Capacitors & Capacitance – Formula & other FAQs

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Capacitors are components of electrical circuits that temporarily store electric charge. The addition of a capacitor into a circuit has two possible effects: either introducing a time delay into the circuit; or storing electrical energy for a short period of time.

Use of capacitors: Capacitors are used extensively in electrical and electronic timing circuits, in power circuits, for smoothing electrical signals, and as part of the signal-receiving circuits found in radios.

Different types of capacitor
Figure 1 Different types of capacitor

What is there in a capacitor?

Modern capacitors consist of two parallel conducting plates (usually made of metal foils, films, or coatings) separated by a thin insulating layer known as a dielectric (generally made from thin plastic films, electrolytes, ceramics, or metal oxides). Most capacitors are then encased in a metal or plastic housing. Figure 1 shows a selection of different capacitors.

What are the circuit symbols used for capacitors?

The main circuit symbols for capacitors
Figure 2: The main circuit symbols for capacitors

There are several different circuit symbols for capacitors depending on their type, although they are all based on the same simple pattern shown in Figure 2.




How does a capacitor work?

capacitor plates
figure 3: Capacitor plates

A potential difference from a battery or a power supply connected across the metal plates causes electrons to flow off one plate, back through the battery, and onto the second plate (Figure 3).

One plate becomes positively charged (where electrons are removed), while the plate with the excess of electrons becomes negatively charged.

capacitor plates discharging
figure 4: Capacitor plates discharging

If the capacitor is then disconnected from the source of potential difference, the charge will stay on the plates until a conducting pathway allows the excess electrons to flow off the negatively charged plate and back onto the positive plate, until the two plates have equal charge again (Figure 4).

The conducting pathway could be a different part of the circuit (controlled by a switch) or the charge could gradually leak away to the surroundings.


What is Capacitance? – definition

The ability of any object to store charge is called capacitance.
Capacitance is given the symbol C, and the SI unit is the farad (F). The capacitance of a capacitor depends on the area of the metal plates, the distance between the plates, and the electrical properties of the material separating the plates.

Equation of Capacitance | Formula

The amount of charge, Q, that can be stored on a capacitor depends on the size of the capacitance, C, and the potential difference, V, across the capacitor causing the separation of the charge.
The relationship among Q, C, and V is presented by this equation:
Q = VC

The capacitance of a capacitor can then defined by the following equation:
C= Q/V

So one farad is equal to one coulomb per volt.
Actually, 1 F is quite a large capacitance, and useful ‘real-life’ capacitors have capacitances measured in microfarads (μF), nanofarads (nF), or picofarads (pF).

Measuring the capacitance of a capacitor

Digital coulombmeter measuring charge
figure 5: Digital coulombmeter measuring charge

A fully discharged capacitor is connected to a variable d.c. power supply and is then gradually charged to different potential differences. A digital coulombmeter is then used to measure the charge stored on the capacitor at each potential difference
(Figure 5).
The data from this experiment (potential difference and charge stored) can be recorded in a table and you can plot a graph of Q against V from that dataset.

Capacitors & Capacitance – Formula & other FAQs
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