ELECTROMAGNETISN

1. Charges and force between charges

Electricity is related to charges. What is charge? Charge is a strange thing given in nature. It is not seen or felt ordinarily. For example if you touch a table you feel no effect due to charge.

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Around 200 years back, charges were discovered to be in matter by the simplest process of rubbing two substances. Much later they came to understand the fundamental essence of charges. This happened when they discovered that all matter is made up of atoms and molecules (molecules are only atoms joined together).

In an inch of any material, there are crores and crores of atoms / molecules ! ( see the next chapter for the details) Now electricity is a property of electrons and protons. Electrons and protons are in atoms. Electrons are negatively charged, protons are positively charged and there is a third type of particle called neutrons. Neutrons have no charge. The negative and positive are only names to say that there are two kinds of charges. The protons and neutrons reside in a central nucleus. The electrons orbit around the nucleus. Now, the property of these two kind of charges (positive and negative) is that positive and positive repel (go away from each other), negative and negative repel and negative and positive attract! This is given in nature !

When two substances are rubbed with each other, electrons from one substance (Say A) escape into another (Say B). B has now more electrons than protons so it has negative charge. B has lost electrons, so it has more protons than electrons.

So it has positive charge. Thus A and B attract each other since there are unlike charges. If A is taken to similar negatively charge body C, A and C repel. If B is taken close to a similar positively charged body D then B and D also repel. Repulsion happens because like charges repel. The force between charges (the repulsion or attraction) is called electrical force. The closer the charges are, greater the force. More the charges, more the force. This is expressed in the equation f = k q1 q2 / r2 , where q1 and q2 are the charges of the two bodies and r is the distance between them. This is called Coulomb’s Law.

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2. Currents, Ohm’s Law and Batteries


Currents are flow of charges in a conducting wire. How can such a flow of charges be created. Currents can be created using batteries. It was Volta created the first battery.

Working of a battery :

A battery has three main parts: a positive electrode (terminal), a negative electrode, and a liquid or solid separating them called the electrolyte. The positive and negative electrodes are separated by the chemical electrolyte. It can be a liquid, but in an ordinary battery it is more likely to be a dry powder. When you connect the battery to a lamp and switch on, chemical reactions start happening. One of the reactions generates positive ions and electrons at the negative electrode. The positive ions flow through the electrolyte to the positive electrode. Meanwhile, the electrons flow around the outside circuit to the positive electrode and make the lamp light up on the way.

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parts of a battery

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Principle of a battery

understand the mathematics of current. The current in a wire is directly proportional to what is called a potential difference between two points. Potential difference is defined as the work done to carry a unit positive charge from one place to another.

The more the potential difference, more the amount of current. V a directly proportional to i. So V = i x R Where R is the resistance in the wire to the flow of current. Current is measured in amperes and potential is measured in volts. Resistance is measured in Ohms. This relationship between potential and current is called Ohm’s Law.

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3. Magnetic effect of current


We have learnt about charged particles. There are positive and negative charges. We know that like charges repel and unlike charges attract. We have also seen that a current can be created through a battery and we have also understood Ohm’s Law.

It was first observed by Oersted that a magnetic needle is affected near a current carrying wire. He accidentally discovered this phenomenon while giving a lecture. It was one of the greatest turning points in the history of science !

Current in a wire brings about a magnetic effect in the space around it. To put it simply current in a wire behaves like a magnet. What is the direction of the magnetic field created by current carrying wire. This is given by the right hand thumb rule. If you imagine holding the current carrying wire in your right hand and with your thumb pointing in the direct of the current, then the curl of your fingers gives the direction of the magnetic field. See figure.

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4. Electro magnetic induction


We have seen that a current carrying wire produces a magnetic effect. Is the reverse true ? Is there a relationship between magnets and currents ? Michael Faraday wondered about it and tried many experiments and finally discovered that if you move a magnet near a wire, current is induced in it. This is called electro magnetic induction and this was used to make a generator.

So in effect we have two principles. The current carrying wire brings about a magnetic effect and moving magnet induces current ! These are the fundamental laws of electricity and magnetism !

Electromagnetic induction

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5. The working of motors


Motor work on the principle that a current in a wire creates a magnetic field. What we do is we take a coil bent it into a loop and place this coil between two magnets. The right hand side and left hand side will experience opposite forces of repulsion. Thus they will be turning motion. A turning motion is a called a torque. In practical motors many loops are taken and magnets are replaced by electromagnets.

