Thermal energy Thermal energy is the internal energy of a substance. I.e. Thermal energy is the kinetic energy of atoms and molecules. Heat is the transfer of thermal energy between substances. Until the late 1700s scientists believed that heat was an invisible substance that flowed called caloric.
Count Rumford (aka Benjamin Thompson), 1798, American physicist and statesmen that defected to England during the Revolutionary War, noticed that heat was produced when holes were bored in a cannon. He concluded that the friction from the drill produced heat, not a flow of caloric. Since work is done by drilling, and energy is the ability to do work, energy and heat must be related. Thompson was the first to conclude that heat was not caloric, but a form of kinetic energy. James Prescott Joule, 1840, British physicist, determined the relationship of heat and mechanical work (energy) objects in motion produce heat.
Heat Transfer What is cold? The absence of heat Heat transfer is the movement of thermal energy from a warmer object to a colder object. This results when warmer molecules collide with cooler molecules. Types of Heat Transfer: 1. Conduction Conduction occurs when heat is transferred from one substance to another by direct contact. Fast moving particles collide and speed up adjacent slower moving molecules. Conduction occurs between solids, liquids, gases, and plasma.
What elements conduct heat well? Why? Substances that do not conduct heat well are called insulators. E.g. wood, plastic, rubber anything made of nonmetals 2. Convection Convection is heat transfer in liquids, gases, and plasma by convection currents Warmer molecules expand (become less dense) and rise. E.g. Convection currents in the atmosphere are ascending masses of warm air.
3. Radiation Radiation is heat transfer through space. This is heat transfer by electromagnetic waves (infrared rays). 49% of our Sun s energy output is infrared radiation. Radiation is the most difficult to block and can ignite objects several meters from a flame.
Temperature and Heat Are heat and temperature the same? No. Heat is the transfer of thermal energy. Temperature is a measure of kinetic energy in a substance. I.e. An increase in temperature indicates the addition of thermal energy. Since temperature is a measure of the average kinetic energy of molecules and kinetic energy is energy of motion, then temperature is also a measure of internal motion of molecules. As molecules of a substance are heated, they speed up and move apart. Factors that affect temperature change: 1. Quantity of the mass (g) 2. Heat energy added or lost (cal or J) 3. Specific heat of the substance (cal/g x C o or J/g x C o )
Metric Temperature Scales Celsius (C o ) H 2 O freezes at 0 o C H 2 O boils at 100 o C Kelvin (K) H 2 O freezes at 273 o K H 2 O boils at 373 o K 0 on the Kelvin scale = absolute zero I.e. All molecular motion stops. Conversion of Celsius to Kelvin o C + 273 = o K Measuring Heat Heat is needed to set molecules in motion. Temperature is a measure of that motion. Changes in temperature can be used as an indirect measure of molecular motion. The metric unit of thermal energy is the calorie or joule. A calorie is the measure of the amount of heat required to raise the temperature of one gram of water one degree Celsius (from 14.5 o C to 15.5 o C).
1 calorie = 4.186 J Specific Heat is the ability of a substance to absorb heat energy. H 2 O forms the basis for measuring specific heat (c). I.e. specific heat of water = 1 cal/g. C o All other substances absorb heat at rates different from water. E.g. Ice = 0.5 cal/g. C o Air = 0.25 cal/g. C o Wood = 0.42 cal/g. C o Glass = 0.2 cal/g. C o
Calculating Heat Energy Heat energy lost or gained = mass x specific heat x change in temperature. or Q = mc T Where: Q = heat energy exchanged (cal) m = mass (g) c = specific heat (cal/g. C o ) T = change in temperature ( o C) How many calories are needed to raise 20 grams of water from 10 o C to 15 o C? 20 g x 1 cal/g. C o x 5 o C = 100 cal How much heat is lost by 10 g of copper if it is cooled from 35 o C to 21 o C? c = 0.09 cal/g. C o 10 g x 0.09 cal/g. C o x 14 o C = 12.6 cal
Calorimeter is a device used to measure the heat given off by a substance or a reaction of substances. A container where a reaction takes place is positioned inside another container of water. Heat lost by the substance will be gained in the surrounding water. Heat and Phase Changes Review of phase changes: Solid to Liquid= melting (heat absorbed) Liquid to Solid= freezing (heat released) Liquid to Gas= vaporization (heat absorbed) Gas to Liquid= condensation (heat released)
Heat of fusion is the amount of heat needed to change one gram of a substance from a solid to a liquid. Heat of fusion (L F ) of ice = 80 cal/g Heat of vaporization is the amount of heat needed to change one gram of a substance from a liquid to a gas. Heat of vaporization (L V ) of water = 540 cal/g. Remember, during a phase change, heat may be added or lost but no change in temperature occurs. Adding or removing heat during a phase change will only result in more substance changing phase, not a change in temperature.
THERMAL PROPERTIES OF SELECTED MATERIALS MATERIAL SPECIFIC HEAT 20ºC HEAT OF FUSION @mp HEAT OF VAP. @bp water 1.0 cal/g.o C 80 cal/g 540 cal/g aluminum 0.22 cal/g.o C 94 cal/g ammonia 1.1 cal/g.o C 108 cal/g 2510 cal/g 327 cal/g copper gold 0.094 cal/g.o C 0.032 cal/g.o C 49 cal/g 1130 cal/g 16 cal/g 377 cal/g iron 0.12 cal/g.o C 1510 cal/g 84.52cal/g lead mercury silver 0.032 cal/g.o C 0.033 cal/g.o C 0.056 cal/g.o C 5.5 cal/g 205 cal/g 2.8 cal/g 71 cal/g 26 cal/g 558 cal/g
Thermal Expansion Thermal expansion is best explained in terms of kinetic energy of molecules. I.e. It is the expansion of a substance due to heat. In solids, heat energy added causes: 1. Increase in kinetic energy, 2. More rapid vibration of molecules, 3. Molecules move farther apart. In liquids, heat energy added causes: 1. Increase in kinetic energy, 2. Molecules move faster, 3. Molecules expand enough to weaken molecular bonds. Exception water expands from 4 o C to 0 o C. Ice is less dense than liquid water. Bismuth, gallium, and germanium also have greater densities as a liquid.
In gasses, heat energy added causes: 1. Increase in kinetic energy, 2. Very rapid molecular motion, 3. Molecules expand indefinitely. Uses of Thermal Expansion Alcohol or mercury thermometers use the principle of thermal expansion to measure the temperature of a substance. The expansion of a bimetallic strip, coil, or wire can serve as a switch. E.g. Thermostat, Thermocouple, Heat Limit Switch
Internal Energy Internal energy is energy contained in a substance. Work is associated with force causing an object to move. Therefore, when a substance does work, it loses internal energy. When a substance has work done on it or takes in heat, it gains internal energy.