A capacitor is an electrical component that stores energy in an electric field. The device is made up of two metal plates separated by a dielectric or insulator. When a DC voltage is applied across its terminals, the capacitor draws a current and continues charging until the voltage across the terminals is equal to the supply. In an AC circuit in which the applied voltage is continually changing, the capacitor is continuously being charged or discharged at a rate dependent by the supply frequency.
Capacitors are often used to filter out the DC component in a signal. At very low frequencies, the capacitor acts more like an open circuit, while at high frequencies the device acts like a closed circuit. As the capacitor charges and discharges, the current is restricted by the internal impedance, a form of electrical resistance. This internal impedance is known as capacitive reactance and measured in ohms.
What is the value of 1 Farad?
The farad (F) is the SI unit of electrical capacitance and measures a component's ability to store charge. A one farad capacitor stores one coulomb of charge with a potential difference of one-volt across its terminals. The capacitance can be calculated from the formula
where C is the capacitance in farads (F), Q is the charge in coulombs (C), and V is the potential difference in volts (V).
A capacitor the size of one farad is quite large as it can store lots of charge. Most electrical circuits won't need capacities this large, so most capacitors sold are much smaller, typically in the pico-, nano-, and micro-farad range.
The mF to μF calculator
Converting millifarads to microfarads is a simple operation. One can use an online mF to μF calculator, or download a capacitor conversion chart pdf but solving mathematically is an easy operation. One millifarad is equivalent to 10-3 farads and one microfarad is 10-6 farads. Converting this becomes
One can convert picofarad to microfarad in the same way.
Capacitive Reactance: The Resistance of a Capacitor
As a capacitor charges, the current through it quickly and exponentially drops off to zero until its plates are fully charged. At low frequencies, the capacitor has more time to charge and pass less current, resulting in less current flow at low frequencies. At higher frequencies, the capacitor spends less time charging and discharging, and accumulating less charge between its plates. This results in more current passing through the device.
This "resistance" to current flow is similar to a resistor but the crucial difference is a capacitor's current resistance – the capacitive reactance – varies with the applied frequency. As the applied frequency increases, the reactance, which is measured in ohms (Ω) decreases.
Capacitive reactance (Xc)is calculated with the following formula
where Xc is the capacitive reactance in ohms, f is the frequency in Hertz (Hz), and C is the capacitance in farads (F).
Capacitive Reactance Calculation
Calculate the capacitive reactance of a 420 nF capacitor at a frequency of 1 kHz
At 10 kHz, the capacitor's reactance becomes
It can be seen that a capacitor's reactance decreases as the applied frequency increases. In this case, the frequency increases by a factor of 10 and the reactance decreases by a similar amount.
About the Author
David Latchman is a science communicator and blogger with a background in physics. He also hosts the Science Bloggers Podcast where he interviews and talks to other science communicators. He also maintains the Science vs. Hollywood blog where he looks at the portrayal of science in popular culture.
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