Page 3 - English Class X_cbse new (FINAL).cdr

TOPIC - 1

Coulomb’s law

Sub Topic: 1.1

S. Solution Marking S. Solution Marking
No. Scheme No. Scheme

1. (a) 1. [1]

[1] cont.  2Kq2  2 −4Kq2  2 [1]
 l 2  l 2  [3]
Fnet = +  + 2  2Kq2 −4Kq2 cos120º
  l2 l2

 Kq2  2  Kq2  2
 l 2   l2 
= ( 4 + 16) + 8

 Kq2  2
 l2 
= (20 + 8)

[1] = Kq2 28
l2

(b) Potential energy of the system = 4KQq + KQ2 + Kq2 [1] =2 7 Kq2
a 2a 2a l2

[3]
(b)

OR U= 1  qA i qB + qBqC + qCqA 
(a) 4πε0  AB BC CA 

PE initial = − −8Kq2 − 4Kq2 + 2Kq2 = −10Kq2
l l l l

[1] PEfinal = 0 10Kq2
l
Workdone = PEfinal − PEinitial =

1