DYNAMICS

INTRODUCTION

In chapter 3, we discussed kinematics of moving bodies. In this chapter we will deal with dynamics of the bodies. Dynamics tells us answer to such question as: why do bodies move from rest and why do they accelerate or re accelerate? For such motion push or pull types external agency is required, which acts on a body in the motion either brings to the rest or external agency is required, which when acts on a body in a motion either brings it to  the rest or causes it to accelerate. On the other hand, if the external agency acts on a body at rest, the body will start to move. This pull or push is called force. Galileo Galilee (1564- 1642) studied the motion of bodies on an inclined plane and concluded that the body is continued to move with the same velocity if no external force acted on it. Later, Sir Isaac Newton (1642- 1727) studied the problem of motion in detail and enunciated three laws of motion, which cover the motion of all bodies- big or small. The three laws are named after him.

1.   NEWTON’S LAWS OF MOTION

The following three laws of motion were originally formulated by Newton and were first published in 1686.

1.   First law of motion: Every body continuous in its rest or of uniform motion in a straight line, unless external force act on it.

2.   SECOND LAW OF MOTION: The rate of momentum of a body is directly proportional to the impressed force and takes place in the direction in which the force acts.

3.   THIRD LAW OF MOTION: Action and reaction are always equal and opposite to each other.

2.   INTERPRETATION OF THE FIRST LAW

The first law of motion has two parts:

I)                Every body continues in its state of rest, unless external force acts on it.

II)            Every body continues in its state of uniform motion in a straight line, unless external force acts on it.

The first parts of the law are experienced everyday, because a body lying on a table continues to remain there until it is pushed (or pulled).

The second part of the law may seem contrary to our common experiences. A rolling ball comes to rest after sometimes. It should be realized that the ball comes to rest because of air- resistance and also due to the friction from the surface it rolls on.

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ELASTICITY

      

INTRODUCTION

Every thing around us, with us and without us is matter. Everything in the materials world consists of matter. Atmosphere, air, water, trees, rocks, flesh, blood, bones, etc are the examples. Matter is every where in nature. It occupies the space and knowledge about it can be obtained using our organs of sense and instrument. It has mass and/or possesses energy. It is not spirit.

Matter is composed of particles called molecules and atoms. The size of the typical molecules is about 3×10ˉ10 m. Mechanical properties of matter are the topics our study in this and in the following two chapters.

The quantity of matter is a given substance is called it mass. Mass remain same even when its volume, shape or size change due to the external force, provided we don’t loose matter in the process. For example, substance like wool, cotton, rubber etc. can be compressed or stretched by Applying external force. But their masses do not change. Liquid and gases also flow, but their mass remains constant.

Constituents if matter

Matter is made up of a very large number of molecules. Each kind of matter has its own distinctive molecules. A molecule, in general, is composed of smaller particles called atoms. Atoms of elements were supposed to be invisible and were regarded as elementary particles with which all matter was assumed to be building up. In the beginning of the last century researches established the existence of more elementary particles, e.g. electrons, protons and neutrons. In the modern theory of matter an atom is made of a nucleus and electrons revolve around it. The nucleus is made of protons and neutrons.

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VAN DE GAFF GENERATOR

A Van d Gaff generator is a device for generating very high voltage and collecting extremely high amount of charge. It is based on of discharging action of points and accumulation charges by a large hollow spherical conductor. In 1929, Robert J. Van d Gaff designed these machines, which was later named after him.

It consists of a large hollow metallic dome mounted on an insulating stand. There are two types pulley, one inside the dome and other is near the bottom as shown in the figure 32.17. A belt of insulating materials, e.g. Silk, goes over the pulley. The lower pulley is driven by an electric motor (or, by hand for a small demonstration machines). There are two metals brushes having a number of sharp needles near the two pulleys. The needles are pointed towards the belt without touching it. The upper brush is connected to the metallic dome from inside whereas the lower brush is connected to a source of high positive potential.

