******* Have a Nice Day***


Image result for electricity

Dear Friends,

In continuation of our Science Article on Relativity Theory, Now we try to understand some basic concepts of another important topic of Science. The topic is shocking.  Yes It is Electricity.

Electricity is one of the most important blessings that science has given to mankind. It has also become a part of modern life and one cannot think of a world without it.  

Let us Start .....

An introduction to


Basic Electricity
As you may aware, in this world, everything is made up of atoms, the smallest particle. Dalton is the first to propose an atomic theory. As per his theory, the atom is the smallest particle which cannot be divided further.  
Scientists like J.J.Thompson, Rutherford, James Chandwick proved that an atom can be further divided into particles. Hence the name proton, electron and neutron. Every atom is made up of these 3 particles. Further particles which are called sub atomic particles are also found, which is out of scope for the present topic. For the time being, it is enough that all things are made up of atoms and each atom is made up of electrons, neutrons and protons. It is worth to note here that J.J.Thompson was a Nobel Prize winner and it is more important that 7 of his research students had won the Nobel Prize! Also, he was the chair-person of Royal Society which conferred FRCS to Srinivasa Ramajujan, the gifted mathematician from India.
Thales of Greek observed that when amber rubbed attracted tiny pieces of straw. This is named as “electricity”. Charles Augustus Coulomb better known as Coulomb with some scientists observed that some materials like glass, sealing wax and ebony acquired the property of attracting things. They named them as “electric charge”. They also found that there are two types of charges, “positive” and “negative”. The names “positive” and “negative” are contributed by Benjamin Franklin. It may be added that, he invented the lightning arrester. Also he was the Postmaster General of US postal department. When a glass rod is rubbed with silk cloth, it acquired a positive electric charge. On the other hand when an ebonite rod is rubbed with fur a negative charge is developed.
The lightning conductor is working on the principle of these charges. We might have seen that a tall metal rod with some spikes is placed on the top of tall buildings. This is called the lightning conductor. This prevents the building from damage due to lightning. The lightning conductor was devised by Benjamin Franklin.
It is a long copper rod placed at the top of the building. This rod passes through the building to the ground. The bottom of the rod is connected to a copper plate which is buried deeply in the ground. At the top of the copper rod, a number of spikes are connected.
When there is lightning, the air makes the clouds negatively charged. The cloud passes through the building. Due to this negative charges, the tip of the spikes are induced with positive charges. (Action of point charges). These positive charges neutralize most of the negative charges of the cloud. If at all some negative charges of the cloud are attracted by the spikes, they will travel to the ground and passes through the earth and hence the building is prevented from damage due to lightning.
C:\Users\sys\Desktop\electric diagrams\Spike-Lightning-Arrester.jpg
Lightning conductor
The flow of electrons is called the electricity. One way of generating electricity is, as already mentioned, rubbing glass rods and allowing the electric charge to accumulate in the glass rod. This electricity is static. That is it does not flow.
We know that for doing any work, we need a force, which may be a push or a pull. Water in a horizontal tube will not flow, but will be in rest. If we lift one side of the tube, the water will flow. Likewise, in order to drive the electrons to flow, we need a force. The force required is called Electro Motive Force, EMF in short. It is measured in “Volts”. The amount of electrons flow is called the current and is measured in “ampere”. If we push a book along the surface of the table, we may feel an opposing force by the table. This opposing force is called friction. Similarly, when the electrons flow through the wire or conductor, it offers an opposing force against the flow of electrons. This opposing force is called the resistance. It depends on various factors. It is measured in “ohms. “
