# NCERT Solutions For Class 12 physics Electric Dipole

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Electric Dipole: A system containing two equal and opposite charges separated by a finite distance is called an electric dipole.

Electric Dipole moment: Electric dipole moment is defined as the product of magnitude of one of the charges and length of the dipole say ‘2a’ i.e.

• The electric dipole moment is a vector quantity.
• It is directed from – q to + q i.e.from negative charge to the positive charge.
• SI unit of electric dipole moment isCoulomb- meter (Cm).
• The net charge on an electric dipole is zero.

Electric force and Torque on an Electric dipole in a uniform electric field:

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Consider an electric dipole placed in a uniform electric field of strength E.The net electric force is zero but a torque is experienced by the dipole. So, the dipole will rotate till it becomes parallel to the electric field.

Now torque =   Force × perpendicular distance

=   qE× BC

=   qE × 2a SinƟ

=   (q × 2a)E SinƟ

= pESinƟ

Torque in vector form is

• The total force on an electric dipole placed in a uniform electric field is zero.

• Maximum value of torque acting on the dipole is when Ɵ = 900

Torque = pE × Sin 900

= pE × 1

= pE

Question: What happens when an electric dipole is placed in a non-uniform electric field?

Ans: When an electric dipole is placed in a non-uniform electric field, the dipole will have both rotational and translational motion. When the dipole becomes parallel to the electric field, the rotational motion will stop.

Question: An electric dipole consists of a positive and negative charge of 4µC each placed at a distance of 5mm.Calculate dipole moment.

Ans:

q = ±4µC = ±4 × 10-6C

p = q x 2a

2a = 5mm = 5 × 10-3 m

= 4 × 10-6 × 5 × 10-3

= 2 × 10-8 C- m

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August 22, 2019