Electrostatics

Ø Electrostatics is a branch of physics which deals with static electric charges. Since classical physics, it has been known that some materials such as amber attract lightweight particles after rubbing.

Ø The Greek word for amber or electron, was the source of the word 'electricity'.

Ø Electrostatic phenomena arise from the forces that electric charges exert on each other. Such electricity produced by friction is called as frictional electricity.

Ø If the charges in a body do not move, then the frictional electricity is also known as Static Electricity.

Two kinds of charges

i. If a glass is rubbed with a silk cloth, glass acquires positive charge while the silk cloth acquires an equal amount of negative charge.

ii. If an ebonite rod is rubbed with fur, it becomes negatively charged while the fur acquires equal amount of positive charge.

These classification of positive and negative charges were termed by American scientist, Benjamin Franklin.

Like and Unlike charges-Experimental verification:

A charged glass rod (can be positive or negative. Here in figure glass is positively charged) is suspended by a silk thread such that it swings horizontally. Now another charged glass rod of same charge is brought near the other end of the suspended glass rod. It is found that the ends of the two rods repel each other as shown in the figure.

Two charged rods of same sign

In the figure, F is the force exerted by the glass rods to move away from one another.

Now if the glass rod is replaced with negatively charged ebonite rod, the two rods (i.e. the glass rod and the ebonite rod) attract each other as shown in the figure.

Two charged rods of opposite sign

In this figure, F is the force exerted by glass rod and ebonite rod to attract each other.

The property of attraction and repulsion between charged bodies have many applications such as

Ø Electrostatic paint spraying,

Ø Powder coating,

Ø Fly-ash collection in chimneys,

Ø Ink-jet printing,

Ø Photostat copying (Xerox) etc.

Conductors and Insulators

According to the electrostatic behavior, materials are divided into two categories:

· Conductors

· Insulators (dielectrics)

Ø Bodies which allow the charges to pass through are called conductors. Eg: Metals, human body, Earth etc.

Ø Bodies which do not allow charges to pass through are called insulators. Eg: Glass, mica, ebonite, plastics etc.

When some charge is transferred to the conductor, it readily gets distributed over the entire body of the conductor. On a contrast when some charge is transferred to an insulator, the insulator remains the same (i.e. there is no transfer of charges in the non-conductor).

This property of material tells us why an electric comb gets electrified when we comb a dry hair. This is not the case a metal body.

Charge on the metal leak through our body to the ground as both are conducting materials. When we bring a charged body in contact with the ground, the excess charges present in the body disappears by causing a momentary current to pass through the ground connecting the conductor. This process of sharing the electric charges with the ground is called as Earthing or Grounding. This prevent humans from electrical shock when the appliances are grounded. A thick metal plate is buried deep into the earth and thick wires are drawn from this plate; these are used in buildings for the purpose of earthing near the mains supply.

The electric wiring in our houses has three wires: live, neutral and earth. The first two carry electric current from the power station and the third is earthed by connecting it to the buried metal plate. Metallic bodies of the electric appliances such as electric iron, refrigerator, TV are connected to the earth wire. When any fault occurs or live wire touches the metallic body, the charge flows to the earth without damaging the appliance and without causing any injury to the humans; this would have otherwise been unavoidable since the human body is a conductor of electricity.

Charging by Induction

The above figure explains clearly about the effect of charging by induction.

Consider two metal spheres which are uncharged. Bring these bodies nearby which are supported by an insulating stand.

Bring a positively charged rod near any one of the uncharged sphere such that the rod shouldn’t touch the other sphere. The free electrons present in the spheres are attracted towards the rod. This leaves an excess of positive charge on the rear surface of the other sphere. Both kinds of charges are bound in the metal spheres and cannot escape. The left surface of the first sphere has an excess of negative charge and the right side of the second sphere has an excess of positive charge. However, not all of the electrons in the spheres have accumulated on the left surface of the first sphere. As the negative charge starts building up at the left surface of the first sphere, other electrons are repelled by these. In a short time, equilibrium is reached under the action of force of attraction of the rod and the force of repulsion due to the accumulated charges. This process is called induction of charge and happens almost instantly.

The accumulated charges remain on the surface, till the glass rod is held near the sphere. If the rod is removed, the charges are not acted by any outside force and they redistribute to their original neutral state.

Even if the spheres are kept at some distance when the positively charged rod is present, the spheres are found to be oppositely charged and opposite to each other.

When the rod is removed and the spheres are separated, the charges in the spheres are equally distributed within the spheres.

In the above process the positively charged rod doesn’t lose its charges, contrary to the process of charging by contact.

Properties of electric charges

The properties of electric charges are

I. Quantization of electric charges,

II. Conservation of electric charges,

III. Additive nature of charges.

All these properties are explained below

I. Quantization of electric charges:

The fundamental unit of electric charge is the charge carried by the electron and its unit is Coulomb. The value of e is 1.6 x 10^-19 C .

In nature, the electric charge of any system is always an integral multiple of the least amount of charge. It means that the quantity can take only one of the discrete set of values. The charge, q= ne.

Here n is an integer and e is the elementary charge whose value is mentioned above.

II. Conservation of electric charges:

Electric charges can neither be created nor destroyed.

According to the law of conservation of electric charges, the total charge in an isolated system always remains constant. But the charges can be transferred from one part of the system to another, such that total charge always remains conserved.

In the above diagram, the negative charges from object A is transferred to the object B. Object A loses electrons (negatively charged) and object B gains the electrons (negatively charged). Hence the total number of negative charges are balanced.

Another example: Uranium can decay by emitting alpha particles and get transformed to thorium. It is given as

₉₂U²³⁸ → ₉₀Th²³⁴ + ₂He⁴

Total charge before decay = 92e whereas total charge after decay = 90e + 2e. Hence the total charge is conserved i.e. it remains constant.

III. Additive nature of charge:

The total electric charge of a system is equal to the algebraic sum of electric charges located in the system. For example, if two charged bodies of charges +2q, -5q are brought in contact, the total charge of the system is -3q (-5q + 2q = -3q ).