HEAT & TEMPERATURE By Dr. Karamjit Singh Senior Lecturer

1. HEAT & TEMPERATURE By Dr. Karamjit Singh Senior Lecturer Govt. Polytechnic College For Girls Patiala Email:karamjit_gpw@yahoo.com Mobile:- 9914029020; 9501029020

2. What is Heat The Universe is made up of matter and energy. Matter is made up of atoms and molecules and energy causes the atoms and molecules to always be in motion - either bumping into each other or vibrating back and forth.

3. The motion of atoms and molecules creates a form of energy called heat or thermal energy which is present in all matter. Even in the coldest voids of space, matter still has a very small but still measurable amount of heat energy.

4. Energy can take on many forms and can change from one form to another. Many different types of energy can be converted into heat energy. Light, electrical, mechanical, chemical, nuclear, sound and thermal energy itself can each cause a substance to heat up by increasing the speed of its molecules. So, put energy into a system and it heats up, take energy away and it cools. For example, when we are cold, we can jump up and down to get warmer.

5. Examples of various types of energy being converted into thermal energy (1) Mechanical energy is converted into thermal energy whenever you bounce a ball. Each time the ball hits the ground, some of the energy of the ball's motion is converted into heating up the ball, causing it to slow down at each bounce.

6. (2) Thermal energy can be transfered to other objects causing them to heat up. When you heat up a pan of water, the heat from the stove causes the molecules in the pan to vibrate faster causing the pan to heat up. The heat from the pan causes water molecules to move faster and heat up. So, when you heat something up, you are just making its molecules move faster.

7. 3) Electrical energy is converted into thermal energy when you use objects such as heating pads, electrical stove elements, toasters, hair dryers, or light bulbs. A thermal infrared image of a hair dryer and a flourescent light bulb

8. (4) Chemical energy from the foods we eat is converted into heating our bodies. (5) Light from the sun is converted to heat as the sun's rays warm the earth's surface. (6) Energy from friction creates heat. For example when you rub your hands, sharpen a pencil, make a skid mark with your bike, or use the brakes on your car, friction generates heat. A thermal infrared image of a pencil after being sharpened

9. A thermal infrared image of hot brakes in a car

10. The more energy that goes into a system, the more active its molecules are. The faster molecules move, the more heat or thermal energy they create. So, the amount of heat a substance has is determined by how fast its molecules are moving, which in turn depends on how much energy is put into it.

11. What is Temperature The atoms and molecules in a substance do not always travel at the same speed. This means that there is a range of energy (the energy of motion) among the molecules. In a gas, for example, the molecules are traveling in random directions at a variety of speeds - some are fast and some are slow.

12. Temperature is a measure of the average heat or thermal energy of the particles in a substance. Since it is an average measurement, it does not depend on the number of particles in an object. In that sense it does not depend on the size of it. For example, the temperature of a small cup of boiling water is the same as the temperature of a large pot of boiling water. Even if the large pot is much bigger than the cup and has millions and millions more water molecules.

13. Heat and Temperature We have all noticed that when you heat something up, its temperature rises. Often we think that heat and temperature are the same thing. However, this is not the case. Heat and temperature are related to each other, but are different concepts.

14. Perhaps the reason the two are usually and incorrectly thought to be the same is because as human beings on Earth our everyday experience leads us to notice that when you add heat to something, say like putting a pot of water on the stove, then the temperature of that something goes up. More heat, more temperature - they must be the same, right? Turns out, though, this is not true.

15. Temperature is a number. That number is related to energy, but it is not energy itself. • Temperature is a number that is related to the average kinetic energy of the molecules of a substance. • Read that last sentence carefully. It does not say that temperature is kinetic energy, nor does it state exactly what is the relation between temperature and kinetic energy. • Here is the relation: If temperature is measured in Kelvin degrees, then its value is directly proportional to the average kinetic energy of the molecules of a substance. Note that temperature is not energy, it is a number proportional to a type of energy.

16. Heat, on the other hand, is actual energy measured in Joules or other energy units. Heat is a measurement of some of the energy in a substance. When you add heat to a substance, you are adding energy to the substance. This added heat (energy) is usually expressed as an increase in the kinetic energies of the molecules of the substance. If the heat (energy) is used to change the state of the substance, say by melting it, then the added energy is used to break the bonds between the molecules rather than changing their kinetic energy.