From the early days, the human race tried to make the EMF. The starting point to the development of electric cells is the experiment by Luige Galvani and his wife Lucia on a dissected frog hung from iron railings with brass hooks. It was observed that, whenever the leg of the frog touched the iron railings, it jumped and this led to the introduction of animal electricity. The first who succeeded in devising a cell was Alessontro Volta of Italy. He devised the first cell in the year 1800. It is called the volta cell.  Subsequently Leclanche cell, Daniel cell were developed. The cell we use in wall clocks, torch lights etc. are called dry cells.
Refer the following diagram which represents the volta cell. In the volta cell the EMF was produced by chemical reaction. It has two rods called the electrodes. For simplicity we can call them as terminals. One is made of copper and other of zinc. Both are placed inside a glass vessel. The vessel contains dilute sulphuric acid. When the terminals are connected externally by a small piece of wire, current flows from copper to zinc terminal. The copper terminal is positive and the zinc terminal is negative. In physics, the positive terminal is called the “anode” and the negative terminal is called the “cathode”. Thus in this cell, the energy produced by the chemical reaction among copper, zinc and sulphuric acid, is converted into electrical energy. This was able to produce an EMF of 1.08 volts. The improved forms are Leclanche cell and Daniel cell.
C:\Users\sys\Desktop\electric diagrams\voltaic cell.png
Volta cell
C:\Users\sys\Desktop\electric diagrams\leclanche cell.jpg
Leclanche Cell
C:\Users\sys\Desktop\electric diagrams\daniell cell.JPG
Deniel cell
The cells are classified into primary and secondary cells. The cells which cannot be recharged are called the primary cells and which can be recharged is called the secondary cell. Volta, Leclanche, Daniel and dry cells are primary cells. The cells in mobile phones, car batteries etc are called the secondary cells, since they can be recharged. They are called in different names viz. Lead acid accumulators, lithium cell etc. In this cells also, chemical reaction takes place and chemical energy is converted to electrical energy. While charging, the chemical reaction takes place in the opposite direction and the cell is charged.
The EMF produced by these cells is very low. They are of the order around 1.5 volts only. The EMF is called the Direct current, shortly DC. The drawback of the DC is that they cannot be transmitted to a long distance. The advantage is that they can be stored and we can use whenever we require.
In our school days we have studied about magnets. Practically we might have used magnets at least on few occasions with toys.
A magnet has two ends, called poles. The end which always points the north direction is called the North Pole. The end which always points the south direction is called the South Pole. Around the magnet, there are invisible lines which are called magnetic lines. These lines start from North Pole of the magnet and end at the South Pole. The area in which the magnetic lines are available is called the magnetic field.
The scientists James Clarke Maxwell and Hans Christian Oersted established the relationship between magnetism and electricity. Oersted proved that electricity creates magnetism and magnetism creates electricity. He found that when current is passed through a metal conductor, magnetic lines are produced around the conductor. A devise which generates electricity using magnetism and mechanical energy is called a dynamo or generator.
It was Micheal Faraday, who invented the first dynamo to produce EMF using magnetism. As already stated, there are magnetic lines around a magnet, which is called the magnetic field. Faraday discovered that EMF is produced in a coil which is placed and made to move in the magnetic field. This is because when the coil is moved in the field, the magnetic lines are disturbed or the lines are broken.
Faraday observed that EMF is produced when a metal conductor is moved in a magnetic field or a magnet is moved near the metal conductor. The following experiment conducted by Faraday is very important in the generation of electricity.
Before proceeding to this experiment, let us know what a galvanometer is. A galvanometer is an instrument which looks like the speedometer in the car or a two wheeler. This is used to find the presence of electricity in a circuit. The needle in the galvanometer deflects whenever there is electricity.
C:\Users\sys\Desktop\electric diagrams\faraday-law-1.jpg

