Chapter 17-2 Capacitance
1 of 9
By: Diego Hernandez, Diego Ayala, and Dilpreet Kahlon.
© Boardworks Ltd 2009

Formula Name
Capacitance
Capacitance for a Parallel-
Plate Capacitor in a Vacuum
Electrical Potential Energy
Stored in a Capacitor
Equation Explanation
𝑄
C=
∆𝑉
Capacitance is equal to the charge on each plate divided by the potential difference.
C= ɛ 𝑜
𝐴 𝑑
Capacitance is equal to the permittivity of a vacuum multiplied to the quotient of the area of one of the plates and the distance between the plates.
PE electric
=
(½)(Q)(∆V)
Electrical Potential Energy is equal to half of the product of the charge on one plate and the final potential difference.
© Boardworks Ltd 2009 2 of 9

Measurement
Capacitance
Name Definition
C
The ability of a conductor to store energy in the form of electrically separated charges
Electric
Potential
V How energetic charges are
Units SI Unit
F
V
F
V
© Boardworks Ltd 2009 3 of 9

Capacitance is the ability of any body to store an electrical charge. Any material that can be made to hold a charge experiences capacitance.
Charges may be placed on these materials by static electricity or by an electric current.
“ Capacitor ” is another name for these kinds of objects that are charged by electrical currents. As charge is added to a capacitor, the potential energy inside of the capacitor increases. Capacitors tend to look like this:
(Circuit board symbol)
© Boardworks Ltd 2009 4 of 9

The capacitance of a capacitor depends on two things, its charge (Q), and change in potential difference (V).
Normally, a large charge will produce a smaller potential difference, and thus, a larger capacitance.
Reversely, a small charge produces a bigger potential difference, and a smaller capacitance.
Capacitance is measure in Farads (F), though normally you will find that the capacitance is in microfarads (µF) because of the small sizes of capacitors on a circuit board.
© Boardworks Ltd 2009 5 of 9

Electric Potential is the amount of work it takes to move a charge from one of the capacitor’s parallel plates, to the other. Once a charge moves to another plate, a small potential difference is created between the two plates. As the next charges flow through this potential difference, the electrical potential energy of the entire system increases.
Capacitance always depends on the size and shape of its capacitor. For a capacitor with nothing between its plates, we use the C= ɛ 𝑜
𝐴 𝑑 formula to calculate its capacitance. In this equation “ɛ " is what is known as a permittivity constant . When followed by a sub 0, the permittivity constant refers to that of a vacuum, which is valued at 8.85x10
-12
C
2
/Nm
2
. The other two factors in the equation are the area of one plate, and the distance between both.
6 of 9 © Boardworks Ltd 2009

Practice 17B #1,a
A 4.00 µF capacitor to a 12.0 V battery.
What is the charge on each plate of the capacitor
1
∆V = 12.0 V 2
C =
4.00µF
3
Q =
4
(C x ∆V)
5 Q = (4.00x10
-6
F)(12.0V)
6
7 of 9
= 4.80x10
-5 C
© Boardworks Ltd 2009

Practice 17B #1,b
If the same capacitor is connected to a
1.50V battery, how much electrical potential
1 energy is stored? (refer to previous example)
2
C =
4.00µF ∆V = 1.5 V
3
PE electric
4
= (½)(C)(∆V) 2
5 PE electric
= (½)(4.00x10
-6
F)(1.5V)
2
6
8 of 9
= 4.5x10
-6 J
© Boardworks Ltd 2009

1. What is capacitance?
A.
Capacitance is the ability of any body to store an electrical charge.
2. What is the value of the permittivity constant?
A.
8.85x10
-12
C
2
/Nm
2
3. What is electric potential?
A.
Electric Potential is the amount of work it takes to move a charge from one of the capacitor’s parallel plates, to the other
© Boardworks Ltd 2009 9 of 9