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Review: Kirchoff’s Rules
 Activity 13C
 Achieved level:
 Qn. 1;
 Merit:
 Qn. 2, 3, 4, 5, 6
 Excellence:
 Qn. 3 d, 6 b) iv.
 Challenge Problem on paper at the front
 Two further challenge problems in your workbooks
DC Electricity:
Capacitors and Capacitance
Demonstration: Charged Parallel Plates
 Observe the electric field established
between the parallel charged plates.
 What can we say about the charge
stored in the plates?
 What happens to the charge on the
plates if we increase the applied
voltage?
 The pair of parallel “plates” is a
capacitor.
Capacitance:
 The amount of charge stored on the plates of the capacitors is linearly proportional to
the potential difference across the plates.
Q V
 The constant of proportionality is called the capacitance.
Q  CV
Q
C
V
 Q is the charge on the plates, in coulombs (C)
 V is the potential difference across the plates, measured in volts (V)
 Capacitance is measured in farads (F) (coulombs per volt)
 Capacitance is a physical property of the capacitor.
 Capacitance is a measure of how much charge can be stored on the plates of a capacitor
Practice Problem:
a) How much charge can be stored on the plates of a 100 μF
capacitor when it is connected to a 12 V battery?
b) How many electrons have been moved by the battery
from one plate to the other? (the charge on an electron is
1.6 ∙ 10-19 C)
Capacitors
 A capacitor consists of a pair of
conducting plates which store charge
when a potential difference is applied
across them.
 Because it breaks the circuit, current
circuit symbol
cannot flow across a capacitor.
 When a battery is connected to a
capacitor, it pulls charge from one plate
and pushes it onto the other plate.
 When there is a potential difference
across a capacitor, an electric field will
be set up between the plates. Its strength
is equal to the voltage applied divided by
the distance between the plates:
V
E
d
Factors Determining Capacitance
 How can we increase the capacitance of a parallel-plate
capacitor? I.e. how can we increase the amount of charge that
can be stored on the plates for a given applied voltage?
 Increase the plate area (it will be easier to put more charge on
the plate)
 Reduce the distance between the plates (the negative plate will
attract charge onto the positive plate more)
 Put an insulating material (dielectric) between the plates (these
become polarised and attract more charge onto the plates)
Parallel Plate Capacitor
 The capacitance of a capacitor made from parallel plates of
area A, separated by a distance, d, with no dielectric is given
by:
C
0 A
d
 Where ε0 is a constant called the permittivity of free
space, and has a value of 8.84 ∙ 10-12 Fm-1
Parallel Plate Capacitors
Practice Problem
 Complete problem 1 on page 224 in your red books.
Dielectrics
 A dielectric is a material placed between the plates of the
capacitor to increase its capacitance.
 It does this by a process of polarisation – negative charges within the
dielectric are attracted to the positive plate, while positive ones are
attracted to the negative plate.
Equation for Capacitors with Dielectrics
 εr is called the dielectric
constant
 It is unitless (it is the ratio of
two capacitances)
 It is a measure of how much
the dielectric increases the
capacitance.
C
 r 0 A
d
Practice Problems:
 Complete problems in activity 14A in the Y13 Study Guide
Lichtenberg figures are formed
in special dielectric materials
that have been “filled” with
electrons.
http://www.youtube.com/watch?v=9lHxBQvqlaU
http://www.youtube.com/watch?v=Bf9n7vcBaKo&feature=related
Next Time:
 Combinations of capacitors
 Charging and discharging capacitors
Combinations of Capacitors: Capacitors
in Parallel:
Capacitors connected in parallel in a circuit have an equivalent capacitance equal to sum of the
individual capacitances.
Ceq  C1  C2  ...
Combinations of Capacitors: Capacitors
in Series
The inverse of the equivalent capacitance of capacitors connected in series is equal to the
sum of the inverses of the individual capacitances:
1
1
1
 
 ...
Ceq C1 C2