17. When heat (energy) goes into a substance one of two things can happen: 1. The substance can experience a rise in temperature. The heat (the added energy) can be realized as an increase in the average kinetic energy of the molecules. The molecules now, on average, have more kinetic energy. This increase in average kinetic energy is registered as a number called temperature that changes proportionally with it. Note that this increase in the average kinetic energy of the molecules means that they will now, on average, be traveling faster than before the heat arrived.

18. 2. The substance can change state. For example, if the substance is ice, it can melt into water. Perhaps surprisingly, this change does not cause a rise in temperature. At the exact moment before melting, the average kinetic energy of the ice molecules is the same as the average kinetic energy of the water molecules at the exact moment after melting. That is, the melting ice and the just melted water are at the same temperature. Although heat (energy) is absorbed by this change of state, the absorbed energy is not used to change the average kinetic energy of the molecules, and thus proportionally change the temperature. The energy is used to change the bonding between the molecules.

19. So, when heat comes into a substance, energy comes into a substance. That energy can be used to increase the kinetic energy of the molecules, which means an increase in their temperature which means an increase in their speed. Or at certain temperatures the added heat could be used to break the bonds between the molecules causing a change in state that is not accompanied by a change in temperature.

20. Heat is the total energy of molecular motion in a substance while temperature is a measure of the average energy of molecular motion in a substance. Heat energy depends on • the speed of the particles, • the number of particles i.e. size or mass, and • the type of particles in an object.

21. Temperature does not depend on the size or type of object. For example, the temperature of a small cup of water might be the same as the temperature of a large tub of water, but the tub of water has more heat because it has more water and thus more total thermal energy.

22. It is heat that will increase or decrease the temperature. If we add heat, the temperature will become higher. If we remove heat the temperature will become lower. • Higher temperatures mean that the molecules are moving, vibrating and rotating with more energy. If we take two objects which have the same temperature and bring them into contact, there will be no overall transfer of energy between them because the average energies of the particles in each object are the same. But if the temperature of one object is higher than that of the other object, there will be a transfer of energy from the hotter to the colder object until both objects reach the same temperature. • Temperature is not energy, but a measure of it. Heat is energy

23. Molecular Model of an Ideal Gas Macroscopic properties of a gas were pressure, volume and temperature Can be related to microscopic description Matter is treated as a collection of molecules Newton’s Laws of Motion can be applied statistically The model shows that the pressure that a gas exerts on the walls of its container is a consequence of the collisions of the gas with the walls It is consistent with the macroscopic description developed earlier

24. Ideal Gas Assumptions The number of molecules in the gas is large, and the average separation between the molecules is large compared with their dimensions The molecules occupya negligible volume within the container. This is consistent with the macroscopic model where we assumed the molecules were point-like The molecules obey Newton’s laws of motion, but as a whole they move randomly Any molecule can move in any direction with any speed.

25. The molecules interact only by short-range forces during elastic collisions This is consistent with the macroscopic model, in which the molecules exert no long-range forces on each other The molecules make elastic collisions with the walls These collisions lead to the macroscopic pressure on the walls of the container The gas under consideration is a pure substance All molecules are identical

26. Ideal Gas An ideal gas is often pictured as consisting of single atoms However, the behavior of molecular gases approximate that of ideal gases quite well Molecular rotations and vibrationshave no effect, on average, on the motionsconsidered

27. Pressure and Kinetic Energy Assume a container is a cube Edges are lengthd Look at the motion of the molecule in terms of its velocity components Look at its Impulse -momentum theorem and the average force Assume perfectly elastic collisions with the walls of the container

28. Pressure and Kinetic Energy The relationship between the pressure and the molecular kinetic energy comes from momentum and Newton’s Laws Total force on the wall can be written as The relationship will be:

29. Pressure and Kinetic Energy, So, pressure is proportional to the number of molecules per unit volume(N/V)and to the average translational kinetic energy of the molecules This equation also relates the macroscopic quantity of pressure with a microscopic quantity of the average value of the square of the molecular speed One way to increase the pressure is to increase the number of molecules per unit volume The pressure can also be increased by increasing the speed (kinetic energy) of the molecules

30. Molecular Interpretation of Temperature Now PV = NkBT Also Therefore, the temperature is a direct measure of the average molecular kinetic energy

31. Molecular Interpretation of Temperature, contd. Simplifying the equation relating temperature and kinetic energy gives This can be applied to each direction, with similar expressions for vy and vz

32. Thermometers A thermometer is a device that is used to measure the temperature of a system Thermometers are based on the principle that some physical property of a system changes as the system’s temperature changes.

