HEAT AND TEMPERATURE
TOPICS Introduction Effects of heat Specific heat Basics of thermodynamics
Introduction Heat may be defined as energy in transit from a high temperature region to a lower temperature region. To understand Effects of heat such as change of temperature, expansions of solids, liquids, gases and change of state it is important to know the nature of heat energy.
CONT We know that matter consists of atoms and molecules. These atoms and molecules are in a state of motion. The motion can be translational, rotational and vibrational.
Cont The particles of matter posses kinetic energy due to such motions. The attraction and repulsion between the particles give rise to potential energy.
Cont The sum of the kinetic and potential energies is called internal energy or thermal energy. The internal energy which can be transferred from one body to another is called heat energy or heat.
Cont. The internal energy may be increased by transferring energy to the object from a higher temperature object this is properly called heating. Heat can be felt in terms of physical changes like hotness, coldness or change of state of matter.
Cont. Heat can cause a change in body s temperature by changing gthe internal energy of the body. Heat transfer is conduction, convection or radiation.
Cont..
Cont. Heat energy is produced at the expense of mechanical energy and vice versa. Ex: Rub your palms vigorously, palm becomes warmer. Heat is measurable quantity it is expressed in calorie or Joule. 1 calorie = 4.2 joule
Cont. 1 calorie = The quantity of heat required to raise the temperature of one gram of water by one degree centigrade.
Cont S I unit of heat is Joule Sources of heat The Sun, hot object Burning fire wood & other fuels
HEAT ENERGY FROM THE SUN
HEAT ENRGY FROM BURNING OF WOOD
TEMPERATURE EXPERIMENT: Take three pans, one containing hot water the second Luke warm and the third cold. Suppose you place your one hand in hot and the other in cold water for some time. Then place both your hands in lukewarm water you will observe that the hand which was previously in contact with hot water will now feel cold, where as the other feels hot. HOT WATER LUKE WARM WATER COLD WATER
Cont It thus shows that the sensation of degree of hotness is just relative to our body. So temperature is the property of a body that determines the sensation of hotness or coldness when we touch the body.
ACTIVITIES Activity 01 : Suppose the two pans, one containing a small and other a large amount of water are placed over identical gas burners and heated for length of time. The temperature of the small amount of water will have aerisen higher than that atof the large age amount. In this instance the increase in temperature are not equal.
ACTIVITIES Activity 02: On the other hand suppose the two pans are both initially at 30 o C temperature and that both are to be heated to 60 o C. It is evident that more heat to be supplied to the pan containing larger amount of water. The temperature change is same for both the quantities of heat supplied are very different.
Thermal equilibrium We all know that hot water or milk when left on a table begins to cool gradually. Ultimately, it attains the temperature of the surroundings.
Cont.. Similarly, a glass of ice cooled water when left on a table for some time will no longer remain cold and finally tends to attain the temperature of the surroundings. This shows that when two bodies at different temperatures come in contact with each other the hotter body becomes cooler and the colder becomes warmer.
Cont.. Finally a state is reached when no heat is flow from one body to other. The state when no heat is exchanged between the two bodies in contact with each other is known as the state of thermal equilibrium.
Cont.. At this stage two bodies are said to be at the same temperature.
MEASUREMENT OF TEMPERATURE Measurement of temperature is based on the effect on the change of physical property of the matter due to transference of heat. An instrument based on any of the below thermometric property is called a thermometer.
EXAMPLES Expansion of solids and liquids Increase of volume/ pressure at constant pressure/volume of a gas Change g in electrical resistance of a piece of metallic wire Setting g up of electromotive force at the junction of two dissimilar metals Change g in vapour pressure Change of chemical phase of a system etc.
Types of thermometers Liquid thermometers: It works on the principal of change in volume of liquid with change in temperature. Range of temperature measured is -30 o C to 357 o C Ex : Mercury in glass thermometer (used widely for measurement of ordinary temperature)
CONT.. MERCURY THERMOMETER CLINICAL THERMOMETER
Gas thermometers : It works on the principal of change in the pressure with change in temperature. Range of temperature measured is -200 o C to 1000 o C Ex: Constant volume gas thermometer ( used in bureau of standards and some research laboratories)
CONT Gas thermometer
Resistance thermometer It works on the principal of change of resistance with change of temperature. Range of temperature measured is -200 o C 1200 o C Ex : Platinum resistance thermometer
Cont..
