Electric Charges and Fields

1. Electric Charge Charge is the property associated with matter due to which it produces and experiences electric and magnetic effect.

2. Conductors and Insulators Those substances which readily allow the passage of electricity through them are called conductors, e.g. metals, the earth and those substances which offer high resistance to the passage of electricity are called insulators, e.g. plastic rod and nylon.

3. Transference of electrons is the cause of frictional electricity.

4. Additivity of Charges- Charge are scalars and they add up like real numbers. It means if a system consists of n charges q1, q2, q3 , … ,qn, then total charge of the system will be q1 +q2 + … +qn.

5. Conservation of Charge The total charge of an isolated system is always conserved, i.e. initial and final charge of the system will be same.

6. Quantisation of Charge -Charge exists in discrete amount rather than continuous value and hence, quantised.

Mathematically, charge on an object, q=±ne
where, n is an integer and e is electronic charge. When any physical quantity exists in discrete packets rather than in continuous amount, the quantity is said to be quantised. Hence, charge is quantised.

7. Units of Charge
(i) SI unit coulomb (C)
(ii) CGS system
(a) electrostatic unit, esu of charge or stat-coulomb (stat-C)
(b) electromagnetic unit, emu of charge or ab-C (ab-coulomb)
1 ab-C = 10 C, 1 C = 3 x 109 stat-C

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✅ Concave Mirrors:
By reflection of light, concave mirrors give real, inverted images if the object is beyond the focus and a virtual, erect, enlarged image if the object has a distance less than the focal length from the pole of the mirror.

👉🏻 Uses of Concave Mirrors:
➖ Concave mirrors are used in torches, searchlights, and headlights of vehicles to get powerful parallel beams of light.
➖ Concave mirrors are also used as shaving mirrors to see a larger image of the face.
➖ Dentists use concave mirrors to see bigger images of the teeth of the patients.
➖ Large concave mirrors are used to focus sunlight to produce heat in the solar furnaces.


✅ Convex Mirrors:
By the reflection of light convex Mirrors always give a virtual, erect, diminished image of the object kept infront of the mirror.

👉🏻 Uses of Convex Mirrors:
➖ The convex mirror is used as a side-view mirror in vehicles to give a smaller view of the vehicles coming from behind.
➖ They are used in shops and supermarkets and any other place where there is a requirement for detecting burglars.
➖ Convex mirrors are used in making lenses for sunglasses.
➖ Convex mirrors are used in magnifying glasses, and telescopes.
➖ Convex mirrors are used to reflect street light; because they can reflect over a wide area.
➖ Convex mirrors are kept at the street corners to avoid collisions.