In the above figure there is a coil of wire. The ends are connected to a galvanometer G. A magnet is taken. It is moved from right to left with North Pole directing the coil. When it is moved, the magnetic lines available with the magnet are disturbed and electricity is induced in the coil. The needle of the galvanometer deflects. If we keep the magnet at rest, there is no deflection in the galvanometer. When the magnet is moved away from the coil that is from left to right there is again deflection in the galvanometer but in the opposite direction. From this experiment Faraday formulated the laws of electromagnetic induction.  These laws are called Faraday’s laws of electromagnetic induction. If a metal conductor is moved in a magnetic field or a magnet is moved near the metal conductor or the coil, a current is produced in the coil. In short, the motion (coil or the magnet) and magnetism produces electricity. In fact, magnetism and electricity or the two sides of a coin.
Faraday was the first to construct a dynamo, which produces the EMF, (DC). As per physics, the direction of motion of the metal conductor, magnetic lines and the electricity are in mutually perpendicular directions. We can visualize three mutually perpendicular lines at the top corner or any corner of a room as found in the following figure.  We can see three lines meet at that point. These three lines can be taken as the three mutually perpendicular directions. The direction of electricity is decided by the direction of the magnetic lines and also the direction of motion of the conductor. The following rule says about the direction of the electricity if we know the directions of magnetic lines and also the direction of motion of the conductor. This law is known as Flemming’s right hand rule or generator rule or dynamo rule.
Image result for fleming's right hand rule
Flemming’s right hand rule or generator rule
Stretch the thumb, fore finger and middle finger of the right hand so that they are perpendicular to each other. If the fore finger indicates the direction of magnetic lines and the thumb shows the direction of the motion of the conductor, then the middle finger shows the direction of the induced current. If the magnetic lines are from right to left on this page, if the conductor moves upwards from this page, then the direction of electricity would be along this page from top to bottom of the page.
C:\Users\sys\Desktop\electric diagrams\dc_gen.bmp

