Answer the following question. Two identical point charges, q each, are kept 2m apart in the air. A third point charge Q of unknown magnitude and sign is placed on the line joining the charges - Physics


Answer the following question.
Two identical point charges, q each, are kept 2m apart in the air. A third point charge Q of unknown magnitude and sign is placed on the line joining the charges such that the system remains in equilibrium. Find the position and nature of Q.



`(K(q)(Q))/x = (-K(q)(q))/2`

⇒ `Q = (-qx)/2`

`because (KqQ)/x = (KQq)/y`

x = y

x + y = 2

∴ x = y = 1

`Q = (-q)/2`

Concept: Equipotential Surfaces
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2018-2019 (March) 55/1/3


Two charges 2 μC and −2 µC are placed at points A and B 6 cm apart.

(a) Identify an equipotential surface of the system.

(b) What is the direction of the electric field at every point on this surface?

The top of the atmosphere is at about 400 kV with respect to the surface of the earth, corresponding to an electric field that decreases with altitude. Near the surface of the earth, the field is about 100 Vm−1. Why then do we not get an electric shock as we step out of our house into the open? (Assume the house to be a steel cage so there is no field inside!)

A man fixes outside his house one evening a two metre high insulating slab carrying on its top a large aluminium sheet of area 1 m2. Will he get an electric shock if he touches the metal sheet next morning?

What are the forms of energy into which the electrical energy of the atmosphere is dissipated during a lightning?
(Hint: The earth has an electric field of about 100 Vm−1 at its surface in the downward direction, corresponding to a surface charge density = −10−9 C m−2. Due to the slight conductivity of the atmosphere up to about 50 km (beyond which it is good conductor), about + 1800 C is pumped every second into the earth as a whole. The earth, however, does not get discharged since thunderstorms and lightning occurring continually all over the globe pump an equal amount of negative charge on the earth.)

What is the geometrical shape of equipotential surfaces due to a single isolated charge?

Draw the equipotential surfaces due to an electric dipole. Locate the points where the potential due to the dipole is zero.

Define equipotential surface. 

Find the amount of work done in rotating an electric dipole of dipole moment 3.2 x 10- 8Cm from its position of stable equilibrium to the position of unstable equilibrium in a uniform electric field if intensity 104 N/C.  

Statement - 1: For practical purpose, the earth is used as a reference at zero potential in electrical circuits.

Statement - 2: The electrical potential of a sphere of radius R with charge Q uniformly distributed on the surface is given by `Q/(4piepsilon_0R)`.

A particle of mass 'm' having charge 'q' is held at rest in uniform electric field of intensity 'E'. When it is released, the kinetic energy attained by it after covering a distance 'y' will be ______.

S1 and S2 are the two imaginary surfaces enclosing the charges +q and -q as shown. The electric flux through S1 and S2 are respectively ______.

Equipotentials at a great distance from a collection of charges whose total sum is not zero are approximately.

Which of the following statements is/are correct for equipotential surface?
  1. The potential at all the points on an equipotential surface is same.
  2. Equipotential surfaces never intersect each other.
  3. Work done in moving a charge from one point to other on an equipotential surface is zero.

Can two equipotential surfaces intersect each other? 

Consider a uniform electric field in the ẑ direction. The potential is a constant ______.

  1. in all space.
  2. for any x for a given z.
  3. for any y for a given z.
  4. on the x-y plane for a given z.

The work done to move a charge along an equipotential from A to B ______.

  1. cannot be defined as `- int_A^B E.dl`
  2. must be defined as `- int_A^B E.dl`
  3. is zero.
  4. can have a non-zero value.

Equipotential surfaces are shown in figure. Then the electric field strength will be ______.

What is meant by an equipotential surface?


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