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.

LIQUIDS

1. Introduction:

When we look around the world we see actually 3 different kinds of things – Solids, Liquids and Gases.

What is the difference between solids, liquids and gases?

Solids have a definite shape, are reasonably rigid i.e. don’t change shape too easily.

Liquids do not have a definite shape and takes the shape of the container.

Gases are even more free. They fill the container fully.

Why do solids, liquids and gases have their nature.

If we keep ice in open air, it melts and becomes a liquid! After some more time, it becomes vapour and vanishes! Surely a solid is not all one piece. If it is becoming a liquid, there is an internal nature it has that gets changed. The same thing with liquid becoming gas.

Yes!

There is an internal nature to everything.

During the 19th century an extraordinary fact came to light.

Everything, an inch of any material, is made up of crores and crores of small particles called atoms or combinations of atoms called molecules.

The first indication came from experiments in chemistry and later many many experiments gave a detailed account of the inner working of atoms.

The story of the atomic structure is explained fully in the section of “understanding atomic structure”. In a solid the atoms are bonded tightly and hence outwardly, it is solid!

In a liquid the atoms are not in one place but are bonded!

In a gas the atoms are free!

This is the reason why a solid is a solid, a liquid is a liquid and a gas, a gas! When heated, the bonds between the atoms in a solid become weak and becomes a liquid outwardly. On further heating, the bonds become completely broken and a gas is formed. We discuss change of state in the section of “understanding heat”.

In this section we try to gain a deeper understanding into liquids. Liquids and solids together are called fluids.

Pascal’s law, Archimedie’s Principle, and Bernoullis principle explain the deeper working of liquids. Bernoulli’s principle applies to gases also and will be considered in the next section on, Fluids (2) – gases.


2. Pascal’s Law


Pascal’s Law states that if you apply a pressure at one point in a liquid, it spreads equally in all directions.

What does this mean?

A liquid is a liquid. This means that it does not resist. If you apply a pressure at one point, obviously it will get transmitted to all points in the container and in all directions. It is because a liquid does not resist a force that this happens.

Pressure is force per unit area.

Why do we need a concept like pressure for liquids (gases too) and not just force? Liquids do not have a definite shape. It takes the shape of the container. So we cannot use the concept of force. The effect of the force depends on the largeness of the surface on which force acts.

If the area of the surface is large the effect of force would be less. If the area is small, the effect of force would be more.

Thus both force and area are important and this is pressure, the force per unit area does give the pressure.

If the force is 8 Newton and the area in which the force is acting is 4m2, then the effect of force i.e.


Pressure = 8/4 = 2N/m2

If the force is halved (4 Newton) and the area is halved (2m2)


Pressure = 4/2 = 2N/m2


Both factors force and area of contact comes in to play to determine pressure.

Now, Pascal’s law can be used in a dramatic way in a hydraulic press.

Consider the figure below…

If a force is applied at A, a small force, for a long distance l, it gets transmitted to B, a great force for a shorter distance D,


So f x d = F x D (Pressure is the same on both the surfaces)


Thus a small force for a greater distance causes us to lift heavy objects by giving a greater force with a smaller distance.






3. Archimedes Principle





Archimedes was a Greek and he is famous for having discovered a principle that governs all liquids – the Archimedes principle. Let us proceed how to understand his principle.

It is a common observation that when you push a body into a bucket of water, you experience a force in the opposite upward direction. This means that the water exerts a force on you when you immerse a body in water.

If you just put the body in water, it either floats or sinks.

Now why does a body thrust into water or every liquid experience an upward, resistant force?

Why do some bodies float and some bodies sink?

These questions are answered by the Archimedes Principle. When you put a body in water, that body displaces water. The l….. of the water rises. How much water is displaced?

Obviously the volume that the body has occupied in the water, that much of volume of water is displaced. See the figure 2 below.

Now, the displaced amount of water tries to get back into its on final position and thus gives an upward force to the body. This is called the Buoyant force as it is trying to keep the body up.

Obviously this buoyant force is equal to the weight of water displaced.

This is the Archimedes principle.

Now, when does a body sink?

When the weight of the body is greater than the upward force which is nothing but the weight of water displaced, the body obviously sinks.

When the weight of the body is less than the upward force, the body obviously floats.

So clearly 2 things are important here.
The weight of the body itself.
The amount of water (or liquid) the body displaces.

If ‘2’ > ‘1’, the body floats. When ‘2’ < ‘1’ the body sinks. A work floats on water because its weight is less than the amount of water displaced (which gives an upward thrust).

An iron nail sinks on water because its weight is more than the water it displaces i.e., the upward thrust given to the nails greater than the weight of the nail. But what about an iron ship? Why does an iron ship float and not sink in water?




The ship is heavy but it is big in terms of volume also. When it enters water, the amount and hence weight of water it displaces is also huge and the upward thrust is huge!

The upward thrust in the ship is actually more than the weight of the ship! Thus the ship floats!

Thus we see both the factors acting always when we consider floating / sinking
Weight of displaced water that determines the upward thrust on the body.
Weight of the body itself.

‘1’ > ‘2’ – body floats.

‘1’ < ‘2’ – body sinks.

All bodies whether they are floating or into water experiences thrust upward thrust. This is equvalent to weight loss.

