Electric current is the flow of electric charge, which is what electrical components harness to light up, or spin, or do whatever they do. When current flows into a capacitor, the charges get “stuck” on the plates because they can’t get past the insulating dielectric. Electrons – negatively charged particles – are sucked into one of the plates, and it becomes overall negatively charged. The large mass of negative charges on one plate pushes away like charges on the other plate, making it positively charged.
The positive and negative charges on each of these plates attract each other, because that’s what opposite charges do. But, with the dielectric sitting between them, as much as they want to come together, the charges will forever be stuck on the plate (until they have somewhere else to go). The stationary charges on these plates create an electric field, which influence electric potential energy and voltage. When charges group together on a capacitor like this, the cap is storing electric energy just as a battery might store chemical energy.
Charging and Discharging
When positive and negative charges coalesce on the capacitor plates, the capacitor becomes charged. A capacitor can retain its electric field – hold its charge – because the positive and negative charges on each of the plates attract each other but never reach each other.
At some point the capacitor plates will be so full of charges that they just can’t accept any more. There are enough negative charges on one plate that they can repel any others that try to join. This is where the capacitance(farads) of a capacitor comes into play, which tells you the maximum amount of charge the cap can store.
If a path in the circuit is created, which allows the charges to find another path to each other, they’ll leave the capacitor, and it will discharge.
For example, in the circuit below, a battery can be used to induce an electric potential across the capacitor. This will cause equal but opposite charges to build up on each of the plates, until they’re so full they repel any more current from flowing. An LED placed in series with the cap could provide a path for the current, and the energy stored in the capacitor could be used to briefly illuminate the LED.
Calculating Charge, Voltage, and Current
A capacitor’s capacitance – how many farads it has – tells you how much charge it can store. How much charge a capacitor is currently storing depends on the potential difference (voltage) between its plates. This relationship between charge, capacitance, and voltage can be modeled with this equation:
Charge (Q) stored in a capacitor is the product of its capacitance (C) and the voltage (V) applied to it.
The capacitance of a capacitor should always be a constant, known value. So we can adjust voltage to increase or decrease the cap’s charge. More voltage means more charge, less voltage…less charge.
That equation also gives us a good way to define the value of one farad. One farad (F) is the capacity to store one unit of energy (coulombs) per every one volt.