Because of the high electric field near the needles of the lower brush, the air molecules become charged due to discharging action of points and air in between begins to conduct. The positive charge forms those needles leak through those sharp ends and reaches the belt. The belt moves upwards and carries those sharp ends and reaches the belt. The belt moves upward and carries those charges up. When it reaches the upper brush needles, induction takes place and their interior of the dome develops negative charges and outer surface becomes positively charged to the same extent. Since the upper brush needles are connected to the interior of the dome, the positively charges on the belt are neutralized by the discharging action of those brushes. Thus, electricity neutral belt moves down on other side of the pulley. However, the outer part of the dome accumulates positive charges.

Thus, these machines continuously transfer positive charge of the sphere. Potential (we will study about it in the next chapter 34) of the dome increases continuously as the belt moves and its value may reach higher as 10^6 V until dielectric breakdown stars. After that, it can not be charged further as the air starts to conduct as this potential and charges leak to the earth or nearby metal through air. However, it is possible to increase further the amount of charge on the dome if we enclose it in a highly evacuated chamber.

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CONDUCTOR AND INSULATOR

All the materials are classified into two types- conductor and insulators- based on their ability to conduct charges.

a)   CONDUCTOR

Materials through which electricity (charge) can pass easily is called conductors. Metals, mercury, acidic water, human body, earth etc. are conductors of electricity. These can be charged by friction but the charges they gain don’t remain localized and spread over the whole surface.

 In metals outermost electrons in the atoms are almost free and may move throughout the metal. They are not bound to any particular atom. They can be treated as free electrons. They are the actual carries of electricity in metal. For example, 1c.c. of copper contain approximately 10^23 free electrons! That is why metals are good conductor of electricity. Silver is a very good conductor, so are copper, gold and aluminum.

INSULATOR

There are some materials which don’t allow   electricity to pass through them. Those are called insulators. Glass, wood, ebonite, silk, rubber, sculpture, air etc are insulator or non conductor of electricity. These can gain charges by friction but the charges cannot move to other parts and remain localized.

SURFACE DENSITY OF CHARGES

We saw that the charges distribute themselves over the outer surface of a conductor. But their distribution is not uniform over the surface. The distribution of charges on a surface is characterized by surface charge density at a point on the surface. It is defined as the amount of charge per unit area of this surface surrounding the point. If q is the amount of charge and a is the small area over which the charge is spread, the surface density of charge is given by

σ= q/a

It depends on the shape of the surface of conductor. If the surface has large curvature at some points, the accumulation of charge there is also large. For uniform curvature of the surface, the charge distribution is also uniform. It is also greatly affected by the presence of other conductor in the neighborhood.

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MEASUREMENT OF CHARGE

We can measure electric charge on a charged body with the help of a simple apparatus called electroscope. There are two types of electroscope and they can be used to measure charges and find their nature.

(a)           PITH- BALL ELECTROSCOPE

(b)          GOLD LEAF ELECTROSCOPE

(a)           PITH- BALL ELECTROSCO                                 A pith- ball electroscope consists of a pith- ball        suspended from a support by a single silk fiber) fig 32.6). Pith is a light and spongy materials extracted from the stream of some plants. If it is gilded (overlaid with thin covering of gold) it works better. To test whether the body has got charge it is brought near an uncharged pith- ball. The pith ball will be attracted towards it. If it touches the pith ball, they will be repelled. The repulsion is due to same kinds of charges shared after touching the body, To find the type of charge on it, the pith- ball is charged by known type. Suppose the pith ball is charged positively. The body whose nature of charge is to be determined is brought near. It will be repelled if it is positively charged and will be attracted if it has negatively charges on it.

(b)          THE GOLD- LEAF ELECTROSCOPE (GLE)

This is a sensitive instrument. It can be used to detect charge and its nature. It can also give us a roughly the amount of charges on the body. That is why it is commonly used for demonstration. A typically gold leaf electroscope is shown in the figure32.7.

It consists a vertical metal rod, with a metal disk on top, going inside through an insulated hole on the top lid of a glass box. The lower end of the rod has two thin gold leaves attached to it (Aluminums foils can also be used). The sensitivity of this instrument can be increased by passing two strips of tin foil on the inner side of the glass panes facing the leaves. The tin- foils cover the level of gold leaves and pass down to the base of the instrument so that the electroscope can be grounded. The utility of these foils is to cancel any influence due to outside charges.

