Thermal properties of matter class 11 notes
- 2. HEAT • A FORM OF ENERGY TRANSFERRED BETWEEN SYSTEMS DUE TO TEMPERATURE DIFFERENCES. • HEAT ALWAYS FLOWS FROM A HIGHER TEMPERATURE TO A LOWER TEMPERATURE.
- 3. TEMPRATURE • DETERMINES THE DIRECTION OF HEAT FLOW WHEN TWO BODIES ARE IN CONTACT. • IT IS THE THERMAL CONDITION OF A BODY AND ACTS AS AN INDICATOR FOR HEAT TRANSFER. Heat Temperature Heat is energy in transit. Temperature measures the degree of hotness or coldness of a body. Heat causes changes, while temperature reflects them. Temperature is the effect of heat. Temperature Scales: Celsius, Fahrenheit, and Kelvin •Celsius Scale (°C): • 0°C = freezing point of water • 100°C = boiling point of water • It is divided into 100 equal intervals between these points. •Fahrenheit Scale (°F): • 32°F = freezing point of water • 212°F = boiling point of water • The interval is divided into 180 equal parts. •Kelvin Scale (K): •SI unit of temperature •Absolute zero is set at -273.15°C
- 5. THERMAL EXPANSION • WHEN A SOLID’S TEMPERATURE INCREASES, ITS MOLECULES VIBRATE MORE, THIS, IN TURN, RESULTS IN AN INCREASE IN THE DIMENSIONS OF THE BODY. • THIS INCREASE IN THE DIMENSION IS KNOWN AS THE THERMAL EXPANSION. • TYPES OF THERMAL EXPANSION: • LINEAR EXPANSION: EXPANSION IN LENGTH. • AREA EXPANSION: INCREASE IN SURFACE AREA. • VOLUME EXPANSION: INCREASE IN THE VOLUME OF THE SOLID. Coefficient of Linear Expansion: It is the fractional change in length per degree change in temperature. It is represented by Coefficient of Area Expansion: It is the fractional change in area per degree change in temperature. It is represented by Coefficient of Volume Expansion: It is the fractional change in volume per degree change in temperature. It is represented by Relationship Between Coefficients: =2 and =3 β α γ α
- 6. THERMAL STRESS • WHEN A MATERIAL IS PREVENTED FROM EXPANDING OR CONTRACTING DUE TO TEMPERATURE CHANGES, THERMAL STRESS DEVELOPS. • FOR EXAMPLE, A ROD THAT IS RIGIDLY FIXED AT BOTH ENDS WILL DEVELOP COMPRESSIVE STRESS IF THE TEMPERATURE INCREASES. • THERMAL STRESS FORMULA: Anomalous Expansion of Water • Water behaves differently from most substances. When cooled below 4°C, instead of continuing to contract, it begins to expand. This unique behavior is whywater pipes burst in winter. Thermal Expansion of Gases Gases expand more than solids and liquids when heated. The coefficient of volume expansion gamma for gases is more γ γ dependent on temperature. For anideal gas: PV=RT Where: •P = Pressure •V = Volume •T = Temperature •R = Universal Gas Constant
- 7. HEAT CAPACITY • IS THE AMOUNT OF HEAT REQUIRED TO CHANGE THE TEMPERATURE OF A BODY. IT IS MEASURED IN JOULES PER KELVIN (J/K). • S=ΔQ/ΔT • SPECIFIC HEAT CAPACITY • THE HEAT CAPACITY PER UNIT MASS OF A SUBSTANCE. ITS SI UNIT IS J/KG K. • S=1/M(ΔQ/ΔT) • WHERE: • M = MASS • ΔQ = HEAT ENERGY • ΔT = TEMPERATURE CHANGE • MOLAR SPECIFIC HEAT CAPACITY • DEFINED AS THE HEAT CAPACITY PER MOLE OF A SUBSTANCE. THE SI UNIT IS J/MOL K. • AT CONSTANT PRESSURE AND VOLUME: • CP: MOLAR SPECIFIC HEAT CAPACITY AT CONSTANT PRESSURE. • CV: MOLAR SPECIFIC HEAT CAPACITY AT CONSTANT VOLUME. Water as a Cooling Agent •Water, with its high specific heat, is an efficient cooling agent. •This means that a small amount of water can absorb a large amount of heat with a relatively minor rise in temperature. •Because of this property, water is widely used in cooling systems for automobiles and engines. •If a liquid with lower specific heat were used, its temperature would rise significantly for the same heat absorption, making water the preferred choice.