In the above figure ABCD is a coil. It is placed between the two poles of a strong magnet. The coil is devised so that it rotates about an axis.(imagine the rotation of a table fan). The ends of the coil are connected to two split rings S1 and S2.  There are two brushes B1 and B2 which are kept pressed to S1 and S2. The rings S1 and S2 will rotate with the coil but B1 and B2 are stationary. Assume that the coil rotates in clockwise direction that is in the direction in which the hands of a clock moves.
The coil rotates in clockwise direction. The magnetic lines are from North to South pole. i.e. from left to right in the diagram. The part CD of the coil is moving from top to bottom. If the diagram is drawn on the table, then CD moves from table towards floor. The direction of magnetic lines is from left to right. By the generator rule, the current in this arm CD is from C to D.
At the same time, the arm AB moves from bottom to top. If the diagram is drawn on the table, then AB moves from table towards roof. By the generator rule, the current in this arm AB is from A to B. Hence the current flows in the direction A-B-C-D. Hence, A is positive and D is negative. That is S1 is positive and also B1 is positive. Similarly, S2 is negative and B2 is negative. If we connect B1 and B2 by a small piece of wire, current will flow from B1 to B2.
After half rotation, the arm CD and S2 will be on the left side. AB and S1 will be on the right side. It may be noted that B1 and B2 would be on the same positions.  Now the arm AB is going down and CD is coming up. The magnetic lines are from left to right. As per generator rule, the current in AB is from B to A and for CD it is from D to C . Hence, the current flows in the direction D-C-B-A. S2 is positive and S1 is negative. Now B1 touches S2 and B2 touches S1. Hence, B1 is again positive and B2 is again negative. Hence in the second half rotation also, the current flows from B1 to B2. That is, in this dynamo, the current always flows in the same direction. That is in one direction or unidirectional. This is called the Direct current or DC. If there are large numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil.
As mentioned earlier, the DC cannot be transmitted to a long distance. This type of generator (dynamo) is used in many electronic appliances. It is used in cars and vehicles.
The production of DC is not sufficient and has the own drawbacks. The current actually we are using in almost all places is called the AC (Alternating Current). This can be transmitted to a long distance, can be increased and decreased whenever we need. Now we will see the working of AC dynamo.
C:\Users\sys\Desktop\electric diagrams\AC generator.gif
AC Dynamo/Generator
The first AC dynamo was devised by the Yugoslav scientist Nicholas Tesla. The construction is very similar to that of a DC dynamo. Instead of two rings there are two hollow cylinders fixed at the ends of the coil as in the above figure. They are called slip-rings. In this arrangement, during the first half rotation B1 is positive and B2 is negative. In the second half rotation, B1 is negative and B2 is positive. Thus the current alternates and hence it is called Alternating current. Can you understand the working of this dynamo?
In our day to day life we use electric motors in almost all places. For example, fans, mixie, water pumps etc. There are also two types of motors, the DC and AC motors. The construction and working are similar to generators. The difference is that in generator, we make the coil rotate in the magnetic field and gets the electricity. In the motors, we give electricity to a conductor which is placed in a magnetic field, and we get the mechanical energy, the rotation. The following figure will explain the principle of motors. The important thing is that in the case of motors we use Flemming’s left hand rule, which is explained below.

