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11. Electromagnetic induction
11.3 Capacitance
Steps
Worked example
Practice 1
Practice 2
Read the question carefully.
Two capacitor plates are separated by
a rubber sheet of thickness 3.0 mm
that fi lls the space between them. The
plate dimensions are 0.35 m x 0.45 m.
ε
r
for the rubber is 6.0. Calculate the
capacitance of the arrangement.
Two parallel pates have an area of
0.015 m
2
and are placed 2.0 x 10
-3
m
apart with a vacuum in between.
Calculate the capacitance of this
arrangement.
A capacitor of capacitance 0.5 F has
plates separated by 5 mm of vacuum.
Calculate the total area of the plates.
Write what you know: highlight/
record the information from the
questions including what you
want to fi nd out.
d =
3.0 mm
A =
0.35 m x 0.45 m
C =
?
ε
0
= 8.85 x 10
-12
F m
-1
ε
r
= 6.0
Convert any data to appropriate
values (for example g to kg, or cm
to m)
d =
3.0 x 10
-3
m
A =
0.158 m
2
C =
?
ε = 5.31 x 10
-11
F m
-1
Write down the equation that links
the variables together
C = Aε / d
Rearrange the equation to make
the unknown the subject
C = Aε / d
Substitute your data into the
equation, including the units
C =
(0.158 m
2
x 5.312 x 10
-11
F
m
-1
) / 3.0 x 10
-3
m
Calculate the answer, leaving a
unit and an appropriate number of
sig fi gs (same as data). Check
that the answer makes sense and
underline.
C = 3 nF
This is a possible value of a
capacitor.
11. Electromagnetic induction
11.3 Capacitance
Steps
Practice 3
Practice 4
Practice 5
:
write your own
Read the question carefully.
What is the permittivity of a
capacitor if it has plates of area 4
x 10
-3
m
2
which are separated by
a substance of thickness 3 x 10
-3
m. The capacitance is 500 nF.
If two plates are separated by 1.5
mm, determine the area of the
plates if the capacitance is 10 mF.
The plates are separated by a
vacuum.
Write what you know: highlight/
record the information from the
questions including what you
want to fi nd out.
Convert any data to appropriate
values (for example g to kg, or cm
to m)
Write down the equation that links
the variables together
Rearrange the equation to make
the unknown the subject
Substitute your data into the
equation, including the units
Calculate the answer, leaving a
unit and an appropriate number of
sig fi gs (same as data). Check
that the answer makes sense and
underline.