- 8. CHANGE OF STATE • MATTER EXISTS IN THREE STATES: SOLID, LIQUID, AND GAS. WHEN TRANSITIONING FROM ONE STATE TO ANOTHER, THE TEMPERATURE REMAINS CONSTANT. COMMON STATE CHANGES INCLUDE: • SOLID TO LIQUID (MELTING) • LIQUID TO GAS (VAPORIZATION) • DURING THESE PROCESSES, THE SUBSTANCE ABSORBS OR RELEASES HEAT, BUT ITS TEMPERATURE DOESN’T CHANGE UNTIL THE ENTIRE MATERIAL HAS TRANSFORMED. • MELTING POINT • THEMELTING POINTIS THE TEMPERATURE AT WHICH A SOLID AND ITS LIQUID STATE ARE IN THERMAL EQUILIBRIUM. AT STANDARD ATMOSPHERIC PRESSURE, THIS IS KNOWN AS THE NORMAL MELTING POINT. • REGELATION • REGELATION REFERS TO THE MELTING OF ICE UNDER PRESSURE AND ITS RE- SOLIDIFICATION WHEN THE PRESSURE IS REMOVED. A COMMON DEMONSTRATION INVOLVES PLACING A WEIGHTED WIRE ON AN ICE BLOCK. THE PRESSURE FROM THE WIRE CAUSES THE ICE TO MELT, ALLOWING THE WIRE TO PASS THROUGH WITHOUT BREAKING THE ICE BLOCK. • VAPORIZATION AND BOILING POINT • VAPORIZATIONIS THE TRANSFORMATION OF A LIQUID TO A GAS. • THE BOILING POINTIS THE TEMPERATURE AT WHICH A LIQUID AND GAS COEXIST IN EQUILIBRIUM. AS PRESSURE INCREASES, THE BOILING POINT RISES, AND VICE VERSA. • SUBLIMATION • SUBLIMATION IS THE DIRECT CONVERSION OF A SOLID INTO A GAS WITHOUT PASSING THROUGH THE LIQUID PHASE. EXAMPLES INCLUDE CAMPHOR AND IODINE. • LATENT HEAT • LATENT HEAT IS THE AMOUNT OF HEAT REQUIRED FOR A SUBSTANCE TO CHANGE ITS STATE WITHOUT CHANGING ITS TEMPERATURE. IT IS DEFINED AS: • L=Q/M • WHERE Q IS THE HEAT ENERGY AND M IS THE MASS. THERE ARE TWO TYPES OF LATENT HEAT: • LATENT HEAT OF FUSION (LF)FOR SOLID-LIQUID TRANSITIONS. • LATENT HEAT OF VAPORIZATION (LV) FOR LIQUID-GAS TRANSITIONS.
