NCERT Physics Class 12 Exemplar Chapter 1 Electric Charges and Fields Part 6

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Very Short Answer Types Questions

Q.14 An arbitrary surface encloses a dipole. What is the electric flux through this surface?

Solution: Zero

According to Gauss’ law, the electric flux through an enclosed surface is

The net charge on a dipole is given by , hence

Hence the electric flux through a surface enclosing a dipole .

Q.15 A metallic spherical shell has an inner radius and outer radius . A charge is placed at the centre of the spherical cavity. What will be surface charge density on (i) the inner surface, and (ii) the outer surface?

Solution: A charge is placed at the centre of the spherical cavity. So, the charge induced at the inner surface of the sphere will be and at outer surface of the sphere is .

Tittle: Image of An arbitrary surface encloses a dipole

Charge Induced

Tittle: Image of An arbitrary surface encloses a dipole

The surface charge density on the inner surface

The surface charge density on the outer surface

Hence, (i) , (ii)

Q. 16 The dimensions of an atom are of the order of an Angstrom. Thus there must be large electric fields between the protons and electrons. Why, then is the electrostatic field inside a conductor zero?

Solution:

In any neutral atom, the number of electrons and protons are equal, and the protons and electrons are bound into an atom with distinct and independent existence Electrostatic fields are caused by the presence of excess charges. But there can be no excess charge on the inter surface of an isolated conductor. So, the electrostatic fields inside a conductor is zero despite the fact that the dimensions of an atom are of the order of an Angstrom

The electric fields bind the atoms to neutral entity. Fields are caused by excess charges. There can be no excess charge on the inter surface of an isolated conductor.

Q.17 If the total charge enclosed by a surface is zero, does it imply that the electric field everywhere on the surface is zero? Conversely, if the electric field everywhere on a surface is zero, does it imply that net charge inside is zero.

Solution:

According to Gauss’ law, the flux associated with any closed surface is given by

.. The term qeio on the right side of the equation includes the sum of all charges enclosed by the surface called (Gaussian surface).

In left side equation, the electric field is due to all the charges present both inside as well as outside the Gaussian surface

Thus, despite being total charge enclosed by a surface zero, it doesn’t imply that the electric field everywhere on the surface is zero, the field may be normal to the surface

Also, conversely if the electric field everywhere on a surface is zero