Static Charge

What is called a Static Charge?

Static charge or an Electric charge is a fundamental physical property of any matter that causes it to feel and experience a force of attraction or repulsion when it is placed in an electric field produced by an other charged body. This force of attraction or repulsion is called electrostatic force.

Unit of Static Charge

The SI unit of electric static charge is coulomb.

One coulomb is defined as that amount of charge which repels an equal and similar charge placed at a distance of $1 \ m$ from it in vacuum or air with a force of $\left ( 9 \times 10^9 \ N \right )$ .

Smaller units of static charge are –

Micro-coulomb $( 1 \ \mu C ) = 10^{- 6} C$
Nano-coulomb $( 1 \ n C ) = 10^{- 9} C$
Micro-micro coulomb $( 1 \ \mu \mu C )$ or pico coulomb $(1 \ pC) = 10^{- 12} C$

Polarity of Static Charge

Polarity of a charge determines the nature of charge. There are two kinds of electric charges –

Positive charge.
Negative charge.

Positive Charge –

When a glass rod is rubbed with silk, static charge developed in the glass rod is conventionally considered as a positive charge. It is denoted by plus $( + )$ sign. This type of charge is exhibited by protons.

Negative Charge –

When a rubber rod is rubbed with fur, the static charge developed in rubber rod is conventionally considered as a negative charge. It is denoted by minus $( - )$ sign. This type of charge is exhibited by electrons.

When two like charges ( like charges means – two charges both are either positive or both are negative charges) are brought to adjacent to each other, they experience repulsive forces.

Similarly, when two unlike charges ( unlike charges means – Two charges one of which is positive and other charge is negative) are brought to adjacent to each other, they experience attractive forces.

Electrostatics

The branch of physics which deals with the study of static charge and their electrostatic forces, electric field etc. is known as electrostatics.

It is also known as static electricity.

Application of Electrostatics

A photocopier is based on the principle of electrostatic.
Capacitors or condensers work on the principle of electrostatics.
High electric potential sources, van-de-graff generator etc. work on the principle of electrostatics.
Electrostatic precipitator works on the principle of electrostatics and used to separate fly ash from chimney gas to control the air pollution.
Spray painting and powder coatings furnishing process which are done with the help of principle of electrostatics.
Lightening and thundering are natural phenomenon based on the principle of electrostatics.

Conductors

The materials which allow easy flow of electric charge through them are called conductors.

Silver, Copper, Iron, The earth, Human being, solution of salts, acids and bases etc. are conductors.

When some charge is transferred to a conductor it readily gets distributed over the entire surface of the conductor. This happens due to the presence of a large number of free electrons inside the material of conductor. These electrons are free to move.

In solid substance, free electrons are the carrier of electric charges.
In electrolytes, ions are the carrier of electric charges.

Insulators

The materials which do not allow easy flow of electric charges through them are called non-conductors or insulators.

Examples –

Glass, rubber, plastics, PVC, dry wood etc. are insulators or bad conductors.

If some charge is put on an insulator it stays at the same place. On rubbing also the electric charge developed in insulators remain fixed at the place of origination. These are called static charges.

TO BE NOTED –

A nylon or plastic comb gets electrified on combing dry hair or on rubbing but a metal article like spoon does not. Why?

Reason – A nylon or plastic comb gets electrified on combing dry hair or on rubbing, but a metal article like spoon does not. Because, the metal spoon also produces charge on rubbing similar to nylon or plastic comb, but this charge instantly get leak through our body to the ground as the spoon and our body, both being conductor of electricity. Ground behaves as a huge reservoir of static charges.

Properties of Electric Charges

Basic properties of electrostatic charges are –

Additive property of electric charge.
Conservation of electric charge.
Quantization of electric charge.

Additive property of Electric Charges

Electric charge is additive i.e. total charge on an extended body is the algebraic sum of charges in different regions of that body.

Positive and negative charges are added like real numbers.

EXAMPLE –

Consider that a body has three different charges as –

$q_1 = 8 \ \mu C, \quad q_2 = 6 \ \mu C \quad \& \quad q_3 = - 4 \ \mu C$

Then the net charge on the body will be –

$Q = ( q_1 + q_2 + q_3 ) = ( 8 + 6 - 4 ) = 10 \ \mu C$

Conservation of Electric Charge

The net electric charge (i.e. algebraic sum of positive and negative charges) in an isolated system remains constant. This is called the law of conservation of electric charge.

EXAMPLE – 1

Uranium $\left ( _{92}U^{238} \right )$ decays by emitting $( \alpha )$ particle $\left ( _2He^4 \right )$ and gives Thorium $\left ( _{90}Th^{234} \right )$ .

Therefore, $\quad _{92}U^{238} \quad \rightarrow \quad _{90}Th^{234} \quad + \quad _2He^4$

In this reaction –

Number of elementary charges before decay on Uranium $\quad \left ( {92}U^{238} \right ) = 92$

Number of elementary charges after decay on Thorium & Helium particle $\quad \left ( _{90}Th^{234} \right ) \quad \& \quad \left ( _2He^4 \right ) = 90 + 2 = 92$

Hence, electric charge is being conserved.

EXAMPLE – 2

Calcium $\left ( _{20}Ca^{44} \right )$ bombarded with accelerated proton $\left ( _1H^1 \right )$ gives a neutron $\left ( _0n^1 \right )$ and Scandium $\left ( _{21}Sc^{44} \right )$ .

$_{20}Ca^{44} \quad + \quad _1H^1 \quad \rightarrow \quad _{21}Sc^{44} + \quad _0n^1$

In this reaction –

Number of elementary charges before bombardment in calcium & Hydrogen $\quad \left ( 20 + 1 \right ) = 21$

Number of elementary charges after bombardment in Scandium and neutron particle $\quad \left ( 21 + 0 \right ) = 21$

Hence, electric charge is being conserved.

Quantization of Electric Charge

Electric charge can exist only in discrete packets. The value of smallest possible positive or negative charge is the charge on an electron or proton respectively.

We know that charge on a proton is $( + e )$ and on electron is $( - e )$ . Thus $( e )$ is called an elementary charge.

Value of elementary charge is given by –

$e = 1.602192 \ \times 10^{- 19} C$

Thus, Quantization of charge is the property of an electric charge by virtue of which a charged body can have charge whose value is only an integral multiple of the elementary charge $( e )$ .

Therefore, $\quad q = \pm \ n \times e$ . Where, $( n ) = 1, 2, 3, 4 .......$ and so on.

Thus, the value of $( n )$ can only be an integer. Fraction value is not possible. Thus possible values of charge on a body are $\quad q = 0, \quad \pm \ e, \quad \pm \ 2e, \quad \pm \ 3e$ etc.

Thus, the fraction values for a charge such as $\quad q = 1.5 \ e, \quad 2.5 \ e, \quad \frac {2}{3} \ e$ etc. are not possible.

This is known as Quantization of charge.

TO BE NOTED –

Charge developed on a body is due to the transfer of electrons or protons. Also, fraction of electrons or protons is meaning-less. Hence, transfer of charges always takes place as discrete particles. Each of these particles has a charge of $( 1.602192 \ \times \ 10^{( - 19 )} \ C )$ . Hence, charge developed on a body is always an integral multiple of $[ 1.602192 \ \times \ 10^{\left ( -19 \right )} C ]$ .

See numerical problems based on this article.