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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Electrostatics Know More

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Atomic View of Dielectrics
What happens when a dielectric is placed in an electric field? Depending on the nature of the molecules, either of the two possibilities can occur. Dielectrics can be classified as i) Polar dielectrics and ii) Non-polar dielectrics.

Polar Dielectrics
Molecules of some dielectrics have a permanent electric dipole moment. Such dielectrics are known as polar dielectrics. When such dielectrics are placed in an electric field, the molecular dipoles tend to align with the external electric field.

Polar dielectric - molecules are randomly oriented in the absence of an external field

(a)

Polar dielectric - molecules are partially aligned in an external electric field.

(b)

 

The alignment of the molecules does not become complete due to thermal agitation. The partial alignment of the molecules produces an electric field that is directed opposite to the external field. This field is smaller in magnitude than the external field.

Non-polar Dielectrics
Non-polar dielectric molecules do not have a permanent electric dipole moment. However, such molecules tend to acquire dipole moments due to induction, when they are placed in an external field. This occurs because the external field tends to cause a slight separation of the centres of the negative and positive charges of the molecules.

Non-polar dielectric slab. The molecules are electrically neutral.

(a)

Non-polar dielectric slab in an external electric field. The centres of +ve and -ve charges are slightly separated

(b)

 

However, the slab as a whole remains electrically neutral and there is no excess charge in any element, though the two faces of the slab are oppositely charged. The induced surface charges produce an electric field opposite in direction to the applied field but smaller in magnitude. Thus, both types of dielectrics produce fields that oppose the external field and weaken it.

Tubes of Force: A group of lines of force starting on an area A1 of a positively charged inducting surface and ending on an area A2 of a negatively charged inducting surface is called a tube of force.

Faraday Unit Tube of Force: According to Faraday, one tube of force starts from unit positive charge in free space. Thus, the number of tubes of force originating from a charge + q is q. If
s is the surface density of charge, s Faraday tubes of force arise from unit area of the surface.

Maxwell Unit Tubes of Force: Maxwell suggested that the number of tubes of force starting from unit positions charge in free space is 1/
0. From charge q,
q/
0 tubes will therefore originate. These tubes of force are called Maxwell tubes of force.

Electric Induction: Consider a charge + q in a medium of primitively
0. Imagine unit area around a point in this medium. The number of tubes of induction passing normally through this area is the normal electric induction. The number of tubes of induction starting from charge + q is q. Draw a sphere of radius r with this point as centre.

Number of tubes of induction passing normally through unit area of the sphere = q/4
p0r2.

\ Normal electric induction =

q

4pr2

 

= .

q

4pr2

 = E

E

q

4pr2

E is called the electrie induction or displacement and is denoted by D.
\
= .
does not depend upon the medium. It represents the number of lines of induction per unit normal area.

 

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