C:\Users\sys\Desktop\electric diagrams\fleming's left hand rule.png
Stretch the thumb, fore finger and middle finger of the left hand so that they are perpendicular to each other. If the fore finger indicates the direction of magnetic lines and the middle finger shows the direction of current in the conductor, then the thumb shows the direction of motion of the conductor.
C:\Users\sys\Desktop\electric diagrams\dcmfor.gif
DC Motor
In the above diagram, the magnetic lines are in the direction of B from north pole to south pole. The direction of electricity I, is along the small red arrow heads shown in the figure. By Flemming’s left hand rule, the direction of rotation is clockwise as seen from the figure. If we change the positions of the magnet or the direction of the electricity, the rotation would be in anti-clockwise direction. You might have observed that a table fan, rotates in clockwise direction, if you are in front of it. On the other hand the ceiling fan rotates in anti-clockwise direction.
The working of AC motor is similar to that of a DC motor, which may be seen from the following diagram.

C:\Users\sys\Desktop\electric diagrams\ACmot.gif
AC motor
George Simon Ohm established the basic relationship among the voltage, current and the resistance. The voltage is denoted by V, the current by I and the resistance by R. Ohm’s law states that V/I is a constant. In other words V=IR.
The power of electric appliances is denoted by the unit watt. Power is the product of V and I. That is P=VI. You might have observed that in electrical appliances the power is noted as 40 W, 200 W etc. In our domestic circuit, electricity is supplied at 220 volts. All appliances require this voltage. It is important that different appliances require different amount of current (ampere) and hence the consumption of electricity by different appliances is different. For example an electrical bulb is marked as 110 W and the voltage is 220 volts. From Ohm’s law, P=VI. Hence, I=P/V=110/220=0.5 ampere. On the other hand if a mixie is marked as 880 W, it will consume 880/220= 4 ampere.
The product of voltage and current is called the power (watts). 1000 watts is called a Kilo watt. The product of power and time is called kilo watt hour, which we usually called as Unit. We are paying electric charges for this usage. If an electric bulb 100 W is used form 10 hours, the product comes to 1000 which is 1 unit. This consumption would be high for water heaters, air conditioners etc. since they consume high current.
Transformer is an electrical device used for converting low AC voltage into high AC voltage and conversely from high AC voltage to low AC voltage. A transformer which converts low voltage to high voltage is called a stepping-up transformer and one which converts a high voltage to low voltage is called a stepping-down transformer. The transformer works on the principle of electromagnetic induction.
A transformer has two coils called the primary coil and secondary coil. They are wound on a soft iron core. They are insulated from one another. The input AC is given in the primary coil, and the output is taken through the secondary coil. The following figure refers a transformer. Np, Ns, are the number of turns in the primary and secondary coils. Vp, Vs are the voltages in the primary (input voltage) and secondary (output voltage) coils. Ip, Is, are the input and output currents in the primary and secondary coils respectively.
C:\Users\sys\Desktop\electric diagrams\Transformer.png
The input AC is applied in the primary coil. As we know, the AC changes its direction continuously and hence the current in the primary is continuously varying. It produces a varying magnetic field in the primary coil. This varying magnetic field is also attached to the secondary coil. Hence an EMF is produced in the secondary coil. (Refer the experiment of Faraday).  The input and output of the transformer are connected by the following formula.
K is called the transformer ratio. If K>1 it is step up transformer, if K<1 it is step down transformer. If K=1, it is called the ideal transformer. If the number of turns in the secondary is greater than the number of turns in the primary, it is called step up transformer. If the number of turns in the secondary is less than the number of turns in the primary it is called step down transformer.
From the above formula we have the following relation between the voltage and current in the primary and secondary coils.
The above formula can be written as ESIS=EPIP. From this we can see that when the voltage is increased, the current will be decreased and if the voltage is decreased the current will be increased. This is very important in the transmission of power.
In other words, a step up transformer increases the voltage by decreasing the current. Similarly, a step down transformer decreases the voltage by increasing the current.
In long distance power transmission, there will be power loss due to heating effect. The power loss due to heat is I2RT, where I is the current, R is the resistance and T is the time. This power loss can be reduced by transmitting the power at high voltage with low current, as will be clear from the following examples.
A power of 11000 watt is transmitted at 220 V.
Power P= VI, I=P/V=11000/220=50 A
Power loss =502R=2500R watts.
On the other hand, if this power is transmitted at 22000 V, I=11000/22000=0.05A and hence the power loss =0.25R watts.
Hence it is evident that if power is transmitted at a higher voltage, the loss of energy in the form of heat can be considerably reduced. For transmitting electric power at 11000 W at 220 V the current capacity of line wires has to be 50 A and if transmission is done at 22000 V, it is only 0.5 A. Thus for carrying larger current (50A) thick wires have to be used. This increases the cost of transmission. Also, to support these thick wires, stronger poles have to be erected which further adds on the cost. On the other hand if transmission is done at high voltages, the wires required are of lower current carrying capacity. So, thicker wires can be replaced by thin wires, thus reducing the cost of transmission considerably.
For example 400 MW power produced at 15000 V in the power station is stepped up by a step-up transformer to 230000 V before transmission. The power is then transmitted through the transmission lines which forms a part of the grid. The grid connects different parts of the country. Outside the city, the power is stepped down to 110000 V by a step-down transformer. Again the power is stepped down to 11000 V by a transformer. Before distribution to the use, the power is again stepped down to 230 V or 440 V depending upon the need of the user.
Diagram showing the generation and distribution of AC
C:\Users\sys\Desktop\electric diagrams\Electricity_grid_simple-_North_America.svg.png
The large scale AC generation is done in different ways. In that it is called the armature instead of coils. Turbines are rotated using different forms of energy and turbine is attached to the armature so that electricity is produced. Depending upon the method which is used to rotate the turbine, different names are given. The following are some of the forms of producing electricity commonly used.
Thermal power plant
Coal is burnt in power stations and the heat produced is used to heat water. This water produces steam which runs the turbine.
Hydro power plant
Dams are constructed on the river to obstruct the flow of water and the water is collected in large reservoirs. This water is allowed to flow through large pipes with great force. This force is used to run the turbine.
Wind energy
The wind mill consists of a large structure similar to a fan. It rotates using the speed of the wind. This rotation is used to rotate the armature and thus electricity is produced. A number of wind mills are erected over a large area which is called wind energy farm.  The energy output of each windmill in a farm is coupled together to get electricity on a commercial scale.
Solar energy
A large number of solar panels are constructed over a large area. There are two methods used in solar energy. In one method, the sunlight falling on the panels are directly converted into electric energy using the principle photo electric current. In another method, the heat is used to heat up a special kind of oil, which in turn heats up water to produce steam.
Nuclear energy

