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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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• centroid of gravitational force ## 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? 
2. Name the experiment which established the quantum nature of electric charge. 
3. Ordinary rubber is an insulator, but the special tyres of aircrafts are made slightly conducting. Why is this necessary? 
4. Draw electric field lines around an electric dipole. 
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? 
6. Obtain an expression for the potential energy of two charges q1 and q2 separated by distance r in a electric field E. 
7. Define electric field intensity. What is its SI unit? What is the relation between electric field and force on the charge? 
8. Obtain an expression for the electric potential at a point due to an electric dipole. 
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. 
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? 
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.