- 9. MODE OF HEAT TRANSFER • THERE ARE THREE MODES OF HEAT TRANSFER: • CONDUCTION: TRANSFER OF HEAT THROUGH A MATERIAL BY DIRECT CONTACT. • CONVECTION: TRANSFER OF HEAT IN FLUIDS (LIQUIDS OR GASES) THROUGH FLUID MOTION. • RADIATION: TRANSFER OF HEAT THROUGH ELECTROMAGNETIC WAVES WITHOUT REQUIRING A MEDIUM • CONDUCTION • IN CONDUCTION, HEAT FLOWS THROUGH A MATERIAL DUE TO TEMPERATURE DIFFERENCES BETWEEN ADJACENT PARTS. A CLASSIC EXAMPLE IS HEATING ONE END OF A METAL ROD; THE OTHER END GRADUALLY BECOMES HOT DUE TO CONDUCTION. • THERMAL CONDUCTIVITY • THE RATE AT WHICH HEAT FLOWS THROUGH A MATERIAL IS DETERMINED BY ITS THERMAL CONDUCTIVITY, K. THIS IS GIVEN BY: WHERE: • H IS THE RATE OF HEAT FLOW • A IS THE CROSS-SECTIONAL AREA • L IS THE LENGTH OF THE CONDUCTOR • TCAND TDARE TEMPERATURES AT EITHER END • CONVECTION • CONVECTION IS THE TRANSFER OF HEAT WITHIN A FLUID (LIQUIDS AND GASES) FROM REGIONS OF HIGHER TEMPERATURE TO LOWER TEMPERATURE, AIDED BY THE MOVEMENT OF THE FLUID ITSELF. • SEA AND LAND BREEZES • SEA BREEZE (DAYTIME):THE LAND HEATS UP FASTER THAN THE SEA, AND THE WARMER AIR FROM LAND MOVES TOWARDS THE SEA, WHILE COOLER AIR FROM THE SEA MOVES INLAND TO TAKE ITS PLACE. • LAND BREEZE (NIGHTTIME):WATER RETAINS ITS HEAT LONGER THAN LAND, SO WARMER AIR FROM THE SEA MOVES TOWARDS LAND, WHILE COOLER AIR FROM LAND MOVES TOWARDS THE SEA. • TRADE WINDS • TRADE WINDS ARE STEADY SURFACE WINDS THAT BLOW FROM THE NORTHEAST TOWARDS THE EQUATOR. THIS HAPPENS BECAUSE THE EQUATORIAL REGION RECEIVES MORE SOLAR HEAT THAN THE POLAR REGIONS, SETTING UP A CONVECTION CURRENT. • DUE TO EARTH’S ROTATION, AIR DESCENDS AT ABOUT 30° N LATITUDE AND RETURNS TO THE EQUATOR. • FORCED CONVECTION • IN FORCED CONVECTION, THE MOVEMENT OF THE FLUID IS DRIVEN BY EXTERNAL FORCES SUCH AS PUMPS OR FANS. FOR EXAMPLE, THE AIR HEATING SYSTEMS USED IN HOMES WORK ON THE PRINCIPLE OF FORCED CONVECTION. • RADIATION • RADIATION IS THE TRANSFER OF HEAT THROUGH ELECTROMAGNETIC WAVES, REQUIRING NO MEDIUM. SOLAR ENERGY IS A PRIME EXAMPLE OF RADIATION. • HEAT ENERGY TRAVELS AT THE SPEED OF LIGHT (3 X 10^8 M/S). • WHEN THERMAL RADIATION FALLS ON AN OBJECT, PART OF IT IS REFLECTED WHILE THE REST IS ABSORBED. THE AMOUNT OF HEAT ABSORBED DEPENDS ON THE COLOR OF THE BODY. • BLACK SURFACES ABSORB AND EMIT RADIANT ENERGY BETTER THAN LIGHTER-COLORED ONES. THAT’S WHY WE PREFER TO WEAR LIGHT-COLORED CLOTHES IN SUMMER AND DARKER ONES IN WINTER.
- 10. NEWTON’S LAW OF COOLING • NEWTON’S LAW OF COOLING STATES THAT THE RATE OF HEAT LOSS FROM A BODY IS DIRECTLY PROPORTIONAL TO THE TEMPERATURE DIFFERENCE BETWEEN THE BODY AND ITS SURROUNDINGS, PROVIDED THE DIFFERENCE IS SMALL. THE RATE OF HEAT LOSS ALSO DEPENDS ON THE SURFACE AREA AND NATURE OF THE BODY. • THE MATHEMATICAL EXPRESSION IS: • WHERE T2IS THE BODY TEMPERATURE, TIS THE SURROUNDING TEMPERATURE, AND K IS A CONSTANT.