A process which is called nuclear fission is used in nuclear power stations. It is a process of breaking up of the nucleus of a heavier atom into two parts using neutrons. In this process a large amount is energy is released. This energy is used to heat water to produce steam. The water used in this process is called heavy water.

Can u tell anything absolute ?

Dear Friends,

Relativity -  The Theory which shattered  the year long belief of  the word absolute.

There is no absolute East or West, Right or Left, Up or Down.

Every thing depend upon the observers view.

Here is an Article  about Special Relativity Theory.
The Speciality of the article is

 "Great concept expressed in easy way to understand "

Is It possible for a person to be elder than his parents?
“yes” says Albert Einstein.
Let us see “how”

Science is the religion of all- AlbertEinstein

An introduction to the Special Theory of Relativity

The theory of relativity of time was proposed in the year 1905 by the greatest 20th century physicist, Albert Einstein at the age of 25. It placed him among the giants of human thoughts-Copernicus, Newton and others.
In this article, I wish to explain some of the details about the relativity theory. I have a very little knowledge about this topic.  I propose to explain the tip of the iceberg, with the aid of some books on the subject which I have gone through.
If this article is able to make you curious to go further on this subject, I think this work has served the purpose.
Science makes our life very simple. All the inventions are used by us in our day to day life. To mention a few, mobile phones, computers, and vehicles.
            This world has created many great scientists and inventors. Even though every invention is useful in one way or the other, some of them are superior to the others, in the sense that without them nothing is possible today. In my opinion, the best inventions are wheel, electricity and computers.
            Many scientists proposed scientific concepts, theories etc. They explain the cause for the happenings around us. Some of the theories are very vital, as they have the effect of making the belief of that time upside down. Let us consider few such examples.
`           It was believed that the shape of the earth was flat. Subsequently, it was established that the shape is almost spherical.
            Dalton stated that the atom is the smallest particle which cannot be divided further. Scientists like J.J.Thompson, Rutherford proved that atoms canbe divided further and named those particles as neutron, electron and proton.
            Once light was believed to have wave nature. It was proved that it has particle nature also. The present theory is that light has dual behavior, wave and particle.
            In my opinion, the greatest of all such theories is the “Theory of Relativity” by Albert Einstein.
            Aristotle believed that a constant force (pull or push) or an external agency is required to keep the motion of a body.
            Galileo proved that this is false. He proved by performing a series of experiments with inclined plane that if there is no friction, no external agency is required for the continuous motion of any object. In other words, an object in motion will continue its motion for ever if there were no friction.
            Sir Isaac Newton consolidated these ideas and formulated the famous “Newton’s laws”. This subject is called the “Newtonian mechanics”, which was followed for almost for 400 years.
Albert Einstein introduced the concept of relativity. He published two famous papers, “General Relativity” and “Special Relativity”. This has almost shattered the basics of Newtonian mechanics. 
Let us begin with some important terms used in this article.
Relative and absolute
            Some sentences have a definite meaning and some have not. Observe the following sentences.
1.     The Sun rises in the east. It is absolute. That is the direction in which the sun rises is called the east direction. This is true for all places in the world. Hence it is absolute.
2.     Our post office is on the left side of the street. If we walk from bus stand to temple, it is on our right side. If we walk from temple towards bus stand, it is on the left side. Which is correct? The terms “left” or “right” are relative to the direction in which we walk. So “left” or “right” are relative terms.
3.     The terms “up” or “down” are also relative.

4.     Which is bigger
In the following figure, the cowherd is bigger than the cow. In the next figure the cow is bigger than the cowherd. In fact, both the drawings were the same but drawn from different angles.

Now, let us introduce the following terms, based on Newtonian mechanics.
Rest: An object which is not changing its position from time to time is said to be at rest. For example, our office is at rest since it is at the same place at different times.
Motion: An object is said to be in motion, if it changes its position with reference to time. A bus which is going from Srivaikuntam to Tirunelveli is in motion. The bus is at Srivaikuntam at 5.45 PM, at Puthukudi at 5.50 and at Tirunelveli at 6.45 PM.
            Consider our friend sitting in a train. His brief case is kept in the place for that purpose which is above his seat, and just opposite to him. See the following figure.

            We are in the platform standing near the coach. The train is about to start. If we ask our friend in the train that whether his briefcase is at rest or in motion, he would say that it is at rest. For us also it is in rest.
            Now the train starts and going at a speed of 5 KMPH towards north. Now, if we ask the same question to our friend in the train, he would give the same reply that the brief case is at rest. Also he would say that we are going backwards towards south at a speed of 5KMPH.  But for us, the brief case and our friend are in motion. Both are right and nobody is wrong. Here comes the relativity. “Rest”and “motion” are taken as “absolute” by Newton but for Einstein they are relative. This is the reason for the contradictory observation by us and by our friend.
            From the above we can see that rest and motion are also relative terms.
One and the same place
Consider two friends are travelling in the same coach of train from Tuticorin to New Delhi. Assume that they used to write letters from the coach to their spouses daily at a particular time. Assume that their spouses receive the letters on the next day.
Now we can find whether the term “one and the same place” is absolute or relative.
If we ask the persons in the train, they would say that all the letters are written from the same place, i.e. from the same coach of the train. If we put the same question to the family they would say that those letters are written from different places based on the date stamp of the post offices. Both are right. Hence this term, “one and the same place” is also relative.
Relative speed
Assume we are going in a car at 50 KMPH. Also, another car is going at the same speed and in the same direction nearby our car. If the suspension system of the car and road are good we cannot feel the movement of the cars. All the persons in both the cars would say that the other car is at rest. We might have experienced the same when one bus overtakes another bus, when we are sitting in one bus. The reason for this is the relative speed, which the difference between the speeds of the cars. In this case it is zero (50-50).

When a car (A) going at 60 KMPH overtakes another car (B) going at 50 KM in the same direction, the persons in the car A will feel that the car B is going backwards at the speed of 10 KMPH (60-50). The persons in B would feel that the car A going ahead of their car at the rate of 10 KMPH.