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CHARGE RESIDE ON THE SURFACE OF

On a charged insulator the charges are localized at places where they are supplied. But, in case of a conductor the charges given to any point immediately spread themselves over the surface. Extra charges do not reside inside the volume of a conductor. Three famous experiments show this.

(i)                         BIOT’S EXPERIMENTS

Take a metal sphere mounted on an insulated stand. Charge it with +ve or –ve charge. Then take two metallic hemispheres which exactly fit onto a sphere and are provided with insulating handles as shown in the figure 32.11. Now fit them over the sphere enclosing it completely. Bring this set near the electroscope. The leaves will diverge. When the hemispheres are separated and tested they are found to possess charge. But the sphere does not show any charge on it. This clearly shows that charges spread out over the outer surface and don’t reside in the interior of the conductor.

(ii)                     FARADAY’S BUTTERFLY- NET EXPERIMENTS

A brass ring is mounted on an insulating stand and conical muslin net is attached to the ring. Two silk threads are tried to the cone so that it can be turned in or out, as desired (fig, 32,12). Give some charges to the net and test for the charge inside and outside. It is found that the charge is entirely on the outer side and no charges are found inside.

Now turn the net inside out by pulling the outer thread, and test for the charge. It is found that the charges are present on the outer side.

(iii)                  FARADAY CASE

Faraday built metal cage supported on an insulating base. He entered the cage and sat on the seat which was insulated from its base. It was charged to very high potential. But he could not detect any charge inside the cage even though the outer surface was sparking into air.

These experiments confirm that charges don’t reside into a hollow charged conductor. This assigns can be tested by taking a hollow charged spherical shell with a hole on it and inserting a proof plane inside it. 

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INSULATOR

It is the clear that the inducing charge of the body (rod) produces opposite type of induced charges. It should be noticed that the inducing charge on the glass rod does not change. We can charge many conductors by the same rod i.e. we can obtain limitless supply of electrical charge this way. By connecting positively charged and negatively charged bodies to a bulb we can desire electrical energy. Does this violate the principal of conservation of energy? Definitely not. In removing the inducing rod we must do some work against the attractive forces between the opposite charges on the rod and conductor.

The phenomenon of electrostatic induction allows us to understand the attraction between a charged body and a neutral body. Suppose a positively charged rod A is brought near a small metal sphere B suspended by a thread (fig.32.4). Instantly, free electrons in the sphere will move towards the side, which is near the rod A. The near side be negatively charged and the far side will be positively charged by induction (fig. 32.4) The induced negative charge will be attracted towards and the induced positive charge will be repelled away from the positive charge on A. But, since the induced negative charges on B are relatively closer than the induced positive charge on it, the force of attraction is greater than the repulsive force. So the net force on B will be attractive and the ball will be pulled towards the rod. Note that induction takes place first which result the attraction.

If we bring the a positively charged rod near a conductor ( insulator) the net force will be attractive but the force will be smaller than in the case of conductor. It is because there are no free electrons inside the insulator- all the electrons are bound to their respective atoms. Thus, the negatively charged particles (electrons) are an atom in C is drawn closer ton A than the nucleus. This type of displacement of negatively charged electrons and positively charged nucleus in an atom is called ‘electric polarization’. Insulation is also called dielectrics in this sense. First there is electric polarization and then there is attractive. We shall talk about it in detail later in chapter 35.

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CHARGING A CON DUCTOR BY INDUCTION

Take a metal rod (A.  conductor) AB mounted on an insulating stand as shown in figure 32.3. Bring a positively charged rod towards one end, say A, of the conductor without touching it (fig 32.3(a)).  The end A will develop negative charge and the end B will develop positive charge due to the electrostatic induction. The reason that is the free electrons in the conductor will be attracted towards the end A by positively charged rod and the same number of electrons will be deficient on the side B. End A will be negatively charged and end B will be positively charged.

Now if we connect the end B of the conductor to ground using the metal wire the positive charges will move to the ground. (fig.32.3 (B) (Actually, the electrons come from the ground to the conductor and neutralized those positive charge). But the induced negative charges near A remains there, because these charges are free to move.

