Top

Electric Charge

We can see many experiments in our daily life which show the conformation of the presence of electric forces which can be attraction or repulsion force; for example, comb our dry hair it shows attraction towards paper. Our body is also electrically charged when we rub our shoes on wool or carpet. All these examples show that there is presence of electric charges in nature. It can be of two types that are positive and negative.

We know that electrons are negatively charged particles while protons are positively charged particles of atom. Two same charged particles always repel to each while oppositely charged particles attracts towards each other. So, the protons and electrons are shown attraction force for each other and atom remains electrically balanced. The attraction or repulsion force between particles of same or different charges produce the invisible force on them which creates an electric field. Now we discuss about the electric charges, induced charges, units, and all about electric field.

 Related Calculators Electric Field Calculator

Electric Charge Definition

The word 'electric' is derived from the Greek word 'elektron' meaning amber. The existence of charges were known when charged particles were produced by rubbing (due to friction) of suitable materials. These facts are demonstrated by simple experiments. Electric charge can be defined as the characteristic of a unit of matter.

In order to verify that there are two states of electric charges, first take a rod of hard rubber and rub it with fur. Also rub glass rod with silk. Now, suspend this hard rubber rod with thread and bring glass rod close to the Rubber rod. Now in this experiment we will note that suspended rubber rod and glass rod will attract each other. Here, the rubber has charge which is opposite to the charge on glass rod. On the other hand if two hard electrified rubber rods are brought close to each other they repel each other and they have repulsive forces between them. This shows that glass rod and rubber rod have different electric charges.

This experiment confirms that the rod of hard rubber and glass rod have opposite electric charges. We can conclude from here that, Charges which are same type repel each other and charges which are of opposite type repel each other. It is shown in the figure below:

Franklin used the convention, and according to this convention the electric charge present on the glass rod is positive charge and the charge present on the rubber rod is negative electric charge. The object which attracts the negatively charged rubber rod contains positive charge and any object which repels the rubber rod is negatively charged.

Cosmetics take advantage of electric forces of attraction by including the chemicals which are attracted towards the skin and stay on the face once they are applied.

Types of Electric Charges:
1. Positive Charge
2. Negative Charge

Net Electric Charge

According to Franklin’s model of electricity, Charge never gets created in the process when one object is rubbed against the other. Only, transfer of charges between the objects takes place.

For example in the instance explained in the previous section when glass rod was rubbed with the silk, silk became negatively charged and the glass rod became positively charged. Charges get transferred from glass to the silk. After rubbing, silk has more negative charges (because of the transfer of electrons from glass to silk). Amount of charge gained by one object is equal to the amount of charge lost by the second one. At the atomic level, transfer of electrons takes place. In any uncharged material, number of positive charges are equal to the number of negative charges and the material is neutral.

Charge is conserved and never gets created, only transfer of charges from one device to other takes place due to transfer of electrons. For instance rubber became negatively charges, because due to the transfer of electrons it has more electrons as compared to protons. Therefore net charge on the rubber was negative after rubbing.

Properties of Electric Charge:
1. Charge is conserved.
2. Charge is quantized.

Unit of Electric charge

SI unit of electric charge is Coulomb, One coulomb is equal to about 6.242×1018 e. According to above definition, charge of an electron is approximately equal to -1.602×10-19 C.

One coulomb is defined as the quantity of charge which will pass through the cross section of an electrical conductor while one ampere current is flowing through the conductor in one second. Electric charge is denoted by Q. Electric charge can be measured with electrometer, or with a ballistic galvanometer.

Electric Charge of a Proton

Electric charge of a single proton is equal to + 1.602×10-19 C, where C is coulomb (unit of electric charge). Charge on Proton is positive in nature. Therefore, Electric charge of a Proton = + 1.602×10-19 C

Electric Charge and Electric Field

First we classify the materials on the basis of their electrical conductivity. Conductors are type of materials in which electric charges move freely. On the other hand insulators do not allow the free movement of electric charges. Plastic, Wood, paper, rubber are the examples of insulators and copper, aluminum, iron, steel re the examples of electric conductors. Insulators support static charges but they does not allow free movement of charges(electrons). For example, when an insulator is charged by rubbing, the portion of the insulator where we have rubbed or where the transfer of charges has taken place becomes charged but the charge does not move to the other parts of the insulator. This shows that insulators are not good conductor of charges.

On the other hand, conductors allow free movement of charges. If we charge the conductors by rubbing, charges produced on the small region get distributed to the whole surface of the conductors. For Example, if you rub steel with fur or wool, steel gets charged but if you bring small pieces of paper close to the steel rod will not attract them. Now put wooden handle on some portion of the steel rod so that we can hold the steel rod from wooden handle. Now if we rub the steel rod it will attract the pieces of paper. Here, in earlier case when the steel was charged, charges flow from the steel to the earth through our body because human body is good conductor of charges. But when we put wooden handle, as wood is bad conductor of charges, it does not allow the charges to flow from steel to our body or earth.

Third type of materials is semiconductors, whose properties are between the conductors and insulator. Examples, silicon (Si) and germanium (Ge).

