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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