Because the electrons at the end B have flown from earth, they are called free charge. The electrons near end A are not free to move. They are held by the inducing charges. Therefore, they are called bound charge.

Again, disconnect end B from the earth. The –ve charges will remain near the end A as before under the influence of the inducing charge (fig. 3©.

Finally if we remove the glass rod as well, the negative charge will spread over the surface over the surface of the conductor evenly and the conductor will become negatively charged (fig.3 (d)).

These are the steps to be followed to charge a conductor by the method of induction. In this activity we described methods to charge a conductor negatively. Exactly the same steps should be carried out to charge the conductor positively, with the difference that we should take a negatively charged rod in place of the glass rod above or connect end A to the ground in the previous activities.

It is a clear that the inducing charges of the body (rod) produce opposite type of induced charges. It should be noted that the inducing charge on the glass rod does not change. We can charge many conductors by the same rod i.e. we can obtain limitless supply of the electrical charge this way. By connecting positively charged and negatively charged bodies to the bulb we can desire electrical energy. Does this violate the principal of conservation of energy? Definitely not. In removing the inducing rod we must do same work against the attractive forces between the opposite charges on the rod and conductor.

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TESTING THE CHARGES

Bring a body under test near the disc of an uncharged electroscope. If the leaves remain parallel the body is neutral. The leaves will be diverge if the body has got charge on it. This shows the presence of charges on the body.

To examine the nature of charge, we have to put a known type of charges on the electroscope. It can be done by conduction or by induction. To charge by conduction, the disk is touched by hand so that the leaves collapse completely (if there were any charges). Then the disk is touched by an ebonite rod rubbed with fur. The charges on the rod will be transferred to leaves of electroscope and they diverge. The rod is removed and electroscope is negatively charged. If similar process is carried out with glass rod rubbed with fur. The charges on the rod will be transferred to the leaves of electroscope and they diverge. The rod is removed and electroscope is negatively charged. If similar process is carried out with glass rod rubbed with silk the electroscope will be positively charged.

Charge it by induction one should follow the procedures described in section 5 of this chapter. First touch the disk of the electroscope by hand and the leaves will collapse. Remove the hand. Bring a glass rod rubbed with silk near the disc. The leaves will be diverging. Keeping the rod there ground the disc on the opposite sides. The leaves will collapse. Disconnect the ground first and then remove the glass rod. Again the leaves will diverge and the electroscope will be negatively charged. The same process is carried out with an ebonite rob rubbed by fur to charge the electroscope positively.

After charging the electroscope the body under the test is brought near the disc of the electroscope. If the electroscope was negatively charged and leaves diverge more then the body is negatively charged. But if the divergence diminishes, the body is either positively charged or simply uncharged i.e. neutral. To make sure the disc is touched by hand to make the leaves collapse. Then the body is brought near the disc. If the leaves diverge the body possesses positively charged and if the leaves are unaffected the body is electrically neutral.

It should be clear that attraction does not always confirm the presence of charge, but repulsion does.

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PROPERTIES OF ELECTRIC CHARGE

Let us look at the properties was the of electrical charges.

i)                 The “positive” and “negative” adjectives added to the charges are not just for convenience. In fact, the positive and negative are to be treated algebraically, so that net amount of charge produced in any process is zero. This is summarized as law of conversation of charges produced in a process is zero”.

In our previous discussions we saw that the glass rod is positively charged when rubbed with silk. Simultaneously, the silk also develops negative charge on it. The amount of positive charge developed on the glass rod is exactly equal to the amount of negative charge developed on the silk. Thus total charge on the rod and silk is again zero, which was the case before rubbing. This shows that charges are conserved.

ii)             Like charges repel and unlike charges attract each

Other. A charge is a scaler quantity. Since there are two types of charges the addition must be carried with proper sign. For example if there are various charge distributed over a body, then the total charge on the body will be the sum of the individual charge taken with their proper sign. If +4C, -2C, -5C, +6C (C stands for coulomb, unit of charge on SI) are added the total or net charge is +3C. Addition of charges is similar to the addition of positive and negative numerals.

iii)          Charges are quantized. Scientist like R. A. Millikan

Have shown experimentally that the smallest amount of electric charge is found on an electrons and protons. All other electric charge is simply integral multiples of this smallest amount. This fact is called quantization of electrical charge. The charge of the proton is given by symbol ‘e’ and its value in SI units is +1.6×10ˉ19 C. So, charges on an electrons would be -1.6×10ˉ19 C. (More about it in Modern physics part in class XII). Charges, which are fraction of charge on an electrons or a protons gas not been ordinarily observed in nature.    

    

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ELECTRIC CHARGE OF ITS NATURE

It is a common experience that a comb, after running through dry hair a few times, can attracts small dust particles or small pieces of paper. Similarly if we rub our hands on silk vest it would also attract small pieces of paper of dust particles, feather etc. When we take off acrylic sweater in a dark room we can see the sparks flying. All these observation demonstrate charging of friction.

To study the nature of charge closely let us take a glass rod, rub it with silk clothes and hang it freely by a silk thread. Let us take another glass rod and rub it with silk cloth as in the first case and take it towards the first rod. The suspended rod will swing away. This means that the two charged ends repel one another.

In the next experiment, take a glass rod, rub it with silk and suspended it as before. Take a hard rubber rod, rub it with woolen clothes and bring it near to the suspended glass rod. It is observed that they attract each other.

Here the charges on the glass rod are same in nature because both the glass rod were rubbed with the same material-silk. It is seen that, when brought closer, they repel. So, we conclude that similar charges repel. Charges developed on the rubber rod behave differently- they attracts the glass rod. Therefore, we conclude that charges on the glass rod are different in nature that on the rubber. Also unlike charges attract.

To investigate it further, take a rubber rod, rub it with the woolen cloth and hang it by a silk thread. Bring another rubber rod, rubbed with woolen clothes, near to the suspended rod will be repelled.

This type of experimental has been done with different set of materials. They revel that two different kinds of electrical charges are produced. Two bodies carrying the same kind of electrical charge repel one another. On the other hand two bodies carrying different type of electrical charge attract one another. Thus we conclude that charges repel; unlike charge attract.

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ELECTIC CHARGE OF ITS NATURE

INTRODUCTION

Today, the word electricity is very mush familier to all of us and everybody knows about it use of electrical appliances like motors, fans, heater, radio, television etc. These function on aspects of electricity and magnetism. In this chapter we will study the nature of electrical charges.

Historically study of electricity goes back to 6’th century B.C. Thales, a famous Greek philosopher, was first to notice the attraction caused when different substance were rubbed against each other. He noticed that when the piece of amber, a yellow resinous substance found on the shores of Baltic sea and used for decoration, was rubbed against silk or woolen cloth is attracted light dust particles, lint, light feathers, pieces of leaves etc. This was the modest beginning of science of electricity. However, a concerned interest was shown to this phenomenon only in the 16’th century. DR. Gilbert, a British scientist, reported that the property of attracting light things was acquired due to friction. Bodies having this property were said to be’ electrified’. When the substance is electrified it is said to possess electric charge.

ELECTRIC CHARGE

Just as mass does a property of all matter and causes gravitational attraction, matter is composed of particles possess electric charges that have electric and magnetic interactions.

Electric charge is a physical quantity and it can be measured. However, effect of electrical charges in an object is not observed normally. This is because there are two different kinds of charges, which generally balance each other in any matter. Therefore, matter is electrically neutral and electrical effects and absent. This balance is disturbed when as excess of one or other kinds of charges is created and only then we observe electrical effects.

  

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POSITIVE AND NEGATIVE CHARGE CARRIES

Every matter is composed of a large number of small particles (Atomic hypothesis). It is easy to

 Think that if two charged bodies attract or repel one another, it is due to attraction or repulsion between the constituent particles. This leads us to think that the constituent particles must also be of two types- positively charged particles and negatively charged particles. Since matter is normally electrically neutral, the number of negatively charged particles and that of positively charged particles must be equal in them. But if we add some positively charged particles in a neutral matter, it would become positively charged. Similarly if we take our some negatively charged particles from a neutral matter it also would show excess positive charge. The same thing happens for negatively charged bodies

We have seen that a glass rod rubbed with silk becomes positively charged. There are two possibilities while rubbing, some positively charged particles from silk move to the glass rod, or some of the negatively charged particles in the glass transfer to the silk. What exactly happens? The answer comes from the structure of atoms.

