Basic Electrical Components ( Resistor, Capacitor and Inductor )
Capacitor
A capacitor is a passive two-terminal
electrical component that stores electrical energy in an electric
field. The effect of a capacitor is known as capacitance.
While capacitance exists between any two
electrical conductors of a circuit in sufficiently close proximity, a capacitor
is specifically designed to provide and enhance this effect for a variety of
practical applications by consideration of size, shape, and positioning of
closely spaced conductors, and the intervening dielectric material. A capacitor was therefore historically first
known as an electric condenser.
The capacitance formula of the capacitor is represented by,
The Unit of capacitor is Farad. Farad is a very big unit , that’s
why we us mili farad (mF), micro farad (μF), nano farad (nF) .
Formation of capacitor:
A capacitor is created out of two metal plates and an insulating material called a dielectric. The metal plates are placed very close to each other, in parallel, but the dielectric sits between them to make sure they don’t touch. The dielectric can be made out of all sorts of insulating materials: paper, glass, rubber, ceramic, plastic, or anything that will impede the flow of current. |
| Fig-1: Formation of Capacitor |
The plates are made of a
conductive material: aluminum, tantalum, silver, or other metals. They’re each
connected to a terminal wire, which is what eventually connects to the rest of
the circuit.
How a Capacitor Works :
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. |
| Fig-2 : Working of a Capacitors |
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.
Calculating Charge, Voltage, and Current :
A capacitor’s capacitance – how many farads it has – tells 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:
Q=CV
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.
Calculating Current :
The amount of current through a capacitor depends on both the capacitance and how quickly the voltage is rising or falling. If the voltage across a capacitor swiftly rises, a large positive current will be induced through the capacitor. A slower rise in voltage across a capacitor equates to a smaller current through it. If the voltage across a capacitor is steady and unchanging, no current will go through it.
The equation for calculating
current through a capacitor is:
I = C (dv/dt )
The dV/dt part of that equation is a derivative
of voltage over time, it’s equivalent to saying “how fast is voltage going up
or down at this very moment”. The big take away from this equation is that if voltage
is steady, the derivative is zero, which means current
is also zero. This is why current cannot flow through a capacitor
holding a steady, DC voltage.
Types
of Capacitor
The types of capacitor are as follows:
1. Polarized Capacitor
2.Non Polarized Capacitor
Voltage vs. current in a Capacitor:
As the electrical conductors are not in physical contact, it will not, in the long-term pass direct current. The action is the same as placing a boat paddle against a stream of water – it blocks current flow. However when voltage is first applied to a capacitor current will flow until the capacitor is charged. This is a transient effect |
| Fig -3: Ac and Dc response of a capacitor |
Example of calculating capacitance of a capacitor:
A capacitor is constructed from two conductive
metal plates 30cm x 50cm which are spaced 6mm apart from each other, and uses
dry air as its only dielectric material. Calculate the capacitance of the
capacitor.
Then the value of the capacitor consisting of
two plates separated by air is calculated as 221pF or 0.221nF.
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