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## Why gravitational force of attraction is necessary for a satellite to revolve around earth?

The question was posted by Raj Kumar.

The satellite has a velocity. To revolve around, the direction of velocity must change. In the absence of a force it can move only in a straight line. It is the gravitational force which helps the satellite top change direction and revolve around the earth.

In other words, for a satellite to revolve in a circular path, a centripetal force is required. This centripetal force is provided by the force of gravitation only.

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## Electrostatics : A Practice Test Paper – 1

Time: 45 min

Max. Marks: 25

1. How does the force between two point charges change, if the dielectric constant of the medium in which they are kept, increases? [1]
2. Name the experiment which established the quantum nature of electric charge. [1]
3. Ordinary rubber is an insulator, but the special tyres of aircrafts are made slightly conducting. Why is this necessary? [1]
4. Draw electric field lines around an electric dipole. [1]
5. What would be the work done if a point charge +q is taken from  a point A to B on the circumference of a circle with another point charge +q at the centre? [1]
6. Obtain an expression for the potential energy of two charges q1 and q2 separated by distance r in a electric field E. [2]
7. Define electric field intensity. What is its SI unit? What is the relation between electric field and force on the charge? [2]
8. Obtain an expression for the electric potential at a point due to an electric dipole. [3]
9. Sketch equipotential surfaces for (i) a positive point charge (ii) Two equal positive point charges separated by a small distance (iii) a uniform electric field. [3]
10. (a) State Gauss Law (b) Using it derive an expression for the electric field intensity at a point near a thin infinite plain sheet of charge. (c) A point charge of 2 microcoulomb is at the centre of a cubic Gaussian surface 9cm edge. What is the net electric flux through all the surfaces of the cube? [5]
11. (a) Define electric dipole moment. (b) Derive expression for electric field intensity of a dipole at a point on its axial line. (c) Two charges +10μ C and – 10 μ C are placed 5 mm apart. Determine the electric field at a point P 15 cm away from its centre O on a line passing through O and normal axis of the dipole. [5]