W
=
Li
2
= X 2 X (4)
2
= 16 J
3.
Capacitance:
1)
A capacitor consists of two metallic surfaces or conducting surfaces separated by a
dielectric medium.
2)
It is a circuit element which is capable of storing electrical energy in its electric field.
3)
Capacitance is its capacity to store electrical energy.
4)
Capacitance is the proportionality constant relating the charge on the conducting plates
to the potential.
Charge on the capacitor q V
q = CV
Where `C`
is the capacitance in farads, if q is charge in coulombs and V is the potential
difference across the capacitor in volts.
The current flowing in the circuit is rate of flow
of charge
i = = C
The capacitance of a capacitor depends on the dielectric medium & the physical
dimensions. For a parallel plate capacitor, the capacitance
C =
= €0 €r
A is the surface area of plates
D is the separation between plates
€
is the absolute permeability of medium€0 is the absolute permeabi
lity
of free
space €r is the relative permeability
of medium
i=
= C
=
V =
The power absorbed by the capacitor P = vi = vc
Energy stored in the capacitor
W =
=
dt
=
C
=
Joules
This energy is stored in the electric field set up by the voltage across capacitor.
Conclusions:
1.
The current in a capacitor is zero, if the voltage across
it is constant, that means the
capacitor acts as an open circuit to dc
2.
A small change in voltage across a capacitance within zero time gives an infinite current
through the capacitor, which is physically impossible.
In a fixed capacitor, the voltage cannot change abruptly
A capacitor behaves as short circuit just after switching across dc voltage.
3.
The capacitor can store a finite amount of energy, even if the current through it is zero.
4.
A pure capacitor never dissipates energy but only stores it hence it is called non-dissipative
element.
Dostları ilə paylaş: