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) Q=It

(µC )V ( v) C = Q / V

(µ F)0 60 0

10 60 1.2

20 60 2.4

30 60 3.6

40 60 4.8

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/solid-state-electronics-channel.htm

http://us.c2w.com/quizzes/45045-capacitor-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 = C V

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

Capacitors in Parallel

Capacitors in sires

V Same 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