33. Thermometers contd. These properties include: The volume of a liquid The dimensions of a solid The pressure of a gas at a constant volume The volume of a gas at a constant pressure The electric resistance of a conductor The color of an object A temperature scale can be established on the basis of any of these physical properties

34. Calibrating a Thermometer-Celsius Scale A thermometer can be calibrated by placing it in contact with some natural systems that remain at constant temperature Common systems involve water A mixture of ice and water at atmospheric pressure called the ice point of water 0oC A mixture of water and steam in equilibrium called the steam point of water 100o C The length of the column between these two points is divided into 100 increments, called Celsius degrees

35. Calibrating an unmarked thermometer • Put thermometer into pure melting ice. • After few minutes, marks the position of mercury level; thus • 00 C is obtained. • Put thermometer into steam, hence 1000C is obtained. • Divides the length obtained into 100 divisions.

36. The Constant-Volume Gas Thermometer & The Absolute Temperature Scale The physical change exploited is the variation of pressure of a fixed volume gas as its temperature changes The volume of the gas is kept constantby raising or lowering the reservoir B to keep the mercury level at A constant The thermometer is calibrated by using an ice water bath and a steam water bath

37. Constant Volume Gas Thermometer contd. The pressures of the mercury under each situation are recorded The volume is kept constant by adjusting A. The information is plotted To find the temperature of a substance, the gas flask is placed in thermal contact with the substance The pressure is found on the graph The temperature is read from the graph

38. Absolute Zero The thermometer readings are virtually independent of the gas used If the lines for various gases are extended, the pressure is alwayszero when the temperature is –273.15o C This temperature is called Absolute Zero

39. Absolute Temperature Scale Absolute zero is used as the basis of the absolute temperature scale The size of the degree on theabsolute scaleis the same as the size of the degree on the Celsius scale To convert: TC = TA– 273.15 (19.1) Because ice and steam points are experimentally difficult to duplicate (depend on atmospheric pressure): An absolute temperature scale is now based on two new fixed points

40. One point is absolute zero The other point is the triple point of water Combination of temperature and pressure where ice, water, and steam can all coexist The triple point of wateroccurs at 0.01o C and 4.58 mmofmercury Thistemperature was set to be 273.16 on the absolute temperature scale This made the old absolute scaleagree closely with the new one The units of the absolute scale are KELVINS Absolute Temperature Scale contd.

41. Absolute Temperature Scale contd. The absolute scale is also called the Kelvin scale Named after William Thomson, Lord Kelvin The triple point temperature is 273.16K No degree symbol is used withKelvin The Kelvin is defined as 1/273.16th of the difference between absolute zero and the temperature of the triple point of water

42. Some Examples of Absolute Temperatures The figures to the right give some absolute temperatures at which various physical processes occur The scale is logarithmic The temperature of absolute zero cannot be achieved

43. Fahrenheit Scale Fahrenheit Scale a common scale in everyday use. Named for Daniel Fahrenheit Temperature of the ice point is 32oF Temperature of the steam point is 212oF There are 180 divisions (degrees) between the two reference points Celsius and Kelvin have the same size degrees, but different starting points TC = TK – 273.15

44. Temperature Scales

45. Comparison of Scales Celsius and Fahrenheit have different sized degrees and different starting points To compare changes in temperature Ice point temperatures 0o C = 273.15 K = 32o F Steam point temperatures 100oC = 373.15 K = 212o F

46. Thermometer, Liquid in Glass A common type of thermometer is a liquid-in-glass The material in the capillary tube expands as it is heated The liquid is usuallymercuryoralcohol

47. Mercury Thermometers • The mercury thermometer is a common type of • thermometer in everyday use. • Narrow bore of capillary tube makes the • thermometer more sensitive. • Range : -100C to 1100C (or 00C to 1000C). • Round or oval glass stem serve as magnifying • lens. • the bulb is made by thin glass.

48. Clinical Thermometer • Typical clinical thermometer is • liquid-in-glass thermometer. • Range : 350C to 420C. • It has a constriction for • preventing liquid fall back to • the bulb immediately after • taking the reading. • When taking reading, the bulb • is gently held under the • patient’s tongue.

49. Clinical Thermometers

50. Mercury Thermometer Most of the liquid-in-glass widely use mercury, because 1. more uniform expansion, 2. does not stick to glass. 3. visible meniscus, 4. react quickly to temperature changes, 5. boiling point: 3570C, freezing point = -390C. But its weak points are: • 1. expensive, • poisonous liquid • High freezing point