Sunday, November 18, 2012

Two Kinds of Charges

It was observed that if two glass rods rubbed with wool or silk cloth are brought close to each other, they repel each other Fig. a. The two strands of wool or two pieces of silk cloth, with which the rods were rubbed, also repel each other. However, the glass rod and wool attracted each other.

Similarly, two plastic rods rubbed with cat’s fur repelled each other Fig. b but attracted the fur.
On the other hand, the plastic rods attract the glass rod Fig. c and repel the silk or wool with which the glass rod is rubbed. The glass rod repels the fur.

If a plastic rod rubbed with fur is made to touch two small pith balls suspended by silk or nylon thread, then the balls repel each other Fig. d and are also repelled by the rod.

A similar effect is found if the pith balls are touched with a glass rod rubbed with silk Fig. e.
A dramatic observation is that a pith ball touched with glass rod attracts another pith ball touched with plastic rod Fig. f.

It was concluded, that there are only two kinds of an entity which is called the electric charge. The experiments on pith balls suggested that there are two kinds of electrification and we find that

(i)                 like charges repel and
(ii)               Unlike charges attract each other.

By convention, the charge on glass rod or cat’s fur is called positive and that on plastic rod or silk is termed negative.

The property which differentiates the two kinds of charges is called the polarity of charge.

Note: Only rubbed area of non conducting body gets charged, and this charge does not move to other parts of the body. The  charge is static on rubbed portion only.

Some of these questions which may be asked in your Board Examination 2012-2013

Q1: what are the characteristic of charges acquired by the objects on rubbing against each other?

Q2: Who suggested first that there are two kinds of charges?

Q3: How can you show that there are two types of charges?

Q4: An ebonite rod is rubbed with the fur or wool. What type of charges do they acquire?

Q5Is mass of body affected on charging?

Q6: What is the polarity of charge?

Answer these questions in comment box and help your friends


  1. Replies
    1. In the process of charging a body either gains or loses electrons to get negatively or positively charged. This transfer of electrons doesn’t bring any observable change in the mass of the body. Reason being, the mass of electron is very small compared to the charged body.
      However, we can still calculate the change in mass of a body due to the charging process.
      Suppose a body of mass ‘M’ is given charge +q. Since, charge is quantized, therefore, q = ne.
      So, mass of electrons lost is = n × me [me is the mass of an electron]
      Thus, the new mass of the body is = M – n × me

  2. 1.Same Charge Repel and opposite Attract.
    2.Du Fay. Rubbing rode On a slik cloth and on a Fur.
    4.wool Require positive and Fur Require Negative.
    5.if we Count mass of electrons than yes .
    And vice versa.
    6.the property which differentiate two type of charge called polarity.