Capacitor
Conceptually a capacitor is composed of a pair of plates of conductor
which can be charged by a voltage source. The voltage between the two
plates is proportional to the charge
, but inversely proportional
to the capacity
of the capacitor:
Inductor
Magnetic field (flux) is generated in the space around a current
flowing through a piece of conductor:
The magnetic field around a coil is the superposition of the magnetic
flux generated by each section of the coil:
Electric current is induced in a conductor when there is changing
magnetic flux in the surrounding space.
A time-varying electric current in a coil will cause a time-varying magnetic field in the surrounding space, which in turn will induce electric voltage and then current in the same coil (self-induction) or a different coil in the neighborhood (mutual-induction).
The self-induced voltage across the coil due to a current
is
proportional to the rate of change over time of the total magnetic
flux caused by the current:
The polarity of the self-induced voltage
in a coil is such
that it tends to produce a current which induces a magnetic flux to
oppose the change of the magnetic field that induced the voltage,
thereby opposing any change in current
that is causing the
magnetic flux.
When current
increases, the induced voltage
tends to
resist it, when current
decreases, the induced voltage
tends to sustain it.