Electric charge[class-12]

Electric Charge

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

• Electromagnetism is the phenomenon of the interaction of electric currents or fields and magnetic fields.

• When insulating surfaces are rubbed against each other, a static charge is developed which gets discharged after getting in contact with a conductor.
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• Only one of the two charges(or polarity) gets developed on rubbing – either positive or negative. 
• An object becomes positively charged when it loses the loosely bound electrons to another object while rubbing. The other object gains electrons and becomes negatively charged.
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• When like charges are brought near, they repel each other. Unlike charges attract each other.
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• The charges get neutralized when the two bodies are brought in contact.
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• An example of electric charge generation through rubbing of glass rod with silk and plastic rod with silk is mentioned below:

Charging by Contact

When a charged object is touched to another object, the other object also gets charged with same polarity due to charge transfer. This is called charge by contact.

Charging by Induction

When a charged object is brought closer to another object (not touched), the original object doesn’t lose any charge and the other object gets charged as well with opposite polarity.

 The other extreme end of the newly charged object develops polarity same as that of the charged object. This type of charging is called charge by induction.

Properties of electric charge

Electric charge for a body is considered as Point charges if their size is very small in comparison to the distance between them. 

So the charge is considered to be concentrated at one point. Following are the properties of electric charge in terms of point charges:

1. Additivity of charges:

• Point charges are scalars and can be added algebraically. If q1, q2, q3, … qn, are point charges, the total charge qtot=q1+ q2 + q3+ qn
• Charges have no direction but can be positive or negative.

2. Conservation of charges:

• Total charge in an isolated system is always conserved. When there are many bodies in an isolated system, the charges get transferred from one body to another but the net charge of the system remains same.
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• During rubbing or natural forces, no new charge is created. The charges are either redistributed or a neutron breaks up into proton and electron of equal and opposite charge.
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3. Quantization of charges:
   • The charge is always represented in the form of, q = ne. Here n is an integer and e is the charge (- for electron and + for proton). Magnitude of e = 1.602192 X 10-19 This is called quantization of charge.
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   • SI unit of charge is Coulomb (C).
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   • Quantization is usually ignored at macroscopic levels (μC) because at that point, charges are taken to be continuous.
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