Coloumb's Law and Electric Field[class-12] part=2

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Coulomb’s Law

Coulomb’s law states that Force exerted between two point charges:

• Is inversely proportional to square of the distance between these charges and
• Is directly proportional to product of magnitude of the two charges
• Acts along the line joining the two point charges.

Here ε0 = 8.854 x 10-12 C2 N-1 m-2is called permittivityof free space.

Electric Field

Electric field is a force produced by a charge near its surroundings. This force is exerted on other charges when brought in the vicinity of this field.

• SI unit of electric field is N/C (Force/Charge).
• Electric field due to a charge at a point is the force that a unit positive charge would experience if placed at that point.
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• The charge generating electric field is called source charge and the charge which experiences this field is called test charge. Practically, to keep source charge undisturbed due to the electric field of test charge, the test charge is kept infinitely small.
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• Since F(Force) is proportional to q (Charge), the electric field is independent of q but depends on r (space coordinates).
• The electric field is symmetric in spherical coordinates.
• The concept of Electric field is used to account for the time delay for a charged body to experience force from the field of a source charge.
• Electric field due to system of charges
  According to the superposition principle, the total electric field at a point in space is equal to the vector sum of individual fields present.
• Electric Field Lines
  Electric field lines are a pictorial way of representing electric field around a configuration of charges.
  • An electric field line is a curve drawn in such a way that the tangent to it at each point is in direction of the net field at that point.
  • Electric field is inversely proportional to the square of the distance; hence electric field near the charge is high and keeps on decreasing as we go farther from the charge. The electric field lines, however, remain constant but are very far apart (at higher distances) as compared to lesser distance.
  
  

Electric Flux

Electric flux is the measure of flow of the electric field through a given area. Electric flux is proportional to the number of electric field lines going through a normally perpendicular surface.

• The orientation of area element decides the amount of electric flux. Thus, the area element is a vector.
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• The vector associated with every area element of a closed surface is taken to be in the direction of the outward normal.
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• Area element vector ΔS = ΔSn̂, ΔS is magnitude of area element and n̂ is unit vectorin the direction of outward normal.
• Electric flux, Δφ = EΔS = E ΔScosθ, θ is angle between E and ΔS.
• Unit of electric flux is NC-1m2.
• Total flux through a surface, φ ≈ Σ EΔS