Robertson C.R. Fundamental electrical and electronic principles
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Electric Fields and Capacitors
81
representative of the actual numbers involved. They are shown to aid
the explanation of the charging process that will take place when the
switch is closed. On closing the switch, some electrons from plate A
will be attracted to the positive terminal of the battery. In this case, since
plate A has lost electrons it will acquire a positive charge. This results
in an electric fi eld radiating out from plate A. The effect of this fi eld is
to induce a negative charge on the top surface of plate B, by attracting
electrons in the plate towards this surface. Consequently, the lower
surface of plate B must have a positive charge. This in turn will attract
electrons from the negative terminal of the battery. Thus for every
electron that is removed from plate A one is transferred to plate B. The
two plates will therefore become equally but oppositely charged.
This charging process will not carry on indefi nitely (in fact it will last
for only a very short space of time). This is because as the charge on
the plates increases so too does the voltage developed between them.
Thus the charging process continues only until the p.d. between the
plates, V is equal to the emf, E of the battery. The charging current at
this time will become zero because plates A and B are positive and
negative respectively. Thus, this circuit is equivalent to two batteries of
equal emf connected in parallel as shown in Fig. 3.6 . In this case each
battery would be trying to drive an equal value of current around the
circuit, but in opposite directions. Hence the two batteries ‘ balance out ’
each other, and no current will fl ow.
EE
II
Fig. 3.6
With suitable instrumentation it would be possible to measure the p.d.
between plate B and any point in the dielectric. If this was done, then
a graph of the voltage versus distance from B would look like that in
Fig. 3.7 . The slope of this graph is uniform and has units of volts/metre
i.e. the units of potential gradient
so potential gradient volt/metre
V
d
(3.5)
Now, energy VIt joule, and I
Q
t
amp
82
Fundamental Electrical and Electronic Principles
therefore energy
VQt
t
VQ joule, and transposing this
V
Q
energy
joule/coulomb
i.e. volt
J
C
11 1……………[]
but the joule is the unit used for work done, and work is force
distance, i.e. newton metre,
so 1 J 1 Nm……………[2]
Substituting [2] into [1]:
1 volt 1 Nm/C or
V ⬅
Nm
C
……………[3]
Dividing both sides of [3] by distance of separation d:
V
d
⬅
Nm
Cm
N
C
Referring back to equation (3.2), we know that electric fi eld strength
E is measured in N/C. So potential gradient and electric fi eld strength
must be one and the same thing. Now, electric fi eld strength is defi ned
in terms of the ratio of the force exerted on a charge to the value of
the charge. This is actually an extremely diffi cult thing to measure.
However, it is a very simple matter to measure the p.d. and distance
between the charged plates. Hence, for practical purposes, electric fi eld
strength is always quoted in the units volt/metre
p.d. (volt)
V
0
d
distance
(m)
Fig. 3.7
i.e. volt/metreE
V
d
(3.6)
Notice that the symbol E has been used for electric fi eld strength. This
is in order to avoid confusion with the symbol E used for emf.
Electric Fields and Capacitors
83
Worked Example 3.3
Q Two parallel plates separated by a dielectric of thickness 3 mm acquire a charge of 35 mC when
connected to a 150 V source. If the e ective csa of the eld between the plates is 14 4 m m
2
, calculate (a)
the electric eld strength and (b) the ux density.
A
d 3 10
3
m ; Q 35 10
3
C; V 15 0 V ; A 144 10
6
m
2
(a)
E
EAns
V
d
volt/metre
so kV/m
1
1
50
30
50
6
(b)
D
Q
A
D
coulom /metre
so C/m
b
2
3
6
2
35 0
44 0
243
1
11
1. Ans
3.6 Capacitance ( C )
We have seen that in order for one plate to be at a different potential to
the other one there is a need for a charge. This requirement is known
as the capacity of the system. For a given system the ratio of the charge
required to achieve a given p.d. is a constant for that system. This is
called the capacitance (C) of the system
i.e. faradC
Q
V
(3.7)
or coulombQVC (3.8)
From equation (3.7) it may be seen that the unit for capacitance is the
farad (F). This is defi ned as the capacitance of a system that requires a
charge of one coulomb in order to raise its potential by one volt.
The farad is a very large unit, so in practice it is more usual to
express capacitance values in microfarads ( F), nanofarads (nF), or
picofarads (pF).
Worked Example 3.4
Q Two parallel plates, separated by an air space of 4 mm, receive a charge of 0.2 mC when connected to
a 125 V source. Calculate (a) the electric eld strength between the plates, (b) the csa of the eld
between the plates if the ux density is 15 C/m
2
, and (c) the capacitance of the plates.
A
d 4 10
3
m ; Q 2 10
4
C; V 12 5 V ; D 15 C/m
2
84
Fundamental Electrical and Electronic Principles
(a)
E
EAns
V
d
volt/metre
so
,
kV/m
1
1
1
25
40
325
3
.
(b)
D
Q
A
A
Q
D
A
cou mb/metre
so, metre
thus,
lo
2
2
4
20
5
33 0
1
1
11.
