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4.2. Capacitor
A capacitor in figure 27 is a device that stores energy in the electrical field by charge separation
when appropriately polarized by the voltage. Simple capacitors consist of parallel plates of
conducting material that are separated by a gap filled with a dielectric material. Dielectric
materials that are air or mica contain a large number of electric dipoles that become polarized
in the presence of electric field. The charge separation caused by the polarization of the
dielectric is proportional to the external voltage and given by the following equation:
q(t)=Cu(t)
Where the symbol, 
C
, represents the capacitance of element. The unit of measurement for
capacitance is the farad or farads (F).
1F =10
6
μF =10
12
pF
When the charge or voltage of capacitor changes, the produced current in circuit is given as:
i =
dq
dt =C
du
dt
This equation is given on the base of the same direction of current and voltage; otherwise there
should be a negative symbol in this equation.
The capacitance of capacitor is determined by the permittivity of the dielectric, 
ε
, that fills the
gap between the parallel plate, the size of the gap between the plates, 
d
, and the cross-section
area of the plates, 
A
, as
C =
εA
d
When a constant voltage is exerted on the both sides of capacitor and its current is zero, this
capacitor is considered as an open circuit or DC circuit. In physical structure, capacitor consists
of two conducting surfaces that store charge, separated by a thin insulating material that has
a very large resistance.
For the equation 
idt =Cdu
, if voltage is produced by both sides of this equation, the following
equation could be given as:
∫
0
t
uidt =
∫
0
u
Cudu =
1
2 Cu
2
Above equation demonstrates that the electric energy increases with the increase of voltage
on the capacitor, and in the course, the capacitor component acquires electric energy from
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
209

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electric source. Formula 
1
2 Cu
2
is the electric energy in the capacitor. When voltage reduces on
the capacitor, electric energy reduces. Namely, capacitor releases electric energy to electric
source. Hence, capacitor is an energy storage element in circuit.