Download capacitor

capacitor
They are important in both electric &
electronic circuit
• Have you seen any of them before ?
Function :
• Capacitors can store and then release
electrical energy .
• The value of a capacitors is often measured in
micro-farads .
• The amount of energy stored by the capacitor
depends on :
How much charged is moved on to the plates .
• The conducting plates are separated by an
insulating material which is called the
Dielectric
Charging a capacitor :
What happens when
the switch closed ?
Discharging capacitor :
if the battery is removed , we can
discharge the capacitor
+
_
Why does the direction of
charging current in the opposite
way of current of the battery ?
The charging & discharging processes
happen very quickly .Can we slow
them down ?
Capacitance
• The ability of a capacitor to store charge on its
plates.
• The large the capacitance , the more charge
the capacitor stores for each volt applied
across its plates . In fact :
• Capacitance = charge / voltage
• C=Q/V
How can we measured the charge on
the capacitor plates ?
Measuring capacitance :
Complete the table :
t(s)
I(µ A)
0
60
0
10
60
1.2
20
60
2.4
30
60
3.6
40
60
4.8
Q=It
C) µ(
V ( v)
C=Q/V
(µ F)
The energy of a charged capacitor
•
A
B
Questions :
• Q1:What happens to the switch in position A?
• Q2:What happens to the switch in position B?
• Q3:Where did this energy come from ?
• The p.d. across the capacitor and the
size of its capacitance , both affect
the energy stored .
• Energy stored =0.5Xcapacitance X
(voltage)2
W = 0.5 X C X V2
OR
W = 0.5 X Q X V
Q1
• A capacitor is charged using a current of
20µA from a 5volts supply .After 55s it is fully
charged.
• Calculate :
• A. Q?
• B. C?
• C. W?
More Qs.
• http://electronics.howstuffworks.com/solidstate-electronics-channel.htm
http://us.c2w.com/quizzes/45045capacitor-quiz
Capacitor in parallel
Here are 3 facts that you should know
for
• The p.d. across each capacitors is the same &
equal to V .
• The total Q is the sum
Q = Q1+Q2
• The total C is the sum
C = C1+C2
Q=CV
Capacitor in series
Here are 3 facts that you should know
for
• The Q. across each capacitors is the same &
equal to Q .
• The total V is the sum
V= V1+V2
• The total C is the sum
1/C = 1/C1+ 1/C2
V
Capacitors in
Parallel
Same
Capacitors in
sires
V = v1+v2
Q
Q = Q1+Q2
Same
C
C =C1+C2
1/C = 1/C1 + 1/C2
Qs.
• Q1: Calculate the charge on each capacitor in
this diagram :
• If :
• V=5volt
• C1=500 micro farad
C2=300 micro farad
Qs.:
Pg. 273
,
Q:6
Home Work :
Pg. 273
, Q :1-3-4-5