capacitor

They are important in both electric & electronic circuit

• Have you seen any of them before ?

Where can we find the capacitors ?

We can find it in radio, TV sets , and computers .

Although there are many different types of capacitors , but they all work in the same way , and that which make it easy to use .

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 .

properties

• All the capacitor have two leads the leads connect to two plates ,the conducting plates are separated by an insulating material which called the dielectric

• The pates in capacitor need to have quite a large area

• How is this achieved • By makes the insulator rolled up

Capacitors• In its simplest form, a capacitor consists of two metal plates, separated from

each other by an insulator (air, mica, paper etc.)• A charged capacitor is shown below.

• The capacitor plates have an equal but opposite charge of magnitude Q.• The net charge on the two plates is zero.

+Q+++++++

-Q-------

Charging a capacitor :What happens when

the switch closed ?

Discharging capacitor :if the battery is removed , we can discharge the capacitor

+

_

The charging & discharging processes happen very quickly .Can we show them down ?

Chart

Q

t

Q

t

Charging

Discharging

Why does the direction of charging current in the opposite way of current of the battery ?

Q

• Why current equal zero ?• Because p.d of battery =p.d of capacitor but

in opposite direction

• The charging & discharging processes happen very quickly .Can we slow them down ?

• (if we place a resistor in series with capacitor the capacitor charge and discharges more slowly

• Also we put a variable resistor to keep the current in steady value

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• Solve example1 page 267

How can we measured the charge on the capacitor plates ?

i.t=q

Measuring capacitance :

Complete the table :C = Q / V

(F)V ( v) Q=It

(C )I(A) t(s)

0 60 0

1.2 60 10

2.4 60 20

3.6 60 30

4.8 60 40

• A B

Questions :

• Q1:What happens to the capacitor when the switch in position A?

• Q2:What happens if you move the switch to 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

The energy of a charged capacitor

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?

Use these link to find more about our lessons

• 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 capacitors in parallel

• 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 sires Capacitors in Parallel

V = v1+v2 Same V

Same Q = Q1+Q2 Q

1/C = 1/C1 + 1/C2 C =C1+C2 C

Qs.

• Q1: Calculate the charge on each capacitor in this diagram :

• If :• V=5volt• C1=500 micro farad C2=300 micro farad

Home Work :Pg. 273 , Q :1-3-4-5-6