Thermoelectric thermometer It works on the principal of change of thermo emf with change in temperature. Range of temperature measured is -200 o C 3000 o C Ex : Cu Fe thermo ocouple Sb bi thermo couple
CONT. Thermoelectric thermometer
Radiation pyrometers It works on the principle of amount of radiation falls.
Bimetallic strip thermometer It works on the principle of linear expansion of solid with temperature.
Temperature scales A number of temperature scales are in use. These scales have a lower fixed point (LFP) which is the temperature of pure ice at one atmosphere pressure. It is also called ice point.
Temperature scales Also these scales have upper fixed point (UFP) which is the temperature of steam at 1 atmospheric pressure. It is also called steam point.
Cont It measures the average Kinetic Energy of a substance. In other words, it measures the average motion of the molecules in a substance. The interval between the ice point and the steam point is divided into different number of divisions for different scales as shown in the table below.
cont.. celsius scale o C Fahrenheit h scale o F Kelvin scale K
Cont..
Different types of temperature scales Name of the scale Symbol for each degree Lower fixed point Upper fixed point No of divisions on the scale Reaumur o R 0 o R 80 o R 80 Celsius o C 0 o C 100 o C 100 Fahrenheit o F 32 o F 212 o F 180 Rankine Ra 460Ra 672Ra 212 Kelvin K 273 K 373K 100
Relation between the temperature on different scales
The zero of the Kelvin scale is called absolute zero. The Kelvin scale is often termed as the absolute scale. In common use, the absolute zero corresponds to 273 o C. However its exact value is -273.16 o C
Different temperature scales have the same reading at following temperatures Celsius and Fahrenheit at 40 o C = 40 o F Fh Fahrenheit hit and Kelvin li at 574.25 o F = 574.25 K Fahrenheit and Reaumur at -25.6 o F =-25.6 o R
Different temperature scales have the same reading at following temperatures Reaumur and Celsius at 0 o R = 0 o C. At no temperature the Celsius scale can have the same reading Kelvin li scale.
QUESTIONS 1. Write the differences between heat and temperature ANS : Si no Heat Temperature 1 It is a form of energy It is a measure of degree ofhotness of a body 2 Heat is responsible for temperature It is one of the effects of heat 3 It is the sum of energies of all It is the average kinetic molecules energy of the molecules 4 S I unit is joule S I unit is Kelvin
CONT 2. What temperature is the same on Celsius scale as well as on Fahrenheit scale. Ans : 40 3. Why a thick glass tumbler does break when a hot liquid is poured in it? Ans: The inside id of the glass expands faster because the heat comes to it faster than the outside so it cracks under pressure.
Cont 4. When can we say that our body is hot? Ans: If heat energy flows from any body to our body. 5. When can we say that our body is cold? Ans; If heat energy flows from our body to any body.
Cont 6. When the temperature increases what happens to molecular vibrations. Ans: Increases 7. What is the value of volume of a gas at zero Kelvin. Ans: zero
Cont 8. What is the value of pressure of a gas at zero Kelvin Ans : Zero 9 Write the relation between temperature in Kelvin scale and Celsius scale Ans: K = 273 + o C
CONT.. 10) What does the temperature scale on the thermometer measure. Ans: It measures the average Kinetic Energy of a substance. In other words, it measures the average motion of the molecules in a substance. 11) The Fahrenheit and Kelvin scale agree at a reading of Ans : 574
Cont.. 12) A body A is in thermal equilibrium with the body B and B is in thermal equilibrium with C Ans: - A and C are in thermal equilibrium 13. Normal body temperature on Fahrenheit scale is 98.6 o F what is the same temperature on Celsius and Kelvin scale? Ans :- T = 37 o C T = 310 o K
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