction, then reversing and flowing in the opposite direction. The capacitor is charging in one direction, discharging, and then charging in the opposite direction. This process continues until the supply is disconnected. The bulb will be continuously ON. Current flows in the wires, but no current flows through the dielectric — the insulating material between the plates. So the conclusion is: a capacitor appears to pass AC. It doesn't actually conduct through the dielectric, but the continuous charging and discharging makes it behave as if AC can flow through the circuit. Now let's look at how we combine capacitors. First, capacitors in parallel. When you connect capacitors in parallel, you are effectively increasing the area of the plates. The total capacitance, which we call CT, can be found by adding the individual capacitances: CT equals C1 plus C2, and so on. So for parallel, you just add them up. Next, capacitors in series. Capacitors in series have effectively increased the distance between the plates, and therefore the total capacitance has decreased. The total capacitance is found by using the formula for resistances in parallel — that's the reciprocal formula. So one over CT equals one over C1 plus one over C2, and so on. This gives you a total capacitance that is smaller than the smallest individual capacitor in the series. So to summarise: a capacitor blocks DC once fully charged, stores energy in an electric field, appears to pass AC by continuously charging and discharging, and when you combine them, parallel adds capacitance, series reduces it using the reciprocal formula.