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6. The working of generators


Generators work on exactly the opposite principle. Here the loop is given a motion so that electric current is induced in it.

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7. Electromagnetic waves (Spectrum)


We have two principles till now. Firstly we saw that when a current flows in a wire it becomes like a magnet creating a magnetic field around it. When such a wire in the form of a loop is kept between two magnets, we have a motor.

Secondly we saw that if there is relative motion between a magnet and a wire a current is induced in the wire. This principle is used to make generators.

Now, the current in a wire is steady. What if the current oscillates ? In other words, what if the current is not steady. A wondrous thing happens ! The changing current creates a changing magnetic field. But according to Faraday’s Law this changing magnetic field should create in turn electric field. But the changing electric field again creates a changing magnetic field and so on.

So when a current is not steady, electric and magnetic fields are created and they propagate outwards as waves. And these waves are called electro - magnetic waves. Maxwell found out the velocity of such waves and it was exactly equal to the velocity of light. Thus they discovered that light waves actually are electro magnetic waves. Electro magnetic waves have various frequencies and wave lengths and the whole range is called the electro magnetic spectrum.

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LIGHT

1. WHAT IS LIGHT AND WHAT ARE ITS PROPERTIES?


Light is one of the most mysterious things on the world. We don’t see light, we in fact, see because of it. Anything heated becomes luminous, is itself glowing, and enables us suddenly to see things around it. If you are sitting in a dark room, by an can’t see anything. When you switch on the light, the bulb glows and other objects are seen! How, what is happening? Clearly the bulb is the source of light and if we close our eyes, we can’t see, obviously. Something mysteriously is happening between the source of light (here bulb) and our eyes.

What is it?

Something is travelling between the source and our eyes. And this something seems to travel on straight lines. Otherwise why would there be shadows if anything obstructs that something coming from the source of light. (here bulb) Then, there are colours. It is as if light comes in a variety of colours. A red short is red, may be, because that mysterious something is a type that causes the sensation of red.

Red is red. We can’t question it. It is that particular quality. What we are doing here is trying to ask what causes red. That which causes on us the sensation of red is “red light”. In the same way violet light, yellow light, blue light, green light, indigo light, orange light.

What about black colour and white colour. White is a mixture of all colours and black is that which gives off no colour. When light, that invisible thing, falls on a particular object, the object absorbs some colours and gives off a colour. The colour given off, if red, will make us see red. Similarly with other colours. If the object gives off all colours, we see white. If the object absorbs all colour and gives off no colour, we see black.

If the object lets the light pars through completely the object is not seen eg: glass, water. What if the object does not take anything and reflects the light from an object completely. Then it becomes a mirror!

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See the figure. The light from object A travels on straight line and hits the mirror. The mirror doesn’t take anything and simply reflects it and then it reaches our eye. Now comes the catch!

What do we see! Since all the light from A has reached us, we see A but where do we see A?

From childhood our brain knows only to ‘see’ objects in straight lines. So it sees the object at A as shown on the figure!

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See figure(2) the object itself has the eye. Then we see ourselves at a obstance which the same distance ‘d’ but ‘inside’ the mirror! In a way the objects that we see is a hallucinatism but it is a hallucination due to physics and it’s reality not due to a ghost! Let us now consider another and last strange phenomenon that we see with our naked eye.

Why does the win in a glass of water appear to be above than it’s normal bottom? Why does a stick seem to bend in water? The answer is that light bends when travelling from one medium to another. (Here from water to air). So what? What if it bends?

If we take two rays - ray 1 and ray 2 and if it bends and reaches our eyes, our eyes don’t know that. It sees on straight lines only and that means that it hallucrates and sees the object above than the normal. Even the bottom comes up.

A spom bends and bulges from the same reason.

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So these facts give us some ideas about light which are fairly obvious. But it is a wonder, isn’t it?

Its strange - this thing that travels in straight lines, bends when travelling from one medium to another, is composed of various components which cause various sensations of colours in us!

Light travels in straight lines

Light consists of various colours

Light is reflected from norms

Light is refracted (as it is called, the act of bending) when from one met to another.

When Light strikes one face of a prism, the different colours of light refract by different amounts! Thus they emerge from the other face with the colours separated. Rainbows are formed that way (the raindrop acts as prism)

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The principle of refraction is used in lenses to make things bigger.