Electric Field

Electric field E is stated as the ratio of the electric force F which is acting on a charge (positive test charge) q0 placed at that point, and divided by the magnitude of the charge q0:

E = $\frac{F_{e}}{q_{0}}$

Units of Electric Field : As we know that the electric filed is defined as work done to move a charge of 1 coulomb. So, the unit is Newtons per coulomb (N/C).

In the definition of electric field explained above, electric fields are produced by some external charge-it is produced by q0. q0 experience electric field of some external charge. So, we can say that electric field is the property of the source. For example, every electron has its electric field.
Electric fields is the Superposition Principle. It states that if one or more fields is present at a point, then the net field is the vector sum of individual fields. Then, we can represent it as
Enet = E1 + E2 + E3 + E4 + .........

Charging by Induction

The Process of charging a body by keeping it near a charged body is called the Charging by induction.

The following are the main steps involved in charging by induction.
1) First the charged body is brought near the neutral body.

2 )Then the charged body attracts the opposite charges towards it which leads to the concentrations of charges to the ends.

3) The other end of the neutral body is earthed. The electrons present in the earth neutralizes the negative charges present on the body.

4) This makes the neutral body as a positively charged body.

Electric Dipole

A system which has two equal and opposite charges which are divided by a very small distance is called electric dipole and the two charges can be attached by a line called dipole axis.
Electric dipole moment is used for characterizing the electric dipole. The electric dipole moment is represented by $\overrightarrow{p}$ which is vector quantity.

The magnitude of the electric dipole moment is equal to the product of the magnitude of the charges and the distance taken from the charges. The direction of the magnitude dipole moment is the direction of dipole axis which is from the negative charge to positive charge.

For the above circuit, the charges of this electric dipole are –q and +q which is divided by a distance 2a. Now the magnitude of electric dipole moment is represented as
p = q . 2a
This has a direction from negative charge –q to the positive charge +q.

In Vector form, the magnitude of the electric dipole moment is written as
$\overrightarrow{p}$ = q . 2 $\overrightarrow{a}$The unit of electric dipole: coulomb $\times$ meter.

Electric Flux

Electric flux is a measure of the number of electric field lines passing through an area. The concept of the flux of a vector is not limited to electric fields but can be applied to any vector field.
Electric flux is different for Uniform and Non uniform electric field.

For Uniform electric field:
The figure shows field lines passing through a rectangular surface of area A perpendicular to the field lines.

(1) The electric flux passing through this surface is given by the product of electric intensity and the surface area perpendicular to the field lines.
$\phi$ = EA

where $\phi$ denotes electric flux and A denotes the surface area.
Suppose the surface is not perpendicular to the field lines, then the electric flux is given by the equation
$\phi$ = EA cos q
where, q is the angle between the direction of electric field E and the normal drawn to the surface in the outward direction.

(2) When the normal to surface is parallel to the electric field as shown in the figure,

The electric flux is
$\phi$ = EA cosq, (q = 0)
$\phi$ = EA cos0
$\phi$ = EA

(3) The electric flux becomes zero if the normal to the surface is perpendicular to the electric field as shown in the figure.

i.e. $\phi$ = EA cosq, (q = 900)
$\phi$ = EA $\times$ 0 = 0

For Non uniform field:
Let us now calculate the electric flux passing through a surface when the applied field is not uniform. The surface is usually divided into a large number of small area dA such that the electric field remains constant over that surface as shown in the figure.

The electric flux passing through dA = E dA cos q.
d$\phi$ = $\vec{E}$ $\vec{dA}$

$\therefore$ Total electric flux = $\int_{S}$ $\vec{E}$ $\vec{dA}$. The S.I unit of electric flux is Nm2/c2.

Electrostatics

Electrostatics is the study of charges at rest in general and the forces, fields, and potentials associated with static charges in particular.

Electrostatics Attraction / Repulsion have many industrial applications in electrostatic loudspeaker, electrostatic spraying of paints and powder coating, fly ash collection in chimneys, xerox copying machine, for the design of a cathode-ray tube used in television and radar etc.

Static Electricity

The two basic kind of electricity that exist in nature are:
1. Static electricity
2. Current electricity.
Static electricity is nothing but the collection of uncontrolled electrons which are passing from one body to another body in a movement which is sudden or momentary. Example:
1. The clothes taken out from the dryer and they are stick together
2. One can get a shock after walking on the carpet and then suddenly touching something.

Electroscope

Electroscope is a device used for detecting an electric charge and identifying its polarity. It consists of a vertical conducting rod passing through a rubber stopper fitted in the mouth of a glass vessel.

Two thin gold leaves are attached to lower end of the rod. When a charged object touches the metal knob at the outer end of the rod, the charge flows down to the leaves.The leaves diverge due to repulsion of the like charges they have received. The degree of divergence of the leaves gives a measure of the amount of charge.

To detect a charge on a rod 'A' or 'B' bring the rod near to the metallic disc or cap of the electroscope. In either case the leaf diverges as shown in the diagram. Here L represents the leaf.

Equipotential Lines

The Equipotential line or the equipotential surface is the surface which is specified by a single value of the voltage on all the points on the surface and all these points are said to be at single potential with respect to the given conductor.

For each Equipotential point or surface there exists a corresponding equipotential line and this line is a curved path which traces only the equal potential value on the surface with respect to a conductor.