ELECTRICITY

Electricity is a form of energy that is associated with electric charges that exists inside the atom.

Electric Current:-

· The flow of electric charges constitute an electric current.

· An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is ‘I’.

Charges

What is a charge? What is an elementary charge?

· Charge or electric charge is the basic property of matter that causes objects to attract or repel each other. It is carried by the subatomic particles like protons and electrons.

· Charges can neither be created nor be destroyed.

· There are two types of charges: positive charge and negative charge. Protons carry positive charge and the electrons carry negative charge.

· There is a force of attraction or repulsion between the charges. Unlike charges attract each other and like charges repel each other.

· Electric charge is measured in coulomb (C).

· Small amount of charge that can exist freely is  called elementary charge (e). Its value is 1.602  × 10-19 C. This is the amount of charge possessed by each proton and electron. But, protons have positive elementary charge (+e) and electrons have negative elementary charge (-e). Since protons and electrons are equal in number, an atom is electrically neutral.

Transfer Of Charges

How does charges get transferred from one substance to another?

Electrons in the outermost orbit of an atom can be easily removed. They can be transferred from one substance to another. The substance which gains electrons become negatively charged and the substance which looses electrons becomes positively charged.

Transfer of charges takes place in the following three ways

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Transfer By Friction

When an ebonite rod (rod made by vulcanized rubber) is rubbed with fur, the fur transfers electrons to the ebonite rod because the electrons in the outermost orbit of the atoms in fur are loosely bound as compared to the ebonite rod. The ebonite rod which has excess electrons becomes negatively charged and the fur which has deficiency of electrons is positively charged. Thus, when two materials are rubbed together, some electrons may be transferred from one material to the other, leaving them both with a net electric charge.

Transfer By Conduction

When the ebonite rod is rubbed with woollen cloth, electrons from the woollen cloth are transferred to the ebonite rod. Now ebonite rod will be negatively charged. When it is brought near the paper cylinder, negative charges in the rod are attracted by the positive charges in the cylinder. When the cylinder is touched by the rod, some negative charges are transferred to the paper. Hence, the negative charges in the rod are repelled by the negative charges in the cylinder. Thus, charges can be transferred to on object by bringing it in contact with a charged body. This method of transferring charges from one body to other body is called transfer by conduction.

Transfer By Induction

The process of charging an uncharged body by bringing a charged body near to it but without touching it is called induction. The uncharged body acquires an opposite charge at the near end and similar charge at the farther end..

When a positively charged rod is brought near an uncharged rod, negatively charged electrons are attracted towards it. As a result there is excess of electrons at nearer end and deficiency of electrons at the farther end. The nearer end of the uncharged rod becomes negatively charged and far end is positively charged.

Flow Of Charges

· Let us suppose, there are two metallic spheres; one having more negative charge (excess of electrons) and the other having more positive charge (deficiency of electrons). When you connect them both with the help of a metallic wire, excess electrons from the negatively charged sphere will start flowing towards the positively charged sphere. This flow continues till the number of electrons in both the sphere is equal.

· Here, the positively charged sphere is said to be at higher potential and the negatively charged sphere is said to be at lower potential.

· Hence, electrons flow from lower potential to higher potential. This is known electric current (flow of electrons).

· The difference between these potentials is known as potential difference, commonly known as voltage.

· Before the discovery of electrons it was considered that electric current is due to the flow of positive charges. Flow of positive charge is called conventional current. Conventional current flows from higher potential to lower potential.

Electroscope

What is an electroscope? Define the principle on which electroscope works.

§ An electroscope is a scientific instrument used to detect the presence of electric charge on a body.

§ There are two types of electroscope: pith-ball electroscope  and goldleaf electroscope.

§ An electroscope is made out of conducting materials, generally metal.

§ It works on the principle that like charges repel each other.

Gold Leaf Electroscope

§ Gold and silver are used in electroscope because they are the best conductors of electric current.

Structure Of Electroscope

§ It is made up of a glass jar. A vertical brass rod is inserted into the jar through a cork.

§ The top of the brass rod has a horizontal brass rod or a brass disc.

§ Two gold leaves are suspended from the brass rod inside the jar.

Working Of Electroscope

When the brass disc of the electroscope is touched by a charged object, electric charge gets transferred to the gold leaf through the rod. This results in the gold leaves moving away from each other. This happens because both the leaves have similar charges.

Charging

Transfer of charge from one object to another is called charging.

Electrical Discharge

The gold leaves resume their normal position after some time. This happens because they lose their charge. This process is called electrical discharge. The gold leaves would also be discharged when someone touches the brass rod with bare hands. In that case, the charge is transferred to the earth through the human body.

Lightening And Thunder

How is lightening caused? What is thunder?

§ Lightning is produced by discharge of electricity from cloud to cloud or from cloud to ground. During thunderstorm air is moving upward rapidly. This air which moves rapidly carries small ice crystals upward. At the same time, small water drops move downward. When they collide, ice crystals become positively charged and move upward and the water drops become negatively charged and move downward. So the upper part of the cloud is positively charged and the lower part of the cloud is negatively charged. When they come into contact, electrons in the water drops are attracted by the positive charges in the ice crystals. Thus, electricity is generated and lightning is seen.

