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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 27
Total: 34
In a certain region of space, electric field is along the z-direction throughout. The magnitude of electric field is, however, not constant but increases uniformly along the positive z-direction, at the rate of 105 N C–1 per metre. What are the force and torque experienced by a system having atotal dipole moment equal to 10–7 C m in the negative z- direction?
Solution:  
As electric field increases in positive z-direction from A to B, So
EB = EA+
dE
dR
 . 2a
So, the net force on the electric dipole in electric field is
Fnet = FB−FA = q (EB−EA) or Fnet = q [EA+
dE
dr
.2a−EA]
Fnet = q . 2a
dE
dr
 = p .
dE
dr
 or Fnet = 10−7×105 or Fnet = 10−2 N directed in direction of
→
FB
 i.e., from B to A or along –z direction.
As the two forces on charges of electric dipole are collinear and opposite, so net torque on it is equal to zero.
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