It is known form school science that very light particles- electrons- continuously revolve around the nucleus in an atom. Electrons are negatively charged particles. The mass of nucleus is very large in comparison to the mass of an electron. The charges on the protons, which makes the nucleus, and on an electron are numerically equal, but they are of opposite types.

Nucleus is made of neutrons and protons (except for hydrogen in which the nucleus consists of just one proton). Neutrons are electrically neutral. In an atom, the number of protons inside the nucleus is equal to the total number of electrons revolving around it. An atom has equal amount of positive and negative charge and hence, it is electrically neutral.

When two object are rubbed, only one electrons move from atoms in one substance and transfer to the other objects, because they are light ( and therefore, easy to remove). When electrons are removed the objects becomes positive charged. This indicates deficiently of the electrons in it. On the other hand if we put some electrons on a substance, it becomes negatively charged. Clearly, it is an indication of excess electrons on it. Therefore, ordinary electrons and not protons are responsible for electrification of any object. 

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THERMISTOR

Thermionic is a heat sensitive resistor usually made from semiconductor. It is a non-ohmic conductor and possesses higher –ve temperature coefficient for resistible (ρ). As the resistible increases (decreases) then corresponding resistance will increase (decrease). Here, due to –ve resistance resulting increase of current in the circuit. This determines the temperature of the hot body.

Thermistors with high negative temperature coefficient are used as resistance thermometer in very low temperature measurements of the order of10 K. Higher resistance at low temperature leads to accurate measurement of temperature in this range.

In general we can say that the use of thermistor for temperature measurements gives more accurate result in the low temperature region than measurement temperature using a platinum resistance thermometer.

THERMAL EQUILIBRIUM AND ZEROETH LAW OF THERMODYNAMICS

It is a common knowledge that a hot body looses heat and becomes cooler when put in contact with a cold body. Heat transfer always takes place from a body at higher temperature to another at lower temperature. This process continues until the temperatures of the two bodies become equal. The two bodies are said to be in thermal equilibrium. For example, take two glasses of water at different temperature (say) 40ºC and 20ºC.When they are mixed, the temperature of the first falls down and the temperature of the second rises. This process is continued until their temperatures become equal and exchange of heat stops. Its shows that thermal equilibrium is the stage where flow of heat between two bodies or systems stops; and this happens only when the bodies concerned are at same temperature. Two bodies in thermal contact are said to be in thermal equilibrium if there is no net transfer of heat them.

  

Sometimes whether two systems A and B are in the thermal equilibrium can be inferred without pulling them in contact.

It can be done by making use of third system C. Suppose that C and A are in the thermal equilibrium and that C and B are in thermal equilibrium. Then A and B should be in the thermal equilibrium. A great deal of indicates that if two systems, then they are in thermal equilibrium with each other. This postulate is called the Zeroeth law of thermo dynamics. The importance of the Zeroeth law is that it allows a useful definition of temperature. All bodies in thermal equilibrium have same temperature. A hot body is assigned higher value of temperature than a cold body. Heat flows from the body at higher temperature to the body at lower temperature. On the basis, the Zeroeth law defines temperatures of a system as the property which determines whether it is in equilibrium with other bodies or it is not.

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THERMO- COUPLE

When two wires of different metals are joined to make a loop and the junction are kept at different temperatures, a potential difference develops between them. As a result a current flows through wires. This potential difference is measured and used to find temperature difference between the junctions. Thermometer based on this principal are called thermo-electric thermometer or thermo-couples.

The junction when maintained at constant temperature difference. The temperature can be well estimated. This type of thermometer is widely used to measure temperature of the furnace used in a heavy duty heating or high temperature heating.

Simple measurement of thermo-emf using potentiometer (detail can be found in chapter 33 of this book), leads for the calibration and determination of temperature in some cases galvanometer is also used to measure the deflection produced due to thermo emf. The deflection produced at diff temp difference give rise of a calibration curve. Hence with the help of calibration curve unknown temperature of the body can be determined.

