SmartLearner

Capacitor 1

1. What is a capacitor

This is simply a device that can store electric charge (electrical energy).
It is also called a condensor.

2. What is it made of?

It consists of:
(a) two conducting sheets
(b) separated by an insulating material.
The insulating material is called a dielectric.

3. What is the symbol for a capacitor?

two parallel lines of equal length

4. How is it connected in a circuit, and what happens to the current from the battery?

Electrons flow from the negative terminal of the battery to the first plate of the capacitor. This plate of the capacitor is called the negative plate.
The electrons collect here. As this plate becomes more and more negative, this plate then opposes the continued flow of electrons from the battery to itself, (like charges repel). Hence the current being supplied from the battery decreases as the capacitor is being charged.

The other plate is connected to the positive terminal of the battery and is called the positive plate.
This plate loses electrons to the battery.

As a result of this process, the capacitor develops a potential difference.

5. What happens to the current from the battery as the capacitor is being charged?

The current decreases.

When the capacitor is fully charged, the current from the battery becomes (virtually )zero.
The ammeter reading would be zero.
The capacitor would have the same potential difference as the battery that charged it.

6. Why is a capacitor useful?

When it is connected in a circuit, it acts similar to a battery.
It is able to produce a large amount of current in a short time. This makes it very useful, example for a camera flash.

7. What is CAPACITANCE?

Capacitance is a measure of the ability of a capacitor to store charge.
The symbol for capacitance is C.
The unit for capacitance is farad or F.
Typical capacitance values are very small hence uF (10^-6 F)is often used.

There are TWO main equations.
Equation 1

From the equation, it is noticed that the unit for capacitance (F) is also C.V^-1.
Please remember that the UNIT for Q(charge) is C(coulombs). This must not be confused with the symbol for capacitance C.

Equation 2

Permittivity of free space
8,85 x 10^-12F.m^-1
A is the area of a plate
d is the distance between the plates

8. What factors affect capacitance?

(a) The AREA of the plates.
The larger the area of the plates, the larger the capacitance.

(b) The DISTANCE between the plates.
The smaller the distance between the plates, the greater the capacitance.

9. Basic Information

A capacitor is a...

passive electrical component
that can store energy
in the electric field between a pair of conductors (called "plates").

The process of storing energy in the capacitor...

is known as "charging",
and involves electric charges of equal magnitude, but opposite polarity
which builds up on each plate.

Capacitors...

are often used in circuits as energy-storage devices.
They can also be used to differentiate between high-frequency and low-frequency signals.
This property makes them useful in electronic filters.

Quick Notes

A capacitor can store energy.
The process of storing energy is called charging.
The charges on each plate are always of equal magnitude but opposite polarity.
A capacitors ability to store charge is measured by its capacitance.
Capacitors are also called condensers.
A dielectric increases the capacitance of a capacitor.
Unlike a battery capacitors do not produce electrons, only stores them.
Once charged, the voltage of a capacitor is the same as the battery.
A capacitor can block DC but not AC.