# KPK Board 12th class Physics Ch 11 Electrostatics short questions answers

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The electric potential is constant through a given region of space. Is the electric field zero or non zero in this region explaining?

When electric potential is constant throughout a given region of space it is not necessary that the field must be zero. Electric field can exist in a region where electric potential is constant.

Explanation:-As electric field is defined as the negative of potential gradient i.e.

E=-ΔV/Δr

So, if in any region of space potential V is constant, then:

ΔV=0

Hence      E=0

Example is interior of a hollow conducting spherical shell. There also exists a case in which they potential are constant in a non-zero electrical field. Example is a given between the plates of a charged capacitor in which a potential is a constant on a potential line and electric field also exist in which is constant on an equipotential line and electric field also exist which is directed from positive plate to negative plate.

Mathematically,

ΔV=E. ΔràΔV=EΔrcos

ΔV=EΔrcos90

ΔV=0

So ΔV=0 means V=constant although E is not equal 0

So, it is concluded that it is a not necessary for constant electric potential that the electric field must also be zero.

if a point charge of mass m is released in a non uniform electric field with field lines pointing in same direction will it make a rectilinear motion?

Yes, it will make rectilinear motion.

Explanation:

A non- uniform electric field is that field in which magnitude of field lines and direction varies, and if the field lines are pointing in the same direction then only magnitude of the field will vary. Now according to the statement it is mentioned that the electric field is non uniform with field lines pointing in the same direction. So in this case if a point charge of mass m is released, electric force of varying force will act on the charged particle and it will make rectilinear motion (motion in a straight path) with certain acceleration.

What is the relationship between voltage and energy? More precisely what is a relationship between potential difference and electric potential energy?

potential energy is equal to the charge multiplied by the electric potential.

Explanation: whenever a charged particle is moved from lower potential to higher potential work has to be done on it. This work is stored in the body in the form of potential energy. It is represented by U ‘and is measured in Joules.

Potential difference V between two points is the potential energy U Per unit charge q not in moving it from lower potential to higher potential it is measured in volts.

Mathematically,

The relationship between potential energy and potential difference can be explained as

U= qxV

voltage always measured between two points why?

Voltage is always measured with respect to a reference so you need two points, one the reference and other the point where it is being measured.

Explanation: voltage is a difference in a potential between two points. Where one point is a higher potential and the other lower potential, therefore the difference is what matters that and it termed as potential difference or voltage. If ΔV is the potential difference between points A and B then

V=U/q=W/q

Where ΔU   the potential energy difference between points A and B and w is the work done in moving a unit charge between these two points.

How are units of volts and electron volts related how to they differ?

Volt is the unit of electric potential, potential difference are electric or elect motive force (e.m.f) defined as the difference of potential between the two points on a conductor carrying a constant current of 1 ampere when the power dissipated between the points in 1 watt. It is named after

Electron volt is the unit of energy 1 EV is equal to the amount of energy. One electronic world by accelerating through the potential difference of 1 volt it is a usually used as a measure of a particle energies although it is not an s I unit the SI unit of energy is Julie. 1 EV is equal to 1.6 x 10-19 raise to power minus 19 job It may help to realize that the electric potential is a property associated with the  field in the space while the energy is associated with the particle you place into the field and it depend on the particle.

In what region of space is the potential due to the uniformly charged sphere is the same as that of the point charge in what region does it differ from the that of the point charge?

Explanation:  Electric potential due to uniformly charged Sphere in the same everywhere around the space as that of a point charge, while it is different inside this is fair as that this point charge. Explanation: electric potential due to the point charge Q at the distance R is given by the equation                   V=Kq/R(1)                                                                                                                                                                    Now consider a uniformly charged sphere of radius R having charge q the electric potential inside this Fair at distance R is;                                                                                                                                        V=KQ/r(2)

However for outside this fear at a distance R electric potential is given by;                                      V=KQ/R(3)                                                                                                                                                                            This is a because uniformly charged sphere behave like a point charge for a point playing outside this fare the equation and equation to show that it is equal for all points lines outside the sphere as that of the point charge while it differ from all those part lie on the surface are inside the surface of sphere.

