When the positive and negative charges melt on the capacitor plates, the capacitor is charged.
A capacitor can maintain its electric field (retain its charge) because the positive and negative charges on each of the plates attract each other but are never in contact with each other.
At one point the capacitor plates will thus be filled with fillers that they can not accept more. There are enough negative charges on the plate that you can reject all the others trying to join. This is the point where the capacitance (Farad) of a capacitor comes into play, ie the maximum charge amount of the capacitor that is able to memorize (Class y capacitors).
If you create a path in the circuit that allows the charges to find another route, they will leave the capacitor, and it will discharge.
For example, in the circuit, a battery can be used to induce an electrical potential through the capacitor. This will cause equal but opposite charges on each of the plates, until they are so full and will not allow the current to flow. An LED placed in series with the capacitor could provide a path for the current, and the energy stored in the capacitor could be used to briefly illuminate the LED.
Calculation of charge, voltage and current
The capacitance of a capacitor (how many farads is) tells you how much charge it can store. The charge depends on the difference in potential (voltage) between the plates. This relationship between charge, capacity, and tension can be found with this equation:
Q = CV
Charge (Q) stored in a capacitor is the product of its capacity (C) and the voltage (V) applied to it.
The capacitance of a capacitor should always be a constant, known value. Thus we are able to adjust the voltage to increase or decrease the charge. More voltage means more charge, less voltage less charged.
This equation also gives us a good way to define the value of a farad. A farad (F) is the ability to memorize a unit of energy (coulomb) for each volt.
We can take the voltage / charge / capacity equation to take another step forward and find out how the capacity and the voltage affect the current, because the current is the charge flow. The ratio of a capacitor between voltage and current is this: the amount of current through a capacitor depends on both the capacity and how quickly the voltage is increasing or decreasing. If the voltage across a capacitor increases rapidly, a large positive current will be induced through the capacitor. A slow increase in voltage across a capacitor is equivalent to a smaller current passing through it. If the voltage at the ends of a capacitor is constant and immutable, no current will pass through it (Class y capacitors).
The dV / dt of the equation is a derivative (an elegant way to say instantaneous rate ) of voltage over time, and is equivalent to saying “how fast the voltage goes up or down right now”. It is deduced from this equation that if the voltage is constant, the derivative is zero, which means that the current is zero. This is the reason why the current cannot flow through a capacitor with a constant voltage.
Types of capacitors
There are all kinds of capacitor types out there, each with certain characteristics and drawbacks that make it better for some other applications.
When deciding on the types of capacitors there are a handful of factors to consider:
- Size – Size both in terms of physical volume and capacity. It is not uncommon for a capacitor to be the largest component of a circuit. They can also be very small. More capacity typically requires a larger capacitor.
- Maximum voltage – Each capacitor is rated for a maximum voltage that can be dropped through it. Some capacitors could be rated for 1.5V, others could be sized for 100V. Exceeding the maximum voltage will usually cause the capacitor to be destroyed.
- Leakage current – capacitors are not perfect. Each cap is prone to leakage some small amount of current through the dielectric, from one terminal to another. This very small current loss (usually nA or less) is called losses. Dispersion causes the energy stored in the condenser to slowly but surely flow away.
- Equivalent Series Resistance (ESR) – The terminals of a capacitor are non-conductive 100%, they will always have a small amount of resistance (usually less than 0.01Ω) to them. This resistance becomes a problem when a lot of current flows through the cap, producing heat loss and electricity.
- Tolerance – Capacitors also can not be done to have an exact, precise ability. Each cap is rated for their nominal capacity, but depending on the type, the exact value can vary anywhere from ± 1% to ± 20% of the desired value.