Between 630.74ºc and freezing point of gold(1064.43ºc), the international temperature scale is expressed in terms of emf of a thermo couple. Wires used in a thermocouple for the measurement of temperature are platinum and platinum-rhodium(90% pt and 10% rh) alloy. While measuring the thermo EMF using potentiometer special care should be taken. That is the thermo EMF are not set up at the junction of thermo couple wires and copper wire connecting the potentiometer. This effect can be avoided by making three junction as shown in the figure. 15.6. The junction of copper lead and thermocouple leads are maintained at same temperature say either at temp or room temperature and junction of two thermocouple wires is brought in contact with the hot body whose temperature has to be estimated. The EMF E of the whole system system is then equal to the EMF of two platinum, platinum rhodium junctions, one in ice and next at hot body.

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TEMPERATURE SCALE

Before going into details of measurement of temperature it is essential to fix its unit. The most common unit used today is the Celsius or centigrade scale. Fahrenheit scale is also used in several countries, For example, in the United States. The mostly used unit in scientific work in the Absolute or Kelvin scale. Ranking scale is not used now.

Lower and upper fixed points in both Celsius and Fahrenheit scales are chosen as freezing and boiling points of water respectively at standard pressure. On the Celsius scale, the frizzing point of water is chosen to be 0ºc and the boiling point of water is 100ºc. On the Fahrenheit scale, the frizzing point is taken at 32ºf and the boiling point at 212ºf. A practical thermometer is calibrated by placing it in a carefully prepared environment at each of two temperature are marking the position of the mercury. The distance between lower fixed point and upper fixed point is divided into 100 equal parts in the Celsius scale where as the same distance is divided into 180 equal parts in the Fahrenheit scale.

THE LIQUID THERMOMETER

As it has mentioned before that the basic of thermometer practically used is based on the principal of expansion or contraction of liquid (change into volume) due to heating or cooling of it respectively. A proper choice of liquid is always essential. Those liquid which is used a thermometer construction called Thermometric liquid and must have following basic character tics

CHARACTER TICS OF THERMOMETRIC LIQUID

1.   It must have low specific heat, so that it absorbs small amount of heat from the body whose temperature is being measured.

2.   It must be good conductor of heat so that it can take heat quickly from a body.

3.   It must be easily visible in the capillary tube.

4.   The liquid should not wet the wall of the capillary tube.

5.   The liquid must have uniform expansively (coefficient of expansion) over wide range of temperature.

6.   The freezing point and baling point of the liquid must be appropriate, so that it remains as liquid in the range of measurement.

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DIFFERENCE BETWEEN HEAT AND TEMPERATURE

HEAT	TEMPERATURE
1. It is a form of energy and measures the total energy of all molecules in the substance.	1. It is a measure of degree of hotness or coldness of a body.
2. Its unit is joule in SI and calorie in CGS.	2. The unit of degree Celsius or Kelvin or degree Fahrenheit.
3. Two bodies can be in a thermal equilibrium without having equal thermal energy.  Their temperature are equal in case.	3. Two bodies cannot be in thermal equilibrium if they are at different Temperature.
4. Flow of heat doesn’t depend upon the heat energy contained in bodies in thermal contact.	4. Flow of heat depend upon the temperature difference between two bodies. Heat always flow from a body at higher temperature to a body at lower temperature.
5. It is a total kinetic energy of all molecules in a substance.	5. It is a measure of average kinetic energy of molecules in a substance.

Whether a body is hot or cold can

Be detected by touching it with our hands. But this judgment may be misleading. Consider an example. There are two buckets of water where one is slightly colder than other. When we put our hands separately into them one hand would fell cold and other would fell hot. Now take Third buckets of water with its temperature lying in between previous two. If we put both our hands into the buckets one would fell hot and other would feel cold.  Since the sensation given by our organ of sense may be misleading, we need an instrument to measure temperature objectively. This instrument is called thermometer.

There are many kinds of thermometer and their operation depends on some property of matter those changes with temperature. Most common thermometer is constructed on the expansion of a material with increase in temperature. The first thermometer was constructed using the idea of Galileo, using the principal of expansion of a gas. Nowadays, common thermometer consists of a hollow glass tube filled with mercury or alcohol. Some accurate thermometer is made using electrical properties of materials such as resistance thermometers, thermocouples and thermostats.

