## Short Notes of SSC JE | Capacitance

Q.1. The two charged conductors are touched mutually and then separated. What will be the charge on them?

Ans. The charge on them will be divided in the ratio of their capacitances. We know that q= CV. When the charged conductors touch, they acquire the same potential. Hence, q∝C.

Q. 2. The plates of a charged capacitor are connected to a voltmeter. If the plates of the capacitor are separated further, what will be the effect on the reading of the voltmeter?

Ans. V = q / c and C = ε0A /d
As the capacitor plates are separated, C decreases. Since the charge on the plates remains the same, the value of V increases. Hence, the reading of the voltmeter will increase.

Q. 3. Any conducting object connected to the earth is said to be grounded. Explain.

Ans. The earth is an electron source or sink and is arbitrarily said to be at zero potential. A conducting body connected to the earth is also at zero potential or “ground potential”. Alternatively, the capacitance of the earth is so large that the removal of electrons from it or the supply of electrons to it makes no difference either in the charge or potential of the earth.

Q. 4. Now do a spark discharge occur between two charged objects?

Ans. The air between the two charged objects is subjected to an electric field. If the potential gradient in the intervening air column becomes high enough, the air is ionized and a conducting path is formed for free electrons which move across to discharge the surfaces. Stored electric potential energy is dissipated as heat, light, and sound.

Q. 5. If a solid dielectric is placed between the plates of a capacitor, its capacitance increases. Is there any other advantage of solid dielectric?

Ans. There are other two advantages of a solid dielectric. First, it helps in keeping the plates close together without touching. Secondly, we can now charge the capacitor to a higher potential (V = q/C).

Q. 6. Can you place a parallel plate capacitor (consisting of two plates) of I farad in your almirah?

Ans. No. Suppose the two plates of the capacitor are separated by as small a distance as 1 mm.
C = ε0A / d
or A= C d/ε0 = 1 x (1 x 10-3 ) / 8.854 x10-12 = 1.1 x 108 m2
This area is equal to the area of a square having each side more than 10 km. Modern technology, however, has permitted the construction of 1F capacitors of very moderate size.

Q. 7. Given a solid metal sphere and a hollow metal sphere. Which will hold more charge? Both spheres are of the same radius.

Ans. Both spheres will hold the same charge. It is because the charge remains on the outer surface of a charged conductor (whether solid or hollow) and the spheres have equal surface areas.

Q. 8. Two spheres of different capacitances are charged to different potentials. They are then joined by a wire. Will total energy increase, decrease or remain the same?

Ans. The two spheres are at different potentials. Therefore, when they are connected by a wire, there will be a redistribution of charge (i.e., the flow of charge through the wire) till the two spheres attain the same potential. Due to the flow of charge through the connecting wire. some energy will be lost heat. hence, the total energy after connecting the spheres will decrease.

Q.9. In. Can there be a potential difference between two adjacent conductors which carry the same positive charge?

Ans. Yes. We know that V =q/C. The capacitance depends upon the dimensions of the conductor. If the two conductors are of different shapes and sizes, they will be charged to different potentials when given the same charge.

Q. 10. What are the differences between conductors and dielectrics?

Ans. (i) Conductors have a large number of free electrons while dielectrics have practically no free electrons.
(ii) When a conductor is placed in an external electric field, there is no electric field inside the conductor. however, when a dielectric is placed in an electric field, its molecules are polarised. The effect of this polarisation is to weaken the applied electric field within the dielectric.
(iii) The dielectric constant of conductors is infinity while that of dielectrics is finite.
(iv) The dielectric strength of conductors is zero while that of dielectrics is finite.
(v) There is no limit to the current that a conductor can carry, provided that it can be kept cool enough. However, there is a limit to the electric flux that a dielectric will carry without breaking down.

Q. 11. Is the name condenser appropriate for a capacitor?

Ans. The name condenser is given to the device due to the fact that when p.d. is applied across it, the electric lines of force are condensed in the small space between the plates. In fact, a capacitor is a device used for ‘condensing’ or accumulating the charge on plates. The term condenser is misleading and, therefore, the term capacitor is preferred.

Q. 12. Why do capacitors connected in series result in less total capacitance?

Ans. When capacitors are connected in series, they act as though we were adding to the thickness of the dielectric. Whenever the spacing between plates is increased, the capacitance decreases. Even the formula for series-connected capacitors reveals this fact.

Q.13. Why do mast variable capacitors use air as the dielectric?

Ans. most variable capacitors have an air dielectric because o the following reasons:

1. Air is the most perfect dielectric material known.
2. There is minimum waste of energy and consequently, efficiency is very high.
3. The ‘all out (i.e., minimum) capacitance is very low.

Q. 14. Although the energy stored in a capacitor is very small. yet it can provide a quantity of power. Why?

Ans. It is because it can provide a large current for a short period of time. That a capacitor when used as an energy source, has a very low internal resistance. As a capacitor is used in an electronic photo flash to fire the Xenon flash tube. while the capacitor only stores 20 J to 40 J of energy, it is discharged by the bulb in about 1 ms or less. Since P =W/t, the power involved in removing 20 J from a capacitor in 1 ms is P = 20/1 x 10-3 =20 x 103 W = 20 kW.

Q. 15. Can you connect a capacitor directly to a d.c. source?

Ans. No, because there is no resistance to limit the current in the circuit. An uncharted capacitor is equivalent to a short circuit so far as d.c. a voltage source is concerned. Therefore, unless a resistor is connected in series with a capacitor, the flow of current when a voltage source is first connected to it is limited by the small internal-. resistance of the source. The surge of current that flows when no resistor is present may be great enough to damage the capacitor or the source or both.

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