62 2
33m or mm 1 . Ans
(c)
QCV
C
Q
V
C
coulomb
so, farad
thus, F
20
25
6
4
1
1
1. Ans
3.7 Capacitors
A capacitor is an electrical component that is designed to have a
specifi ed value of capacitance. In its simplest form it consists of two
parallel plates separated by a dielectric; i.e. exactly the system we have
been dealing with so far.
In order to be able to design a capacitor we need to know what dimensions
are required for the plates, the thickness of the dielectric (the distance of
separation d ), and the other properties of the dielectric material chosen.
Let us consider fi rst the properties associated with the dielectric.
3.8 Permittivity of Free Space ( ε
o
)
When an electric fi eld exists in a vacuum then the ratio of the electric
fl ux density to the electric fi eld strength is a constant, known as the
permittivity of free space.
The value for F/m.
o
ε
8 854 10
12
.
Since a vacuum is a well defi ned condition, the permittivity of free
space is chosen as the reference or datum value from which the
permittivity of all other dielectrics are measured. This is a similar
principle to using Earth potential as the datum for measuring voltages.
3.9 Relative Permittivity (ε
r
)
The capacitance of two plates will be increased if, instead of a vacuum
between the plates, some other dielectric is used. This difference in
capacitance for different dielectrics is accounted for by the relative
Electric Fields and Capacitors
85
permittivity of each dielectric. Thus relative permittivity is defi ned as
the ratio of the capacitance with that dielectric to the capacitance with
a vacuum dielectric
i.e.
r
ε
C
C
2
1
(3.9)
where C
1
is with a vacuum and C
2
is with the other dielectric.
Note: Dry air has the same effect as a vacuum so the relative
permittivity for air dielectrics 1.
3.10 Absolute Permittivity (ε )
For a given system the ratio of the electric fl ux density to the electric
fi eld strength is a constant, known as the absolute permittivity of the
dielectric being used.
so farad/metr
e
ε
D
E
(3.10)
but we have just seen that a dielectric (other than air) is more effective
than a vacuum by a factor of ε
r
times, so the absolute permittivity is
given by:
εεε
or
farad/metre
(3.11)
3.11 Calculating Capacitor Values
From the equation (3.10):
ε
ε
D
DQA Vd
Qd
VA
E
Ebut / and /
so
also, / capacitance
therefore
QV C
C
d
A
ε
and transposing this for C we have
C
A
d
A
d
ε
εε
or
farad
(3.12)
86
Fundamental Electrical and Electronic Principles
Worked Example 3.5
Q A capacitor is made from two parallel plates of dimensions 3 cm by 2 cm, separated by a sheet of mica
0.5 mm thick and of relative permittivity 5.8. Calculate (a) the capacitance and (b) the electric eld
strength if the capacitor is charged to a p.d. of 200 V.
A
A 3 2 10
4
m
2
; d 5 10
4
m ; ε
r
5.8; V 200 V
(a)
C
A
d
C
εε
or
farad
so pF
8 854 0 5 8 6 0
50
662
24
4
..
.
11
1
1
1
Ans
(b)
E
EAns
V
d
volt/metre
so kV/m
200
50
400
4
1
Worked Example 3.6
Q A capacitor of value 0.224 nF is to be made from two plates each 75 mm by 75 mm, using a waxed
paper dielectric of relative permittivity 2.5. Determine the thickness of paper required.
A
C 0.224 10
9
F; A 75 75 10
6
m
2
; ε
r
2.5
C
A
d
d
A
C
εε
εε
or
or
farad
so metre
8854 0 257575 0
26
..11
1
00 224 0
56 0 056
9
4
.
..
1
1so m mm d Ans
Worked Example 3.7
Q A capacitor of value 47 nF is made from two plates having an e ective csa of 4 cm
2
and separated by
a ceramic dielectric 0.1 mm thick. Calculate the relative permittivity.
A
C 4.7 10
8
F; A 4 10
4
m
2
; d 1 10
4
m
C
A
d
Cd
A
εε
ε
ε
or
r
o
farad
so
s
47 0 0
8 854 0 4 0
84
12 4
.
.
11
11
oo
r
ε 1327 Ans
Electric Fields and Capacitors
87
Worked Example 3.8
Q A p . d . o f 180 V creates an electric eld in a dielectric of relative permittivity 3.5, thickness 3 mm and of
e ective csa 4.2 cm
2
. Calculate the ux and ux density thus produced.
A
V 180 V; d 3 10
3
m ; ε
r
3.5; A 4.2 10
4
m
2
There are two possible methods of solving this problem; either determine the
capacitance and the use Q VC or determine the electric eld strength and use
D ε
o
ε
r
E . Both solutions will be shown.
C
A
d
C
εε
or
farad
so pF
8 854 0 3 5 4 2 0
30
434
24
3
...
.
11
1
1
aand since coulomb then
hence,
QVC
Q
Q
11
1
80 4 34 0
078
2
.
