Charge --Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.

Two kinds of charges : The following simple experiment proves these facts:

1. Rub a glass rod with silk and suspend it from a rigid support by means of silk thread. Bring another similarly charged rod near it. The two rod repel each other.
2. Bring a plastic rod rubbed with wool near the charged glass rod. The two rod attract each other.
3. Now, rub a plastic rod with wool and suspend it from a rigid support. Bring another similarly charged plastic rod near it. There will be repulsion between the two rods, so we can conclude that

1. There are only two king of electric charges-positive and negative.
2. Like charges repel each other and unlike charges attract each other.

 

The property which distinguishes the two king of charges is called polarity of charge.

The charge developed on a glass rod when rubbed with silk is called positive charge.

The charge developed on a plastic rod when rubbed with wool is called negative charge.

Two kind of charges are developed on rubbing

Positive charge Class rod Flannel or cat skin Woolen cloth Woolen coat Woolen carpet

Negative charge Silk cloth Ebonite rod Amber rod Plastic seat Rubber shoes

 

 

 

 

 

 

 

Electronic theory of frictional electricity :

When two different bodies are rubbed against each other electrons are transferred from the material with lower work function to the material with higher work function. The material with lower function lose electrons and becomes positively charged while other becomes negatively charged which gain the electron and mass is decreased or increased according to proportion.

Work Function : The energy required to remove an electron from the surface of a material is called its work function.

Earthing and safety : when a charged body is brought in contact with the earth, its entire charge passed to the ground in the form of a momentarily current. This process in which a body shares its charges with the earth is called grounding and earthing.

Electrostatic induction : it is the phenomenon of temporary electrification of a conductor in which opposite charges appears its closer ends and similar charges appear at its farther end in the presence of a nearby charged body.

The positive and negative charges produced at the ends of the conducting rod are called induced charges and the charge on the glass rod which induced these charges on the conducting rod is called inducing charge.

Basic property of electric charge :

1. Additivity
2. Quantization
3. Conservation

Additivity  of electric charge : It means that the total charge of a system is the algebraic sum of all the individual charges located at different points inside the system.

                If a system contains charges q₁, q₂, q₃ ……………….q_n, then its total charge is given by

 q = q₁, q₂, q₃ ……………….q_n.

quantization of electric charge: it means that the total charge q of a body is always an integral multiple of a basic quantum of charge (e), i.e.

q = ne  where  n=0,1,2,3……..n

cause of quantization: the basic cause of quantization of electric charge is that during rubbing only an integral no. og electrons can be transferred from one body to another.

Law of conservation of charge : The law of conservation of charge which states that

1. The total charge of an isolated system remains conserved.
2. The electric charge can neither be created nor be destroyed, they can only be transferred  from one body to another.

Example:

1. When a glass rod is rubbed with a silk cloth, it develops a +ve charge, but at the same time the silk cloth develops an equal –ve charge. Thus the met charge of the glass rod and silk cloth is zero, as it was before rubbing.
2. Charge is conserved during the fission of a   nucleus by a neutron.

₀n¹ + ₉₂U²³⁵→₅₆Ba¹⁴¹ + ₃₆Kr⁹² + 3₀n¹ + Energy.

Electric charge and Mass:

Electric charge	Mass
Electric charge may be positive, negative or zero.	Mass of a body is always positive.
Electric charge is always quantized.	Quantized of mass is not yet established.
Charge of a body does not depend on its speed.	Mass of a body increases with its speed.
Charge is strictly conserved.	Mass is not conserved by itself as some of the mass may get changed into energy or vice versa.
Electrostatics forces between tow charges may be attractive or repulsive.	Gravitational forces between two masses are always attractive.
Electrostatics forces between different charges may cancel out.	Gravitational forces between different bodies never cancel out.
A charged body always possesses some mass.	A body possessing mass may not have any net charge.