When a coil and capacitor are placed in parallel they will resonate at a certain frequency, a frequency that can be determined by this formula:
Although
to understand why a certain value of inductor and capacitor
resonate at a certain frequency you must understand REACTANCE.
Furthermore once reactance is understood it will be much easier to
understand the formula above and why 159,155 is a constant.
Inductive
Reactance
Inductance
is the characteristic of an electrical conductor that opposes change in
current. As Lenz's law states "The induced emf in any circuit is always
in a direction to oppose the effect that produced it." This opposition
to change in current is called Counter Electromotive Force (cemf).
Inductance can be compared to Inertia. A physical analogy to Inductance
would be moving a heavy load. It takes more work to start the load
moving that it does to keep the load moving, as in reverse it takes
more work to stop that load moving than it does to keep it moving. This
is because that load possesses Inertia, just as current possesses
Inductance
in an electrical circuit.
As an
alternating current flows through an inductor, it is continuously
reversing itself. This causes the same inertial cemf effect
discussed above, only now it is constantly reversing. This opposing
force to the flow of alternating current is called Inductive Reactance
because it is the reaction an inductor presents to alternating current.
The
greater the inductance and the greater the frequency of the alternating
current, the greater the Inductive Reactance. Inductive Reactance can
be found using this formula:
Capacitors also
present an opposition to a.c. When an alternating current flows
through a capacitor each plate changes polarity according to the
frequency. As electrons move from plate to plate they are
limited to that plates storage ability (capacitance). Therefore at a
higher frequency the maximum number of electrons stored will change
plate more often, which means more current will flow. Also, as the
capacitance increases, more electrons change plate every cycle, and
more current will flow.
As you can see, an
increase in frequency will cause a decrease in the capacitor's
opposition and allow more current to flow, and an increase in
capacitance will cause a decrease in the capacitors opposition and thus
allowing more current to flow. This opposition a capacitor
presents to alternating current is called Capacitive Reactance.
Capacitive Reactance can be found using this formula:
C is
Capacitance in farads
Component Symbol 
Frequency 

Lower 
Higher 

Broken Circuit 
Unbroken Circuit 

Unbroken Circuit 
Broken Circuit 