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| Electric Flux |
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| Electric field can be quantitatively described by using the concept of electric flux. |
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| Electric flux is the number of electric field lines penetrating a surface or passing through a surface. |
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| The electric field can be uniform or non-uniform. Let us now try to find the electric flux passing through a surface when the field is uniform. |
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| The figure shows field lines passing through a rectangular surface of area A perpendicular to the field lines. |
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| The electric flux passing through this surface is given by the product of electric intensity and the surface area perpendicular to the field lines. |
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| f = EA where f denotes electric flux and A denotes the surface area. |
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| Suppose the surface is not perpendicular to the field lines, then the electric flux is given by the equation |
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| f = EA cos q |
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| where q is the angle between the direction of electric field E and the normal drawn to the surface in the outward direction. |
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| When the normal to surface is parallel to the electric field as shown in the figure, the electric flux is |
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| f = EA Cos q, (q = 0) |
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| f = EA Cos 0 |
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| f = EA |
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| The electric flux becomes zero if the normal to the surface is perpendicular to the electric field as shown in the figure. |
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| That is, f = EA Cos q, (q = 90o) |
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| = EA Cos 90 |
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| = EA x 0 |
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| f = 0 |
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| Let us now calculate the electric flux passing through a surface when the applied field is not uniform. The surface is usually divided into a large number of small area dA such that the electric field remains constant over that surface as shown in the figure. |
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| The electric flux passing through dA(df) = EdA cosq |
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| S.I unit of electric flux is Nm2/c2 |
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| The electric flux per unit area is defined as the electric flux density. |
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| 01. A point charge of 1.8mC is at the centre of a cubical Gaussain surface having each side 50cm. What is the net electric flux through the surface? |
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| Suggested answer: |
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| 02. An electric dipole consists of +5C and -5C separated by a distance 3cm. Find the flux passing through a sphere of radius 6cm enclosing the dipole. |
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| Suggested answer: |
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