.nnC Ans
D
Q
A
D
coulomb/metre
so C/m
2
0
4
2
78 0
42 0
86
.
.
.
1
1
1
1
Ans
Alternatively:
E
EE
V
d
D
volt/metre
so kV/m and using cou
or
1
1
80
30
60
3
εε llomb/metre
so C/m
2
23
2
8 854 0 3 5 60 0
86
D
D
QDA
..
.
11
1
1
Ans
coulomb
so nC
11 1..
.
86 0 4 2 0
078
64
Q Ans
3.12 Capacitors in Parallel
Consider two capacitors that are identical in every way (same plate
dimensions, same dielectric material and same distance of separation
between plates) as shown in Fig. 3.8 . Let them now be moved vertically
until the top and bottom edges respectively of their plates make contact.
We will now effectively have a single capacitor of twice the csa of one
of the original capacitors, but all other properties will remain unchanged.
Fig. 3.8
88
Fundamental Electrical and Electronic Principles
Since C ε A/d farad, then the ‘ new ’ capacitor so formed will have
twice the capacitance of one of the original capacitors. The same effect
could have been achieved if we had simply connected the appropriate
plates together by means of a simple electrical connection. In other
words connect them in parallel with each other. Both of the original
capacitors have the same capacitance, and this fi gure is doubled when
they are connected in parallel. Thus we can draw the conclusion
that with this connection the total capacitance of the combination
is given simply by adding the capacitance values. However, this
might be considered as a special case. Let us verify this conclusion
by considering the general case of three different value capacitors
connected in parallel to a d.c. supply of V volts as in Fig. 3.9 .
Q
1
Q
2
Q
3
C
1
C
2
C
3
V
Fig. 3.9
Each capacitor will take a charge from the supply according to its
capacitance:
Q
1
VC
1
; Q
2
VC
2
; Q
3
VC
3
coulomb
but the total charge drawn from the supply must be:
Q Q
1
Q
2
Q
3
also, total charge, Q VC
where C is the total circuit capacitance.
Thus, VC VC
1
VC
2
VC
3
, and dividing through by V
CC C C
123
farad
(3.13)
Note:
This result is exactly the opposite in form to that for resistors
in parallel.
Electric Fields and Capacitors
89
Worked Example 3.9
Q Three capacitors of value 4.7 F, 3.9 F and 2.2 F are connected in parallel. Calculate the resulting
capacitance of this combination.
A
In this case, since all of the capacitor values are in F then it is not necessary
to show the 10
6
multiplier in each case, since the answer is best quoted in F .
However it must be made clear that this is what has been done.
CC C C
C
C
1
1
23
47 39 22
08
microfarad
so F
...
. Ans
3.13 Capacitors in Series
Three parallel plate capacitors are shown connected in series in
Fig. 3.10 . Each capacitor will receive a charge. However, you may
wonder how capacitor C
2
can receive any charging current since it is
sandwiched between the other two, and of course the charging current
cannot fl ow through the dielectrics of these. The answer lies in the
explanation of the charging process described in section 3.6 earlier.
To assist the explanation, the plates of capacitors C
1
to C
3
have been
labelled with letters.
C
1
AB CD E
E
F
C
2
C
3
Fig. 3.10
Plate A will lose electrons to the positive terminal of the supply, and
so acquires a positive charge. This creates an electric fi eld in the
dielectric of C
1
which will cause plate B to attract electrons from plate
C of C
2
. The resulting electric fi eld in C
2
in turn causes plate D to
attract electrons from plate E . Finally, plate F attracts electrons from
the negative terminal of the supply. Thus all three capacitors become
charged to the same value.
Having established that all three capacitors will receive the same
amount of charge, let us now determine the total capacitance of the
arrangement. Since the capacitors are of different values then each
will acquire a different p.d. between its plates. This is illustrated in
Fig. 3.11 .
90
Fundamental Electrical and Electronic Principles
V
Q
C
V
Q
C
V
Q
C
1
1
2
2
3
3
; ; volt
and V V
1
V
2
V
3
(Kirchhoff ’ s voltage law)
also,
V
Q
C
volt,
where C is the total circuit capacitance.
Hence, , and dividing by :
Q
C
Q
C
Q
C
Q
C
Q
123
C
1
V
1
V
2
V
V
3
C
2
C
3
QQQ
Fig. 3.11
1111
123
CC C C
(3.14)
Note:
The above equation does not give the total capacitance
directly. To obtain the value for C the reciprocal of the answer
obtained from equation (3.14) must be found. However, if ONLY
TWO capacitors are connected in series the total capacitance may
be obtained directly by using the ‘ product/sum ’ form
i.e. faradC
CC
CC
12
12
(3.15)
Worked Example 3.10
Q A 6 F and a 4 F capacitor are connected in series across a 150 V supply. Calculate (a) the total
capacitance, (b) the charge on each capacitor and (c) the p.d. developed across each.
A
Figure 3.12 shows the appropriate circuit diagram.
C
1
6 F; C
2
4 F; V 150 V