If in the above case, if the cars are going in the opposite directions, the relative velocity would be 110 KM (50+60). In this case, both the cars would separate at the rate of 110 kmph. That is the persons in both cars would feel that the other car is going at a speed of 110 KM.

From the above, we see that when both the cars are at rest or both moving with the same speed and in the same direction the cars seem to be in rest.
In the above circumstances, whether the cars are at rest or at motion?
In the same way, day and night are also relative. While it is day in India, it is night in US.
Consider a train is moving in a straight line with uniform speed. For a person in the train all the objects in the train are at rest. If he tosses a ball up it will drop back to his hands. The person will feel as if the train is at rest.

At the same time, a person in the platform will see the movement of the ball in a different way which is shown in the following diagram.

Some facts about the light
The speed of light is almost 3 lakh KM per second. To have an idea of this speed, if we have a rocket of this speed, we can reach moon in 1 sec and the sun in 8.5 minutes. We can travel around the earth almost 7.5 times in one second. The greatest speeds available are those of a rocket with 12 km/sec. The speed of rotation of the earth on its axis is about 30 km/sec.
Light does not have the relative velocity. Its speed is always the same. It is a constant. It is independent of the source which produces the light whether it is moving or at rest.
Naturally, it is possible to increase or decrease the speed of a body artificially. For example, suppose a bullet is fired from a gun. If we place a box or bag of sand in its way, it will slow down the velocity of the bullet. The speed of the bullet depends on the type of gun and the gun powder used in the gun etc. But for light the speed does not depend on the source.
Sound requires a medium for propagation. But light does not need any medium. Assume a bell and a bulb is placed inside a glass jar. The air inside the jar is constantly pumped out. The sound of the bell gets weaker and after some time we cannot hear the sound. On the other hand, the bulb glows as usual. This shows that light requires no medium.
An experiment was conducted by Albert Michelson to find the speed of light. His experiment showed that light travels in all directions with the same speed. The speed does not vary with anything. It was concluded that the velocity of light is not relative but absolute.
Time is relative. Contraction of time
Assume a train which is 5400000 (fifty four lakh) long which travels at a speed of 240000 (two lakh forty thousand) km/sec in a straight line. Assume the train travels from west towards east direction.
There are two automatic doors one at the first coach and another one at the last coach. There is a bulb in the middle of the train. The doors are so devised so that they open immediately when the light from the bulb reaches the doors.
We will find the observations made by a person in the train and a person in the platform.

Person in the middle of the train
We know that light travels at the same speed of 3 lakh km/sec. The light will reach the door in the first coach in 9 seconds. (2700000/300000). The person will see the door in the first coach opens in 9 seconds. Similarly, he will see the door in the last coach also opens at 9 seconds. Hence the person in the middle of the train will observe that both the doors will open at the same time.
Person in the platform.
For this person, the fist coach is going towards east at the speed of 240000 km/sec and light from the middle of the coach also going in the same direction. For him, the relative speed is (300000-240000)=60000 km/sec. In order to reach the door in the first coach, light has to travel a distance of 2700000 km. It will take 45 seconds (2700000/60000). Thus, he will see the door open in 45 seconds after the bulb is switched on.

As the last coach moves in the east direction, towards the bulb and also the light moves towards the last coach, the relative speed is 540000 (300000+240000). Time taken for the light to reach the door in the last coach is 5 seconds. (2700000/540000). Thus, he will see the door in the last coach will open after 5 seconds after the bulb is switched on.
Hence, two identical functions, opening of the doors in a train will happen at the same time for a person in the train and with 40 seconds interval for the person in the platform.
It will seem to the people on the platform that the doors open at different times, the rear door first and the front door after 40 seconds. How it is?
There is a joke. One cow boy on seeing a giraffe in the zoo exclaimed, “It cannot be”. The reason is that he had never seen any animal other than cow. Likewise, the above phenomenon is unbelievable since we cannot imagine the tremendous speed of 240000 km/sec.
The theory of relativity of time and its effects are usually known as special theory of relativity.
            Assume that we are travelling in a train called Einstein train along an endless railway. The distance between two stations is 864000000(eighty six crore forty lakh) km. The speed of the train is 240000 km/sec. If the train starts from one station, at this speed, it will take 1 hour for the train to reach the second station.
            There are clocks at both stations. A person boarding the train at the first station sets his watch by the station clock. On arriving at the second station he is surprised to find that his watch is slow.  At the repair shop he was told that his watch was in good order.
            To explain the reason, consider the following. Assume that the traveler placed a torchlight on the floor of the train and sends the light towards the ceiling. There is a mirror in the ceiling which reflects the beam of light back to the torchlight. The following figure shows the path of this beam as found by the person in the train and by a person in the platform.

