A capacitor is a two-terminal, electrical component. Along with resistors and inductors, they are one of the most fundamental passive components we use. You would have to look very hard to find a circuit that didn’t have a capacitor in it. A capacitor will charge, store, and discharge electric charge. This electrical charge is stored as voltage and can be used just like any other DC power source while charged.
A capacitor is made out of two metal plates and an insulating material called a dielectric. The metal plates are placed very close to each other, parallel to each other, but the dielectric sits between them to make sure they don’t touch. This allows charge to build up on the plates but never touch.
The schematic symbol for a capacitor looks like the internals of the capacitor. This symbol shows the two plates with a gap in between representing the dielectric.
A traditional capacitor has two terminals, an input and an output, and can be polarized or nonpolarized. A non-polarized capacitor is represented with two straight lines and does not have a positive or negative. A polarized capacitor is represented by a straight and curved line where the straight like is positive and the curved line in negative.
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. This is repeated almost instantaneously until each plate shares only like charges. Since like charges repel each other the individual plates will be absent from opposite charges.
When a capacitor is fully charged, the plates will be fully oppositely charged. In electricity this means that the electric potential of one side and plate of the capacitor will be the same as the circuit on that side of the capacitor, and the other side's electric potential of one side and plate of the capacitor will be the same as the circuit on that side of the capacitor. Usually this means one side will have voltage and the other will be grounded or close to zero volts.
Each capacitor is built to have a specific amount of capacitance. The capacitance of a capacitor tells you how much charge it can store, more capacitance means more capacity to store charge. The standard unit of capacitance is called the farad, which is abbreviated F.
It turns out that a farad is a lot of capacitance, even 0.001F (1 millifarad) is a big capacitor. Usually, you’ll see capacitors rated in the pico- (10^-12) to microfarad (10^-6) range.
The coulomb is defined as the quantity of electricity transported in one second by a current of one ampere. Named for the 18th–19th-century French physicist Charles-Augustin de Coulomb, it is approximately equivalent to 6.24 × 10^18 electrons.
Along with Farads, Capacitors also have another unit which is Voltage. On the capacitor or the order sheet, will be the voltage rating. The voltage rating is the maximum voltage the capacitor can handle before overloading and exploding. The voltage rating should be respected and have a safety rating of at least 1.5. This means for a 9V battery and circuit, a capacitor must be rated for 13.5 or higher.
By definition, a capacitor is not a resistive component. However, resistance is defined as the opposition of current flow. So following that logic there is some “resistance” in a Capacitor, but it is called Capacitive Reactance. Capacitive Reactance is the process of charge building up on one plate which induces a charge in the opposite plate of the opposite sign. However, the resistance is so low, it is usually negligible.
Radial Capacitors will have the units for both capacitance, in farads, and voltage, in volts printed directly on the casing of the capacitor. The printed location of each may be slightly different or shifted because of the manufacturing process. This means that the nominal values may be slightly cut off but should continue somewhere on the casing.
Ceramic Capacitors are slightly different than radial capacitors in that they are smaller in both size and capacitance. All ceramic capacitors are rated in picoFarads which is 10^-12. The physical casing of a ceramic capacitor is too small to write the full farad and voltage rating on the casing. Because of this, manufacturers print a set of numbers and letters that represent the capacitor's value. The last number on a ceramic capacitor is the multiplier which is how many zeros you add to the end of the value. The first set of figures which can be one or two numbers will be its value. If there is a letter on it, that represents its tolerance. Common tolerances are J ±5%, and K ±10%.
A Ceramic Capacitor is known for having a high voltage rating but does not have it printed on the disc. Check the manufactures details for the voltage.
See our Series Circuit and Parallel Circuit to see how you add Capacitance in Series and Parallel.
When positive and negative charges build up on the capacitor plates, the capacitor becomes charged.
At some point, the capacitor plates will be so full of charges that they just can’t accept any more. 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.
Capacitors are used in two main ways: They are used in the replacement of a power source when you need intermediate power in cycles, or in voltage rectifiers. Diode rectifiers can be used to turn the AC voltage coming out of your wall into the DC voltage required by most electronics. But diodes alone can’t turn an AC signal into a clean DC signal, they need the help of capacitors! By adding a parallel capacitor to a bridge rectifier, a rectified signal like this:
It seems obvious that if a capacitor stores energy, one of its many applications would be supplying that energy to a circuit, just like a battery. The problem is capacitors have a much lower energy density than batteries; they just can’t pack as much energy as an equally sized chemical battery.... for now! Slowly we are creating what we are calling super capacitors that have similar energy densities to batteries and could solve all the problems with chemical batteries while maintaining all the advantages and then some. The upside of capacitors is they usually lead longer lives than batteries, which makes them a better choice environmentally. They’re also capable of delivering energy much faster than a battery. A Super Capacitor would revolutionize the way the world interacts with electronics.
For more information on Capacitors and a few experiments to build to further understand them, check out our page on Capacitors in a Circuit.