# Physics Class 12 NCERT Solutions: Chapter 1 Electric Charges and Fields Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 5. When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge.

Answer: Rubbing produces charges of equal magnitude but of opposite nature on the two bodies because charges are created in pairs. This phenomenon of charging is called charging by friction. The net charge on the system of two rubbed bodies is zero. This is because equal amount of opposite charges annihilate each other. When a glass rod is rubbed with a silk cloth, opposite natured charges appear on both the bodies. This phenomenon is in consistence with the law of conservation of energy. A similar phenomenon is observed with many other pairs of bodies.

Q: 6. Four point charges are located at the corners of a square ABCD of side . What is the force on a charge of placed at the centre of the square?

The given figure shows a square of side with four charges placed at its corners. O is the centre of the square.

Where,

(Sides)

(Diagonals)

A charge of amount is placed at point .

Force of repulsion between charges placed at corner A and centre O is equal in magnitude but opposite in direction relative to the force of repulsion between the charges placed at corner C and centre O. Hence, they will cancel each other. Similarly, force of attraction between charges placed at corner B and centre O is equal in magnitude but opposite in direction relative to the force of attraction between the charges placed at corner D and centre O. Hence, they will also cancel each other. Therefore, net force caused by the four charges placed at the corner of the square on charge at centre O is zero.

Q: 7. (A) An electrostatic field line is a continuous curve. That is, a field line cannot have sudden breaks. Why not?

(B) Explain why two field lines never cross each other at any point?

(A) An electrostatic field line is a continuous curve because a charge experiences a continuous force when traced in an electrostatic field. The field line cannot have sudden breaks because the charge moves continuously and does not jump from one point to the other.

(B) If two field lines cross each other at a point, then electric field intensity will show two directions at that point. This is not possible. Hence, two field lines never cross each other.

Q: 8. Two point charges are located apart in vacuum.

(A) What is the electric field at the midpoint O of the line AB joining the two charges?

(B) If a negative test charge of magnitude is placed at this point, what is the force experienced by the test charge?

(A) The situation is represented in the given figure. O is the mid-point of line AB.

Distance between the two charges,

Net electric field at point

Electric 3⧵times 10 ^ -6 field at point O caused by charge,

Along

Where,

Permittivity of free space

Magnitude of electric field at point caused by charge,

along

Therefore, the electric field at mid- point is along .

(B) A test charge of amount is placed at mid-point .

Force experienced by the test charge

The force is directed along line . This is because the negative test charge is repelled by the charge placed at point B but attracted towards point A.

Therefore, the force experienced by the test charge is .

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