Properties of Electric Charges

Key Concepts

• The law of conservation of charge
• The additive nature of charge
• Quantization of charge

Introduction: Properties of Electric Charges 

When two materials are rubbed against one another, electrons can move from one material to the other. This causes one material to become more negatively charged than the other. Objects get charged on rubbing or by conduction. When they are brought close to each other we see that: 

The objects with the same electric charge repel each other. 

The objects with opposite charges attract each other. 

Law of conservation of charge: 

The law of conservation of charge states that a charge can neither be created nor destroyed. However, it can be transferred from one body to another. 

Explanation: 

Charging by rubbing: 

When two objects are rubbed against each other, negative charges (electrons) from one object gets transferred to another object. The object which loses electrons become positively charged and the object which gains electrons becomes negatively charged. 

Conclusion: 

When two objects are rubbed against each other, the transfer of charges takes place, but the total charge of the isolated system remains constant. 

Charging by induction: 

When a charged body is brought near an uncharged conductor, the charged body will attract the opposite charges and repel similar charges present in the uncharged body. As a result of this one side of the neutral body (close to the charged body) becomes oppositely charged while the other is similarly charged. 

Conclusion: 

In the method of charging by induction, the inducing body neither gains nor loses charge. The total charge of the isolated system remains constant. 

Charging by conduction: 

In the case of conduction, electrons flow from one object to the other. 

Conclusion: 

In the case of conduction, electrons flow from one object to the other. However, the total charge of an isolated system remains constant. 

Additive nature of charges: 

Charges given to a system add up like real numbers. If a system contains three charges Q₁, Q₂, and Q₃ then the total charge of the system is Q. 

Total charge = Q = (Q₁ + Q₂ + Q₃) 

If the system has n number of charges, then the total charge Q: 

Q = (Q₁ + Q₂ + Q₃ + ……. + Q_n) 

A charge is a scalar quantity. It has magnitude but no direction.  

A charge can be positive or negative. 

The SI unit of charge is the Coulomb.  

Example: 

A system contains four charges Q₁ = +2C, Q₂ = +3C, Q₃ = -4C and Q₄ = -5C, then find  the total charge of the system. 

Solution: 

Total charge = Q = (Q₁ + Q₂ + Q₃ + Q₄) 

Total charge = Q = (2C) + (3C) + (-4C) + (-5C) 

Total charge = Q = (-4C) 

Quantization of charge: 

• In the 20^th century, Robert Millikan performed an experiment to determine the smallest possible charge in nature. 
• By his famous oil-drop experiment Millikan found that the smallest possible charge in nature is 1.6 x 10^-19 Coulomb. 
• Later it was found that the charge on an electron or a proton is 1.6 x 10^-19 Coulomb. 

• Experimentally it has been observed that the charge on a body either positive or negative is always an integer multiple of 1.6 x 10^-19 Coulomb. 

Electric charges are quantized. They come in packets that cannot be divided.   

The charge of a system is the integer multiple of the basic electric charge on an electron or proton (e). 

Charge on an electron or proton = e = 1.6 x 10^-19 C 

The general formula for quantization of charge is: 

Q = ± ne 

Here Q = charge 

n = an integer that gives the number of electrons or protons 

n = …-2, -1,0,1,2,3………. etc. 

e = charge on an electron or proton 

Summary

Properties of charges:

Law of conservation of charge:

The law of conservation of charge states that a charge can neither be created nor destroyed. However, it can be transferred from one body to another.

Additive nature of charge:

Charges given to a system add up like real numbers. If the system has n number of charges,

then:

the total charge Q (Q1 + Q₂+Q3+…….+Qa)

A charge is a scalar quantity. It has magnitude but no direction. The SI unit of charge is

Coulomb (C)

Quantization of charge:

The charge of a system is the integer multiple of the basic electric charge on an electron or proton (e).

Charge on an electron or proton e = 1.6 x 10-19 C The general formula for quantization of charge is Q = ne Here n is an integer that gives the number of electrons or protons.

n=-2,-1,0,1,2,3………..etc.