DC Electricity:
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Transcript DC Electricity:
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