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HEAT AND TEMPERATURE

Heat is a form of energy. Its nature was explained about 150-200 year ago by Rum ford and joule. It is well known that a body becomes hot when heat is added to it. A hot body contains more thermal energy than a cold body. When we rub our hands sometimes, they become warm. In the process of rubbing, one has to do mechanical work and this mechanical energy has produced heat. It means mechanical energy can be converted to heat. While reading on a bicycle, if you apply breaks, the bicycle slows down and the break shoes become hot. A bullet moving with high velocity is brought to rest when it strikes a target; the target becomes warm. In these examples, heat content in bodies increases when they loose mechanical energy. This heat content may also increase due to conversion of chemical, electromagnetic or other forms of energy. All objects require energy to heat up. This energy goes in increasing the internal energy of the body to have a clear idea about the relationship of internal energy with heat, let us consider the composition of matter.  According to the modern science, matter is made of “atoms” and “molecules”, which vibrate about their mean positions in solids and liquids and thus, possess kinetic energy. In gases, molecules move freely until they collide with the wall of the vessels or among themselves. Therefore, they also possess kinetic energy. Molecules in a hot body vibrate more vigorously than molecules in cooler substance. A hot body, when put in contact with a colder body, will loose energy and the transfer of energy takes place from the hot body to cold one. As a result, the hot body becomes cooler due to decrease in kinetic energy of its molecular motion. At the same time molecules in the colder body gain kinetic energy and warm up.

                  HEAT AND TEMPERATURE

 Heat is a form of energy arising from the molecular motion in matter. This energy gives us sensation of hotness or coldness. For example, we fill hot in front of fire. We fill cold when we touch ice. Temperature is a measure of degree of hotness or coldness of a substance. It is also a measure of the average kinetic energy of molecules in a substance as we will see in kinetic theory of gases (chapter-20). Heat and temperature are different physical quantities. Transfer of heat energy always takes place from a hot body to the cold one. In other words heat flows from a body at higher temperature to a body at lower temperature. Thus, temperature gives the direction of heat flow between two bodies when put in thermal contact.

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DETERMINATION OF G

For the determination of the value of universal gravitation constant G, it is necessary to measure the force of attraction between two bodies each of mass 1 kg separated by 1m is only 6.6×10ˉˡˡ N(extremely small to measure) the experiment must have a very high precision. The first accurate measurement of G was made by a English physicist lord Cavendish in 1798 using a very sensitive torsion balance. Since the significant improvements have been made in determining G particularly by J. pointing and c. v. boys in the 19’Th century. Here, we describe Boys experiment.

BOYS EXPERIMENT

The experimental arrangements in shown in figure 7.3 It consists of two identical gold balls A and B each of the mass m and diameter 0.5 cm suspended from the ends of the mirror RS by fine quartz Fibbers. The plane mirror RS itself is suspended from torsion head H by a fine Quartz fiber W. To avoid the suspended system form external disturbance, it is kept centrally inside a vertical glass tube T.

In order coaxial tube, two identical lead balls c and d each of mass M and of diameter 1.125 cm are suspended in level with A and B respectively in a way that the distance between the centers of pairs A and C and B and D are equal.

Two rubber pads p, p are placed just below the leads balls C and D to prevent damage to the apparatus in case they fall. The apparatus is mounted on a platform such that component of the set up are capable of rotating about their common axis. The reflection of RS is measured by telescope and by scale methods.

PROCEDURE: First of all, the lid of the outer cylindrical tube is rotated so those lead balls lie on the opposite sides of the gold ball but not in line with RS. The gravitational force of attraction between their pairs, being equal and opposite, form a couple causing RS to rotate. Due to this rotation of mirror strips RS the suspension fiber W is twisted and an elastic restoring couple develops in it. As soon as the restoring couple balances defecting couple, the stripe RS will attain equilibrium position. The position of C and D are so adjusted that the deflection of the stripe RS is maximum. The angle of deflection is measured by lamp and scale arrangement.

Next the outer tube is rotated so that the lead spheres lie on the other sides of the gold spheres in exactly similar position to produce maximum deflection. The mean of two deflections is taken.

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