Everything Science Grade 11

CHAPTER 17. ELECTROSTATICS 17.5

Physical properties of the capacitor and ca-

pacitance

ESBHQ

The capacitance of a capacitor is proportional tothe surface area of the conducting plate and
inversely proportional tothe distance between the plates. It also dependson the dielectric between
the plates. We say that it is proportional to the permittivity of the dielectric. The dielectric is the
non-conducting substance that separates the plates. As mentioned before, dielectrics can be air,
paper, mica, perspex orglass.
The capacitance of a parallel-plate capacitor is given by:

C = � 0

A

d

where � 0 is, in this case, the permittivity of air, A is the area of the platesand d is the distance
between the plates.

Example 10: Capacitance

QUESTION

What is the capacitanceof a capacitor in whichthe dielectric is air, the area of the plates is

0 ,001m^2 and the distance between the plates is 0 ,02m?

SOLUTION

Step 1 : Determine what is required

We need to determine the capacitance of the capacitor.

Step 2 : Determine how to approach the problem

We can use the formula:

C = � 0

A

d

Step 3 : Determine what is given.

We are given the area of the plates, the distancebetween the plates andthat the

dielectric is air.

Step 4 : Determine the capacitance

C = � 0

A

d

(17.1)

=

(8, 9 × 10 −^12 )(0,001)

0 , 02

(17.2)

= 4, 45 × 10 −^13 F (17.3)