## Tuesday, July 26, 2011

### Conceptual Questions - Capacitors

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 proportional to 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?
As the capacitor plates are separated, C decreases. Since charge on the plates remains the same, value of V increases. Hence, the reading of the voltmeter will increase.
Q. 3. Any conducting object connected to 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 earth is also at zero potential or â€œground potentialâ€. Alternatively, the
capacitance of earth is so large that removal of electrons from it or supply of electrons to it
makes no difference either in the charge or potential of earth.
Q. 4. How does 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 ionised and 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 thereany 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 high potential (V = q/ C).
Q.6 Given a solid metal sphere and a hollow metal sphere. Which will hold more charge? Bothspheres are of same radius.
Ans. Both the spheres will hold the same charge. It is because charge remains on the outer surface of a charged conductor (whether solid or hollow) and the spheres have equal surface areas.
Q. 7. Two capacitors of capacitances 1 Î¼F and 0.01 Î¼F are charged to the same potential. Which willgive more intense electric shock if touched?
Ans. q = CV. Since V is constant, qâˆC. It means that capacitor having large capacitance will store more
charge. Hence, when 1Î¼F capacitor is touched, the discharging current will be high and you will
get more intense electric shock than in case of 0.01Î¼F capacitor.
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 redistribution of charge (i.e., flow of charge through wire) till the two spheres attain the
same potential. Due to the flow of charge through the connecting wire, some energy will be lost as
heat. Hence, the total energy after connecting the spheres will decrease.
Q. 9. Can there be 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 coolenough. However, there is a limit to the electric flux that a dielectric will carry without breaking down.