## Charging a Body by Induction Method and Coulomb's Law

Subject: Physics

#### Overview

Coloumb's law states that "the force of attraction or repulsion between two charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them." This note provides us an information on charging a body by induction method and coulomb's law

#### Charging a body negatively by using induction method

• Take an isolated conductor with insulated stand.
• Bring a glass rod rubbed with silk near one end of an isolated conductor.
• Connect the remote end of the conductor to the ground with the help of conducting wire. (The process is called grounding or earthing)
• Induced free charge (positive charge) is neutralized by the flow of electrons from the earth.
• Cut the grounding/earthing and finally remove the glass rod.
• The induced bound charge which is negative finally gets distributed throughout the conductor and it is negatively charged.

#### Charging a body positively by using induction method

• Take an isolated conductor with insulated stand.
• Bring a glass rod rubbed with fur near one end of an isolated conductor.
• Connect the remote end of the conductor to the ground with the help of conducting wire. (The process is called grounding or earthing)
• Induced free charge (negative charge) is neutralized by the flow of electrons from the earth.
• Cut the grounding/earthing and finally remove the glass rod.
• The induced bound charge which is positive finally gets distributed throughout the conductor and it is positively charged.

#### Conductors

Those materials having free electrons are known as conductors. Due to the presence of free electrons conductors allows easy flow of electric current through them. Example: metals

#### Insulators

Those materials having all bounded electron are called insulators. Due to lack of free electrons they do not allow the flow of electric current through them. Examples: non-metals

Electric charge per unit area is called surface charge density.

i.e. surface charge density $\sigma = \frac{\text{electric charge(q)}}{area(A)}$

$$\therefore \sigma = \frac {q}{A}$$

Electric charge has a tendency to accumulate at sharp edges, corners and pointed parts to have high surface charge density there. Due to this, the charge can leak by the process of corona discharge.

Difference between electric charge and electric mass

 SN electric charge SN electric mass 1 Electric charges are positive as well as -ve. 1 Electric masses are always positive. 2 It is independent of the motion of the charged body. 2 Mass of an object varies relativistically according to the relation $$M=\frac{M_o}{\sqrt{1-\frac{V^2}{C^2}}}$$ where, Mo = mass of the body at rest M = mass of the body while moving without velocity. 3 Electric charge follows quantization condition. $$\text{i.e.} q = \pm ne$$ 3 Quantization of mass is unknown yet. 4 The force between two charged can be attractive as well as repulsive according to Columb's law. $$\therefore f = \frac{kq_1q_2}{r^2}$$ 4 The force between two masses as given as Newton's law of gravitation is always attractive. $$\therefore f = G\frac{M_1M_2}{r^2}$$
##### Things to remember

Those materials having free electrons are known as conductors.

Those materials having all bounded electron are called insulators.

Electric charge per unit area is called surface charge density.

Electric charge has a tendency to accumulate at sharp edges, corners and pointed parts to have high surface charge density there.

Coloumb's law states that "the force of attraction or repulsion between two charges is directly proportional to the product of charges and inversely proportional to the square of the distance between them."

The dielectric constant or relative permittivity of a medium can be defined as the ratio of the permittivity of a medium and the permittivity of vacuum of free space.

The dielectric constant of a medium can be defined as the ratio between two charges at certain distance in vacuum and the force between the same charges placed at the same distance is such medium.

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• It is a network of social relationships which cannot see or touched.
• common interests and common objectives are not necessary for society.