            From the two figures above, we may conclude that for the observer in the platform, the beam travelled a greater distance than to those on the train. This is because of the movement of the mirror and the torch along with the train. We already seen that the velocity of light is absolute and hence it the constant for the traveler and also for the person in the platform. We see that for the observer in the platform, light travels more distance. From the basic formula, time=distance/speed, we can see that it takes more time for the light to start from the torchlight, reaches the mirror at the ceiling and to return to the torchlight.
            By a simple calculation, we can find the difference of time between the traveler and the person on the platform.Suppose the observer in the platform finds the time for the light from the torch light to reach the mirror and then return back to the torch light to be 10 seconds.  Since the speed of light is 300000 km/sec, and it took 10 seconds to complete its path, the total distance travelled by the beam for this observer is 300000*10=3000000 km. This is the length of AB+BC in the triangle ABC. Hence, the length of AB=length of BC=1500000 km. The distance AC is the distance travelled by the train in 10 seconds, i.e. 240000*10=2400000.
            In right angled triangle ABC, BD is the height of the coach. We can find BD from triangle ABD using Pythagoras theorem. It is 900000 KM.
            For the traveler in the train, the distance travelled by the beam is 1800000 (900000+900000 up and down). Speed of light is 300000. Hence, the time taken by the beam is 1800000/300000= 6 seconds.
            From the above, it could be seen that while 10 seconds elapsed at the railway station, only 6 seconds passed in the train. That is to say, if the train arrived at the next station in 1 hour, for the observer in the platform, it would be only 36 minutes for the traveler. To be clear, if the train started at the first station at 1100 AM, while reaching the next station, the time would be 11.36 AM as per the clock of the passenger and would be 1200 as per the clock of the person on the platform.
            If the speed of the train is increased so that it approaches the speed of light, it is possible that only one minute will elapse on the train whereas one hour will elapse in the platform.
            Now assume that the Einstein train travels along a circular railway. It will return after a certain time to the point where it started. As already discussed, the passenger will discover that his watch is slow. By increasing the speed of the train we may reach a point where only a day passes for the passenger while a number of years elapse for the station master. If the passenger were the father, he would find his son elder than him.
            A light year means the distance travelled by light in one year. Usually the distances of stars are measured in light years because of its enormous distance from earth. Assume a star is at a distance of 40 light years. That is to say, if we go in a rocket at a speed of light, 300000 km/sec, it will take 40 years to reach the star.
            Suppose we fly to the star in an Einstein rocket at a speed of 240000 km/sec, for the people in the earth we will reach the star in 50 years. But as we seen earlier, a time of 10 seconds in platform is only 6 seconds in the train, the actual time taken for this travel is 50*6/10= 30 years.
            If the speed of this Einstein rocket is sufficiently increased, we can reach the star and return to the earth within a minute. But on the earth 80 years will have passed at the same time.
            From calculations, it is observed that if we travel for 6 months at an acceleration equal to that of the earth, we may gain 6 weeks.12 months of flying in a rocket will gain 18 months, for 2 years of travel the gain is 28 years, and if we spend three years in interplanetary travel we will gain more than 360 years.
            This can only be theoretical and is impossible because of the expenditure involved. A rocket of weight 1 ton and flying with a speed of 260000 km/sec consumes 250,000,000,000,000 (250 followed by 12 zeroes.) kilowatt-hours (units), which it takes the world several months to produce. This is only for the travel. At the speed of 260000 km/sec one year in the rocket will be equal to 2 years in the earth.
            The requirement would be 200 times as mentioned above for the starting speed and also for the safe landing of the rocket. This energy is approximately equal to the power that the world produces in several dozen years.

Article by:

Sri R.Hariharakrishnan,

1.     Relativity, The special and the General theory- Albert Einstein

2.     What is the theory of Relativity- L.Landau and Y.Rumer.