The weight acting downward is contracted by a buoyant force upwards.

Apparent weight loss = weight of body – upward thrust (buoyant force)

Thus we clearly see how (and why) bodies behave the way then do when immersed in liquids.

SOUND

1. Introduction - What is Sound?

Sound is a sensation that is always all around us. We hear sounds when objects strike or any disturbance suddenly happens in air. But if we close our ears, we cannot hear any sound. Sounds are of different kinds. Some sounds are loud, other sounds are low or soft. Some sounds have a higher pitch. A scream is at a high pitch. Girls have a higher pitch of sound than boys. We know and understand pitch directly by our sensation of it i.e. we can directly know what is at a higher pitch and what is at a lower pitch. Gradation of pitch is after all the musical notes – “Sa”, “re”, “ga”, “ma”, “pa”, “dha”, “nee”.

Sounds coming from far are heard faintly and those coming from near are heard more clearly. There is also a quality of a sound. We can recognize our friend’s voice by the quality of his voice.

Now these facts seem straight forward but they are actually wondrous facts – in nature. They actually raise many questions. We normally do not ask these questions because we take these facts for granted having got used to them from childhood. We feel they are like that because they are like that.

But these facts suggest many deep questions.

What is sound? This question means that, what is happening really between the source of sound and our ear? When we clap our hands (for example), we hear a distinct sound but the real question is, what happened quickly between the point of clapping and our ear? It is a wnder that something happens, isn’t it? What is it? To put the question in another way, what is the physical basis of our sensation of sound.

The next deep question that can be asked about the facts given before is, when is a sound heard loudly and when softly?

The third question that can be asked is when is the pitch high and when is the pitch low.

You see, something invisible is happening between the source of the sound and our ear.

What is it?

Let us now proceed to answer this question.

The ancients asked these same questions and suspected that air has something to do with propogation of sound.

There is the famous bell jar experiment that can be done.


Take a jar and put a bell with connection in it. When the top is not sealed and the switch is on, the bell rings and we can hear the ringing of the bell.

Now, remove the air inside the jar using a vacuum pump and seal the top. Again put the switch on, we see the bell shaking i.e. ringing but we don’t hear anything.

Yes! Air is part of the process of transmission of sound to the ear.

But the question still remains, what is the role that air is playing? What happens to the air?

In the 10th century a remarkable fact came into light that an inch of any material – solid, liquid or gas is made up of crores & crores of atoms or combination of atoms called molecules.

Keeping this in mind, it is obvious that something internal is happening that causes the disturbance that reaches our ear and causes in us the sensation of sound.

One can understand this by realizing that sound propogation is invisible and something must be happening inside and in the air..

Now the gases in a region, or liquid or a solid contains atoms / molecules in close proximity. Of course they are closer in solids than in liquids and closer in liquids than in gases but atoms are close.

Now atoms when brought together have the property of elasticity. They are like a balls with springs attached in the middle. When we shout, (lets say) or 2 things strike, the molecules immediately beside the source get disturbed and get compressed. But since the molecules are connected like springs and are elastic, they snap back but in going back to their original position, they do not come back exactly to the original position but go an extra distance. This pushes the next immediate air that gets compressed and that too tries to come back to the original position and goes an extra distance and compresses the next immediate air and so on. A wave of compressions (more pressure) and what is called rarefactions (less pressure, molecules more apart) occur.

The above is only for one hit of the source. But the source is continuously giving pressure, so a series of waves is sent out as long as the source is vibrated.

The wave travels out and it is energy and due to the pressure increase loses ultimately it’s energy due to getting converted to heat. Hence we cannot hear sounds (or only very faintly) at a distance.





2. Pitch and loudness depends on frequency and amplitude


Let us try to fully grasp the above point and what it implies.

There are 2 things that are important properties of sound as mentioned earlier – loudness and pitch.

Let us now understand both these facts with our deeper understanding of sound as a series of compressions and rarefactions.

Lets take loudness first. When we speak softly, we are putting less pressure an air so the (maximum) compression (or rarefaction) is less.

When we speak loudly, we are putting more pressure on air so the (max) compression or rarefaction is more. So, the greater the compression from normal state, greater is the volume or loudness!

Take a spoon and bang on the table lightly, the sound is low! Bang it hard, the sound is louder. Again the same principle applies. If the pressure is more, compression is more and hence loudness is more.

A tyre or balloon bursts i.e. gives a loud sound because the air suddenly shoots out, the pressure is more and hence the compression (max) is more and hence the ear grasps that as a loud sound and gives us a loud sensation.

Wherever there is a loud sound there is high pressure i.e. high (max) compression and hence high volume.

The maximum compression is called by the technical word amplitude.

So more amplitude – distance from mean position, the greater is the loudness of the sound.

The next question is with respect to pitch. What about pitch? What determines pitch? How do we relate the deeper nature of sound (as having compressions and rarefactions) with the pitch.

Why do girls have high pitch and boys lower pitch. Why does “Nee” note have higher pitch than “re” note.

The answer is that when there are more vibrations per second i.e., the number of successive compression and rarefaction in one second is more, the pitch is more.

This number of successive compressions and rarefactions per second is called by the name frequency.

Hence if frequency is more, the pitch is high.