§ Sometimes the lower part of the cloud which is negatively charged comes into contact with the positive charges accumulated near the mountains, trees and even people on the earth. This discharge produces lot of heat and sparks that results in what we see as lightning. Huge quantities of electricity are discharged in lightning flashes and temperatures of over 30,000°C or more can be reached. This extreme heating causes the air to expand explosively fast and then they contract. This expansion and contraction create a shock wave that turns into a booming sound wave, known as thunder.

Earthing

§ A safety measure devised to prevent people from getting shocked if the insulation inside electrical devices fails is called Earthing.

§ Electrical earthing can be defined as the process of transferring the discharge of electrical energy directly to the Earth with the help of low resistance wire.

§ Usually an electric appliance such as a heater, an iron box, etc. are fitted with three wires namely live, neutral and earth.

§ The earth wire is connected to the metallic body of the appliance. This is done to avoid accidental shock. Suppose due to some defect, the insulation of the live wire inside an electric iron is burnt then the live wire may touch the metallic body of the iron. If the earth wire is properly connected to the metallic body, current will pass into the Earth through earth wire and it will protect us from electric shock.

§ The Earth, being a good conductor of electricity, acts as a convenient path for the flow of electric current that leaks out from the insulation.

Lightning Arresters

§ Lightning arrestor is a device used to protect buildings from the effects of lightning.

§ Lightning conductor consists of a metallic lightning rod that remains in air at the top of the building.

§ Major portion of the metal rod and copper cable are installed in the walls during its construction. The other end of the rod is placed deep into the soil.

§ When lightning falls, it is attracted by the metallic rods at the top of the building. The rod provides easy route for the transfer of electric charge to the ground.

§ In the absence of lightning arrestors, lightning will fall on the building and the building will be damaged.

Electric Circuit

· We can draw circuit diagrams using these symbols. Symbols for bulbs, cells and switches are shown in figure.

· In a cell, the longer line denotes the positive (+) terminal and the short line denotes the negative (-) terminal. We shall use these symbols to show components in the circuits we draw. Such diagrams are called circuit diagrams.

Types Of Electrical Circuits

Series Circuit

· So, similar looking bulb do not always glow equally bright when connected in series.

· The circuit can be broken at several places. For example, between the cell and the bulb, between the two bulbs etc.

· A series circuit is one in which more than 1 bulbs are connected along 1 path through which electricity flows.

Parallel Circuit

· A circuit in which two bulbs are connected in different places.

Similarity And Difference Between Series And Parallel Circuit

Chemical Effects Of Electricity

Chemical effects of electric current

We know that when an electric current passes through solution it ionizes and breaks down into ions. This is because of chemical reactions that take place when an electric current passes through a solution. Depending on the nature of the solution and the electrodes used, the following effects can be observed in the solution:

1. metallic deposits on the electrodes

2. change in the colour of the solution

3. a release of gas or production of bubbles in the solution

Applications of chemical effects of electric current

Electroplating

· Electroplating is a process in which layer of metal is deposited on another material with the help of electricity.

· Electroplating is used in many industries for depositing a layer of metal with desired characteristics on another metal.

· Different metals used for electroplating are Nickel, Copper, Gold Silver, Tin, Brass, Zinc, Chromium and Platinum.

Process of electroplating

· In order to conduct electroplating right electrodes and electrolytes must be chosen so that metal can deposit over a material.

· For instance, if we want to deposit copper on a material we need an electrolyte that contains copper in it. Similarly, if we need gold on a material we need an electrolyte that contains gold in it.

· Also, we should make sure that the electrode that we are choosing is completely clean.

· The electrodes used are made up of different materials. One of the electrodes is of the same metal of which the electrolyte solution is. The second electrode needs to be the material on which we want to coat another metal.

· For instance, in case we want to plate copper upon brass, one electrode should be of Copper and the other electrode should be of Brass and the electrolyte solution should be any salt which contains copper in it, for example, copper sulphate solution. Consider the diagram given below that describes the process of electroplating of copper.

· Out of these two electrodes the copper electrode acts as the anode (positive electrode) and brass electrode acts as the cathode (negative electrode).

· When electricity is passed through the solution, the copper sulphate breaks down into its ions.

· The copper ions (they have a positive charge) get attracted by the brass electrode while the sulphur ions being negatively charged move towards the copper electrode.

· As a result, copper starts depositing on the brass electrode.

· The process of electroplating takes some time to complete.

· The amount of time that it will take depends upon the strength of the current that is being passed through the circuit and also upon the concentration of the electrolyte.

· As these two are increased the speed of the electroplating process also increases.

Applications of electroplating

· Medical equipment is made up of nickel which is harmful to the human body hence to avoid it from coming in contact with our body a coating of platinum or gold is applied on the surface of nickel.

· Many kitchen equipments, bath taps, parts of cars etc. are covered with chromium coating. Chromium is an expensive metal hence the objects are created with the cheaper metal and chromium coating is provided. Thus, to bring a shining over the objects and prevent them from corrosion chromium coating is used.

· Jewellery makers often make ornaments of less expensive metals and provide a coating of gold or silver upon them.

· The tin cans that are used to store food are actually made up of iron and have a coating of tin on them. Iron can easily react with food and spoil it, however, tin prevents the food from getting reacted with iron and therefore helps in preventing it from getting spoiled easily.

· Bridges and various parts of automobiles are made up of iron because it provides strength. However in order to prevent iron from getting rusted a coating of zinc is provided over it. This method is also called galvanization of iron.