## December 11, 2012

### Answers to Review Questions: Ch # 12 Electrostatics

Karachi Board (BSEK)

Q.12.1 Repulsion is sure test of electrification, explain.
Ans.As we know that the two same charges if come closer they repel each other. for example +ve repel +ve charge and -ve repel -ve charge .and No two bodies which have no charge will never repel each other .Therefore repulsion is sure test of electrification.

Q.12.2 Will a solid sphere hold a larger electric charge than a hollow sphere of the same diameter ?Where does the charge resides in each case?
Ans.A solid metal sphere will hold the same amount of charge as is hold by a hollow sphere of same diameter.Becaus in Both the cases the charge always resides on the suface so that the hollow and solid sphere have the same surface area.

Q.12.3 Explain why it is so much easier to reove an electron from an atom of large atomic mass than is to remove a proton?
Ans.When an atom has large atomic mass so that it will have more orbits which means grater the distance from the nucleas where the proton is present to attaract it.Now if the electron of large atomic mass is remove so that there is weaker force of attaraction with proton therefore it is easy to remove.

Q.12.4 Why it is not crrect to say that the potential diffrence is work done in moving a unit positive charge b/w the points concerned.
Ans.The potential diffrernce is the increase in electric potential energy per unit charge.and as we know that the poteial energy is raised when a unit positive chrge is moved against the electric field.so it is not correct to say that.

Q.12.5 Why is it logical to say thet the potential of an earth connected object is zero.what can be said about charge on earth?
Ans.The earth is taken to be at zero potential.When a charged body is connected to the earth,electron flow will take place such that the cherge of the body is neutralized.As earth is a good conductor(netral body).So that we can say that it is a sink for the electron,electrons flows to earth grately without changing its potential.

Q.12.6 Can an electric potential exists at a point where electric field is zero.Can the potential be zero at a place where electric intesity is zero,Give examples
Ans.Electric potential can exists where the elctric feld is zero.Because the charge resides on the surface ,inside the body the field line cancels each other so that there is no electric intesity and neverthless it do have a potetial difference.
For example :-The point in the middle of the two equal but opposite chrges.But still the electric intesity is towards negetive charge.
Both the potential and intsities are zero for a point at infinity.

Q.12.7 An air capacitor is charged to a certain potential difference it is then immersed in oil what happen to it’s a)charge b)Potential and c) capacitance
Ans.The dielectric constant Ð„r of the oil is grater than that of air.Whan an air capacitor is immersed in oil then
a)Its charge remain constant b)PD b/w the plates decreases c)the Capacitance increase

Q.12.8 Two unlike capacitors of different potentials and charges are joined in parallel.what happens to their pD?How are their charges distributed ?Is the energy of system affected?
Ans.(i)The potential difference will remain same.(ii) The charge is distributed (iii)The energy of the system decrease

Q.12.9. Four similar capacitors are connected in series and joined to 36 volt battery .The mid point of the group is earthened.What is the potential of the terminal of the group?
Ans.If two similar capacitors are connected in series joined to a 36 V battery and if mid point is connected to earth then there is no transfer of charges.Hence the potential difference will remain same because the mid point is b/w the oppositely charged plates of C2 and C3

Q.12.10.A point charge is placed at the centre  of spherical Gaussian surface. Is the Flux Changed?(i)If the guassian surface is replaced by the cube of the same volume(ii) If the sphere is replaced by a cube of 1/10 of this volume.(iii)If the charge is moved from from the centre in the sphere(iv)If the charge is moved outside the sphere(v) If a second chrge is placed inside the sphere.
Ans.(i) No   .(ii) No  .(iii) No   .(iv) Yes  .(v) Yes

Q.12.11. Four capacitors each of 2ÂµF connected in suh a way that the total capacitance is also  2ÂµF .Show what will be the combination.?
Ans. The capacitors should be joined in combinations
(i)                 Two pairs of parallel combination

Q.12.12. A capacitor is charged by a battery .the battery is disconnected and a slab of some dielectric is slipped b/w the plates.Describe what happen to the charge,Potential difference ,capacitance and energy stored?
Ans. The dielectric constant Ð„r of the slab is grater than that of air.
.(i)The potential difference decreases.(ii) The charge remain same (iii)The capacitance increases(iv) The energy stored will decrease

Q.12.13 Answer the questions if the battery is not disconnected.
Ans. The dielectric constant Ð„r of the slab is grater than that of air.
.(i)The potential difference Remains constant.(ii) The charge Increases (iii)The capacitance increases(iv) The energy stored will increase

Q.12.14 A capacitor is connected across a battery.Why does each plate receives a charge of same magnitude?will it be true if plates are of different sizes?
Ans. Because the electrons move from the –ve terminal to the +ve terminal because the –ve terminal is at the high potential and +ve is at low potential.The charges always move from the high to low potential.The same will be the process if the plates are of different sizes.

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