G485 Module 2 Notes
Fill in the gaps as you work through Physics 2 for OCR and the online material.
You should be able to...
Unaware of this
section
I am aware of
this, but need to
do more work
I understand
this area well
Capacitors and Exponential Decay
define capacitance and the farad
select and use the equation Q = VC
state and use the equation for the total capacitance
of two or more capacitors in series
state and use the equation for the total capacitance
of two or more capacitors in parallel
solve circuit problems with capacitors involving series
and parallel circuits
explain that the area under a potential difference
against charge graph is equal to energy stored by a
capacitor
select and use the equations
W = ½ QV and W = ½ C V2
for a charged capacitor
sketch graphs that show the variation with time of
potential difference, charge and current for a
capacitor discharging through a resistor
define the time constant of a circuit
select and use time constant = CR
analyse the discharge of capacitor using
equations of the form
x = x0e(- τ/CR)
explain exponential decays as having a constant-ratio
property
describe the uses of capacitors for the storage of
energy in applications such as flash photography,
lasers used in nuclear fusion and as back-up power
supplies for computers
Capacitance (C)
The charge stored per unit potential difference across it, as given
by C = Q/V.
Farad
Unit of capacitance (F). 1 F is 1 C of charge stored per volt.
Time constant
The time taken for the charge remaining on a capacitor to fall to
1/e of its original value.
It can be found using the expression CR, measured in seconds (s).
Exponential
A function that varies as the power of another quantity.
Construction of a Capacitor
Capacitors are electrical components that
They are constructed from
s
electric charge.
plates that are separated by a
The two main types of capacitors are ceramic capacitors (above right) and
capacitors (above left).
Charging a Capacitor
Draw a circuit with a capacitor being charged by a single cell.
.
Stored Charge
(add your notes here)
Capacitance
On the side of a capacitor you will find its capacitance, and indicator of the amount of
stored charge it can hold.
Capacitance = _____________
C = _____
Capacitance is defined as:
The unit for capacitance is named after
so is called the Farad. 1 farad is
equal to
1F is a LARGE capacitance, typical capacitors usually have a value of
Answer SAQs 3 and 4, page 160
3)
4)
Energy Stored in a Capacitor
If an experiment is carried out where a capacitor is charged then discharged (as you
did in class) the following graph is plotted:
Copy Figure 11.5
The gradient shows the:
The area under the line represents:
The energy stored by a capacitor, W, is equal to:
W=
Derivation
Work is done to charge a capacitor (which will then be equal to the energy stored by it).
This can be derived as follows:
There are 3 equations that can be used for calculating W, in terms of QV, QC or CV.

.

.

.
Answer SAQs 7 and 8, page 161
7)
8)
Capacitors in Parallel
Draw fig 11.9 page 163
There are 3 rules for capacitors in parallel:

.

.

.
SAQ 12 Page 163
12)
Capacitors in Series
Draw fig 11.10 page 163
There are 3 rules for capacitors in series:

.

.

.
SAQ 14 Page 163
14)
Capacitor
Series
Parallel
SAQs 20 and 21 page 167
20)
21)
Resistor