Capacitor

 

Capacitor is Device that Store an Electrical Charge between the Two Plates. A capacitor is made two parallel plates separated by an insulating material called  dielectric. When a voltage source is connected across the plates, a momentary charging current deposits charge on the plates which establishes an electric field  Capacitor when fully charged, can act as a voltage source and can discharge through a Resistor or wire. Thus a capacitor has an abul to store electrical energy. The unit of charge is Coulomb and it denoted by the C.

• two plates of a capacitor have an equal number of free electrons. When the capacitor is connected to a voltage source through a resistor, the electrons of negative charge are removed from plate A, and an equal number are deposited on plate `B'. Plate A becomes positive with respect to plate B as shown in The current enters and leaves the capacitor, but the insulation between the capacitor plates prevents the current from flowing through the capacitor.

As electrons flowing into the negative plate of a capacitor have a polarity opposite to that of the battery supplying the current, the voltage across the capacitor opposes the battery voltage. The total circuit voltage, therefore, consists of two series-opposing voltages.

As the voltage across the capacitor increases, the effective circuit voltage, which is the difference between the battery voltage and the capacitor voltage, decreases. This, in turn, causes a decrease in the circuit current  When the voltage across the capacitor equals the battery voltage, the effective voltage in the circuit is zero, and so the current flow stops. At this point, the capacitor is fully charged, and no further current can flow in the circuit.

ELECTRIC field of acharged capacitor

CAPACITANCE : it's abul to store energy in the form of Electric charge it's called capacitance . capacitance is measure in FARAD and symbol of farad is ( F ) .

SYMBOL OF CAPACITOR : there are three types of symbol 

1:fixed capacitor (non polarized)

Fixed capacitor (non polarized )

2 : polarized capacitor

Polarized capacitor

3: variable capacitor

Variable capacitor

• FORMULA OF CAPACITOR:

                C= Q/V 

    

C=capacitance

Q= store charge

V = voltage across capacity

• TYPES OF CAPACITOR :

There are two types of capacitor

1: material Wise

2: polarity Wise

1: material Wise

I)paper capacitor

In these types of capacitor, dielectric medium is old paper . They can be made in range of .001 micro Farad to 1 µF.

ii)mica capacitor

This capacitor used as dielectrics. Their capacitance ranges from 330 micro Farad to 0.035 micro Farad . They can be used in high temperature applications

iii) ceramic capacitor

In these capacitors is dielectric medium is silicate of magnesia, Barium  Tytenate . Their capacitance ranges from 5 micro farad to 0.5 micro Farad . It's can be used in high frequency appliances

iv) electrolyte capacitor

Electrolyte capacitor

These can be made up to 1000 micro Farad . They cannot be used in AC supply They are used in filtering and cathode

2: polarity Wise capacitor :

I ) polar type capacitor

ii) bipolar type capacitor

• USES OF CAPACITOR : capacitor are used for capacitor bank fitting , energy storage , motor starter  etc.......

• WORK OF CAPACITOR : 

-- capacitor block DC supply and allow AC in the circuit

-- capacitor work as a filter

-- capacitor work as a discharge unit

• SOME IMPORTANT QUESTION AND ANSWER OF CAPACITOR :

1. If the supply frequency  goes from F to 2F, then the reactance of the capacitor will be-

A) 0.5Xc 

B) 2Xc

C) 4Xc

D)  0.25Xc                                         Ans=A

3. the voltage in a short circuited capacitor is-

A ) Maximum

B) Minimum

C) Zero

D) As per capacity                              Ans=D

4. How much is the capacitor connected to the 50 V supply charged in the first 1t?

A) 50V क

B) 41.4 V 

C) 63.2 V 

D)  31.6 V                                        Ans=D

5. Capacitance is the inverse of ... 

A)  Elastance

B) Reluctance

C) Susceptance

D) Impedance.                                   Ans=A

6. Variable capacity is one whose capacity is ....

A) Change with time

B) Change with temperature

C) Can be changed by manually or mechanically

D) Change with voltage                      Ans=C

7. At 25 Va capacitor collects a charge of 0.5 C. Find its capacity

A) 12.5 F

B) 50 F

C)  0.02 F

D) 0.25 F.                                             Ans=C

8. What will be the ratio of the series capacitance of two capacitors of equal capacitance and the total capacitance of parallel capacitance?

A) 1:4

B) 4:1

C) 1:2

D) 2:1                                               Ans=A

9. If the length of a cable is doubled  then its capacitance-

A) Double

B)  Half

C) 1/4.                                              Ans=A

D) 4time

10. Which of the following devices collects energy in the form of an electrostatic field?

A) Resistor

B) Inductor

C) Conductor

D) Capacitor.                                    Ans=D

11. Capacitor capacitance is not affected by......

A)  Dielectric material

B) Area of plates

C) Distance between plates

D) Frequency.                                     Ans=D

12.  in a capacitor the dielectric material replaced by a conductor then......

A) Capacitor too hot due to more eddy current

B) Capacitor will short circuited

C) No effect on capacitor

D) Capacitor can accumulate infinity charge

                                                        Ans=B

13. What is the dielectric material used in variable capacitors?

 A) Ceramic

B) Paper

C) Teflon

D) air                                                   Ans=D

14. Which is wrong about the combination and function of capacitors?

A) For phase spliting-in series with starting winding

B) For pf Improvement - in parallel with load

C)For phase spliting-in series with starting winding 

D) For pf Improvement-in parallel with load

                                                    Ans=E

15. The resistance in the circuit is 6 ohms and the capacitor is 0.5 micro Farad . Find the time constant -

A) 3 ms

B) 30 ms

C) 300 ms

D) 3000 ms                                        Ans=D

16. The energy collected by a capacitor is doubled. What is the increase in voltage applied to it?

A) 41.41%

B) 86.6 %

C) 28.28%

D) 66.31 %                                       Ans=A

17. In a motor circuit you need 5 uf capacity. You have a capacitor of 2.5 uf available. Arrangements will be made to get the required capacity--

A) 2 capacitor in series

B) 2 capacitor in parallel

C) 4 capacitor in series

D) 4 capacitor in parallel                  Ans=B

18. a capacitor

A)  Pass ac & d c

B) Block ac & d c

C) Pass ac, block d c

D) Pass d c , block ac                         Ans=C

20. What does indicate in the Types of capacitors  letter code 432

A) Voltage rating

B) Tolerance

C) Multiplier

D) Types of capacitor                         Ans=B

22. Actual power expenditure in a pure capacitor

A)  Zero

B) Minimum

C) Unity

D)  Maximum                                Ans=A

23. The capacitor can be rated in two ways

A) Farad, kvar

B) Farad, kw

C) Farad, kva                                     Ans=A

24. Which of the following is the type of capacitor

A)  Ceramic 

B) Paper

C) Electrolytic

D) All above                                      Ans=D

25. The opposition of voltage changes in the circuit circuit is called-

A) Capacitance

B) Inductance

C) Conductance

D) Resistance                                  Ans=A