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Electronics Reference

Capacitors

Capacitor Charge

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Q - The charge stored in the dielectric in coulombs (C)
C - The capacitance in farads (F)
V - The voltage across the plates of the capacitor in volts (V)

The amount of charge stored in a capacitor is proportional to the voltage applied. The higher the voltage, the stronger the electric field is and more charge is stored in the dielectric. A capacitor can also store more charge if its capacitance is larger.

 > Capacitor Charge

Capacitance

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The capacitance of a capacitor can only be determined by its physical construction. But if the charge stored and the voltage applied are given, you can calculate the capacitance of a capacitor using this equation.

 > Capacitance

Capacitor Current-Voltage Relationship

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This equation shows the current-voltage relationship in a capacitor where,

i
is the instantaneous current
C is the capacitance of the capacitor
dv/dt
is the measure of the change in voltage in a very short amount of time

The equation also shows that if the voltage applied across a capacitor doesn’t change with time, the current is zero.

 > Capacitor Current-Voltage Relationship

Capacitive Reactance

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Capacitive reactance (XC) is the measure of the capacitor’s opposition to the flow of alternating current. It depends on the amount of capacitance and the frequency of the voltage applied. To determine the reactance of a capacitor, you can use this equation.

 > Capacitive Reactance

Capacitor Stored Energy

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The electrostatic field of the charge stored in a capacitor has electric energy. This is supplied by the voltage source. When the voltage source is removed, the capacitor can discharge current using the energy it's stored. To calculate the energy stored in a capacitor, you can use this equation.

 > Capacitor Stored Energy

Series Capacitors Equivalent Capacitance

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Connecting capacitors in series are just like increasing the thickness of a capacitor’s dielectric. This makes the total or equivalent capacitance (Ceq) of the series capacitors less than the smallest individual capacitance. The calculation of the equivalent capacitance is the same in parallel resistors. It is determined by inverting the sum of the inverses of all capacitances.

 > Series Capacitors Equivalent Capacitance

Parallel Capacitors Equivalent Capacitance

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Connecting capacitors in parallel have the same effect as increasing the area of a capacitor’s plate. The calculation of the total or equivalent capacitance (Ceq) is the same in series resistors, where you simply add the individual capacitances.

 > Parallel Capacitors Equivalent Capacitance
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