- Electric Charge , Basic properties of electric charge and Frictional Electricity
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- Electrical and electrostatic force
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- Coulomb's Law (with vector form)
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- Principle Of Superposition
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- Electric Field
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- Calculation of Electric Field
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- Electric Field Lines
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- Electric Flux
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- Electric Dipole

- Electrical interaction between charged particles can be reformulated using the concept of electric field.

- To understand the concept consider the mutual repulsion of two positive charged bodies as shown in fig (a)

- Now if remove the body B and label its position as point P as shown in fig (b), the charged body A is said to produce an electric field at that point (and at all other points in its vicinity)

- When a body B is placed at point P and experiences force F, we explain it by a point of view that force is exerted on B by the field not by body A itself.

- The body A sets up an electric field and the force on body B is exerted by the field due to A.

- An electric field is said to exists at a point if a force of electric origin is exerted on a stationary charged (test charge) placed at that point.

- If F is the force acting on test charge q placed at a point in an electric field then electric field at that point is

**E**=**F**/q

or**F**= q**E**

- Electric field is a vector quantity and since F = qE the direction of E is the direction of F.

- Unit of electric field is (N.C
^{-1})

Ans. [MLT

- In previous section we studied a method of measuring electric field in which we place a small test charge at the point, measure a force on it and take the ratio of force to the test charge.

- Electric field at any point can be calculated using Coulomb's law if both magnitude and positions of all charges contributing to the field are known.

- To find the magnitude of electric field at a point P, at a distance r from the point charge q, we imagine a test charge q'to be placed at P. Now we find force on charge q' due to q through Coulomb's law.

$$\boldsymbol{F}=\frac{kqq_{'}}{4\pi \epsilon _{0}r^{2}} $$

electric field at P is $$\boldsymbol{E}=\frac{kqq_{'}}{4\pi \epsilon _{0}r^{2}} $$

The direction of the field is away from the charge q if it is positive

- Electric field for either a positive or negative charge in terms of unit vector r directed along line from charge q to point P is
$$\boldsymbol{F}=\frac{kqq_{'}\boldsymbol{\widehat{r}}}{4\pi \epsilon _{0}r^{2}}
$$

r = distance from charge q to point P.

- When q is negative , direction of E is towards q, opposite to r.

**Electric Field Due To Multiple Charges**

- Consider the number of point charges q
_{1}, q_{2},........... which are at distance r_{1P}, r_{2P},................... from point P as shown in fig

- The resultant electric field is the vector sum of individual electric fields as

**E = E**_{1P}+ E_{2P}+ .....................

This is also a direct result of principle of superposition discussed earlier in case of electric force on a single charge due to system of multiple charges.

- E is a vector quantity that varies from one point in space to another point and is determined from the position of square charges.

Class 12 Maths Class 12 Physics

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