HEAT

1. WHAT IS HEAT REALLY AND WHAT ARE ITS PROPERTIES ?


Heat is a sensation that we feel, but what is that sensation and how is it caused? Heat in the earlier times was thought to be a kind of fluid that goes from one body to another. But later they understood that heat is nothing but the jiggling and wiggling of atoms and molecules. Matter is not all one piece. Inside it is made up of crores and crores of atoms. These atoms move randomly and more the movement of the atoms, more the heat. Our skins have evolved to precise heat with a sensation of hot and cold! In a cold night, why do we feel cold? It is because the molecules of air move about at less average energy( called kinetic energy). When we rub our hands - hard lot of heat is automatically produced. Why? Again it is because when you rub your hands together, the atoms inside jiggle more and more and that gives us a sensation of heat! So when pressure is increased then heat is produced. The pressure can be due to friction, electricity. Then there is another thing that heat does , it can change the state from solid to (picture)


Change of State


liquid and liquid to gas! Why does this happen? It happens because in a solid the atoms are fixed in their places. They cannot move about and when heat is given to such a solid, the bonds etween the atoms that keep them fixed weaken and they move about though the link between them does not break. This is a liquid. On further heating, the links that is the bonds are broken completely and that is a gas obviously!

Heat also expands solids, liquids and gases. This is because the bonds between the atoms inside get loosened and the body as a whole expands. (Picture)


Expansion of Solid


Heat is also transmitted in 3 ways - conduction, convection, radiation. When one part of a solid body is heated, the atoms inside that part starts jiggling and wiggling. These in turn moves the atoms beside them and so on. Thus heat is transmitted from one end to the other. This is conduction(see Picture)

Convection is very interesting. Suppose we take a bucket full of water and heat it for sometime. The lower part which is closer to the heat gets heated up. The bonds inside loosen up and the distance between the atoms become greater. When the distance becomes greater it is like the weight is reduced as less is now in the space occupied earlier. The gravitational attraction hence reduces and the water goes up. The colder heavier water pushes into its place (the lower part of the water). Thus heat gets transmitted by actual movement of water. This is called convection. Exactly to the same thing happens causing winds. The air closer to the earth is heated up, becomes lighter due to the atoms going further apart and hence rising due to less gravitational attraction. The colder heavier gas pushes into its place and winds are caused!

There is a third way in which heat is transmitted. They are through waves. There are waves called heat waves or infrared waves. They are part of the electromagnetic spectrum (to be covered in a later section). These waves do not need any medium. The sun’s heat is actually heat waves which travel through space and come to earth !

GASES

1. BERNOULLI’S PRINCIPLE

Statement:

The pressure of a liquid is decreased if the velocity with which it is moving is increased. Why is this so? This is because a flowing liquid (or even a gas) is made up of molecules. When these molecules hit the surface it is a pressure. When the molecules are going fast, the number of hits are less and hence the pressure is less.

This principle is used to make airplanes fly. What we do is we make the wings of the airplane in such a way that air needs to travel a longer distance on the top surface when compared to the bottom surface. Since the air is moving smoothly on both the surfaces, it takes a longer time for the air moving on the top surface to cover the wing as compare to the air that is moving along the bottom surface.

This means that the velocity of the air moving on the top surface is less when compare to the velocity of air moving along the bottom surface. Hence the pressure on the top surface is lesser when compared to the pressure on the bottom surface. So there is a net upward pressure on the bottom surface and airplane flies! (see figure)

2. WHAT ARE THE PROPERTIES AND LAWS OF GASES?

2.1 What is a Gas ?

A gas is a substance that can compressed easily. Gases fill the container in which they are put. Actually the atoms and molecules in a gas are free to move about. There are spaces in between molecules ! That’s why it is a gas ! There are three fundamental variables that define the state of a gas. They are pressure, temperature and volume. These three variables are connected deeply in the form of laws. The first law is Boyles law. This gives the connection between pressure and volume. Pressure is the force per unit area on the wall of container. Volume is the space occupied by the gas. How are these two related and why ?

2.2 Gas Laws

Boyle’s law

If you increase the volume keeping temperature constant, there will be less molecules per unit volume. Hence the hitting of the molecules of the container will be less. Hence the volume is inversely proportional to pressure. This is called Boyle’s law. The next law is Charle’s law. Charle’s law states that the volume of a gas is directly proportional to its temperature provided the pressure is held constant. It is because if you increase the temperature and pressure is kept constant, then the body will increase in volume due to increased movement of molecules.

These are the two fundamental laws that govern all gases.