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# Electric Charges (Coulomb’s Law, Induction, Conduction) – CBSE NCERT Science Lesson

#### Electric Charges (Coulomb’s Law, Induction, Conduction) – CBSE NCERT Science (Physics) Lesson

HISTORY

Some experiments have shown that there are certain pairs of  materials which on rubbing could attract light objects like straw, pith balls and bits of paper.

These objects are said to be electrified.

Coulomb’s Law

Electric Field

Electric Field Lines

Modes of charges

1. By conduction

2. By induction

Coulomb’s Law

According to this law, the force exerted between 2 charges varies inversely as the square of the distance between the charges and is directly proportional to the product of the magnitude of the two charges and acts along the line joining the two charges.

F = K(Q1)(Q2)/  R2

Where K= constant

Validity :

The Coulomb’s Law is valid only when the linear size of charged bodies are much smaller than the distance separating them. If the above is true, then we can ignore the size of the charged bodies and they can be considered point charges.

For example:

Let Q1 = +3C, Q2 = -1C and R= 3m

Then, applying Coulomb’s law ,

F = K|+3C||-1C|/  (3)2  =      K(3)(1)/ (3)2

Electric Field

Electric Field due to a charge Q at a point in space may be defined as the force that a unit positive charge (test charge) would experience if placed at that point.

The S.I. unit of Electric Field is Nm (Newton meter).

E (Electric Field of Q1) = F/ Q2 =  KQ1/ R2

Simple Funda of ELECTRIC FIELD : Jaise every Gunda has his own area, similarly every charge has some area jisme ussi ka raaj chalta hai.

Electric Field Lines

Some lines/curves are drawn in the area of each charge, such that the tangent drawn to the curve gives the direction of electric field at that point.  These lines are known as Electric Field Lines or Lines of Force.

Some important general properties of field lines :

Field lines always start from a positive charge and end at a negative charge. But if there is single charge, it may start or end at  infinity.

In a charge-free region, electric field lines can be taken to be continuous curves without any breaks.

Two field lines can never cross each other. If they did, the field at the point of intersection will not have a unique direction, which is absurd (not possible).

Electrostatic field lines do not form closed loops like the magnetic field lines (due to the conservative nature of electric field).

Modes of Charging

Conduction and Induction

Charging By :

Conduction

Charged object touches the electroscope.
Electroscope ends up similarly charged to the object used to charge it.
The first charge is strong but gets weaker each time the electroscope is recharged.  (This is due to the original object giving up some charge every time it is touched.)

Induction

Charged object does not touch the electroscope.
Electroscope ends up oppositely charged to the object used to charge it.

The first charge is strong and stays strong each time the electroscope is recharged.  (This is due to the original object not losing any charge in the process.)

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