# NCERT Physics Class 12 Exemplar Chapter 1 Electric Charges and Fields Part 7

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Q.18 Sketch the electric field lines for a uniformly charged hollow cylinder shown in Fig.

Solution:

The electric field lines start from positive charges and move towards infinity and meet plane surface normally as shown in the figure below:

Q.19 What will be the total flux through the faces of the cube (Fig.) with side of length if a charge is placed at

(a) A: a corner of the cube.

(b) B: mid-point of an edge of the cube.

(c) C: centre of a face of the cube.

(d) D: mid-point of B and C.

Solution:

(a)Use of symmetry consideration may be useful in problems of flux calculation. We can imagine the charged particle is placed at the centre of a cube of side . We can observe that the charge is being shared equally by cubes. Therefore, total flux through the faces of the given cube .

(b) If the charge is placed at . middle point of an edge of the cube, **it **is being shared equally by cubes. Therefore, total flux through the faces of the given cube .

(c) If the charge is placed at . the centre of a face of the cube, it is being shared equally by cubes. Therefore, total flux through the faces of the given cube

(d) Finally, if charge is placed at . the mid-point of and , it is being shared equally by cubes. Therefore, total flux through the faces of the given cube .