Can the potential of a non-uniformly charged sphere be the same as that the point charge explain?

Yes, at very large distance the potential of non-uniformly charged sphere is the same as that of the point charge.                                                                                                                                                                       Explanation: consider a non-uniformly charged sphere, now at the point lying at a very large distance r from the sphere; the sphere may be regarded as a point charge. so, in that cases the potential is given by;     V=KQ/r                                                                                                                                                                              But inside of the non uniform charge square potential cannot be same as potential for a point charge because last three is non-uniform so the electric field produces will also be non-uniform. Due to which the potential will not be same as that of point charge.

What are equipotential lines and equipotential surface?

Equipotential surface.

A surface on which all points are at equal potential is called a potential surface

equipotential lines

Those lines which are drawn in order to represent equipotential surface are called utensil lines.

Can different equipotential lines cross? Explain.

No different equipotential lines never cross one another.

Explanation:

We know that there is a constant value of a potential associated with each other points of an equipotential lines. Now if two different equipotential lines intersect each other at the point of interaction, there will be two value of potential which are physically not possible. The different equipotential lines cannot cross each other.

Water has a large dielectric constant, but it is rarely used in capacitors. Explain why?

In spite of large dielectric constant, water is rarely used in capacitor due to some of its property.

Explanation: some of the reason which makes water unsuitable for to be used as dielectric material in capacitor as given below.

1. It consists of impurities which make it a conductor and will provide a conducting path for discharging the capacitor.
2. It is corrosive in nature and can reduce the capacitance of a capacitor.

Generally, a solid dielectric is used in capacitors which help in keeping the plates a close to each other without touching. as a result, with the capacitor may be charged to maximum extent.

A capacitor is connected in series with a resistor and charge. Explain why the P.D across the resistor decreases with time during the charging?

As the capacitor charge, the energy required to build up for the charge decrease as a result current is the circuit decreases and the potential difference across three resistor connected in series also decreases.

Explanation: Consider a series RC circuit. When the switch is closed the battery starts charging capacitor through resistor. The capacitor is not charged immediately but the charges developed gradually on the capacitor up to the maximum value of Q. initially very little work is required to transfer charge from one plate to another plate. But the charge start developing then more and more work is required to transfer the charge from one plate to another plate. Current is in the circuit drops which result in the dropping of potential across resistor according to Ohm’s law.

V = IR

Explain why capacitors are of little used for storage of energy for normal domestic purpose of lightning heating as and so on.

Ordinary capacitors can store small amount of charge, which make them a limited to store energy. Therefore, they are little used for storage of energy for normal domestic purpose.

Explanation: while charging a capacitor, one can say that the capacitor is fully charged when the potential between the plates of the capacitor becomes equal to the potential of the battery. But when High Voltage is applied to the capacitor to store more charge, charges jump from one place to another plate making a dielectric conduct. This is called breakdown of capacitor, and the voltage at which the breakdown of capacitor is called breakdown voltage, this phenomenon makes a capacitor unsuitable for storing large amount of charges for a definite time period. Batteries are preferred to store energy two capacitors because of their large capacity the second reason is that, for the smooth operation of electrical appliances; a constant potential source is required. While using capacitor the potential drop with the passage of time in discharging.

Compare the formula of capacitors in series and parallel with those for resistors in series and parallel. Explain why the pattern is different.

Ans: In series combination of the capacitors and resistors the equivalent capacitance and resistance is given by the following equations respectively

1/Ce = 1/C1 + 1/C2 + 1/C3 + . . . and Re = R1 + R2 + R3 + . . .

And in parallel combination the equal length capacitance and resistance is given by the following equation respectively,

Ce = C1 + C2 + C3 + . . . and 1/Re = 1/R1 + 1/R2 + 1/R3 + . . .

The formula pattern is the result of the distribution and equivalence of different patterns in the combination. in series combination in charge our current remain the same while potential split, while in parallel combination charge or current divides and potential remains same.

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