Thermal Physics 1
Thermal Energy The kinetic molecular theory is based on the assumption that matter is made up of tiny particles that are always in motion. In a hot object the particles are moving faster (and at higher energies) than the particles in a cooler object. This is easy to visualize in a liquid or gas where the particles are free to roam around, however, it can be difficult to imagine in a solid. We have to imagine the particles in a solid being held together by tiny massless springs. The particles vibrate back and forth extending and compressing the springs while their average position in the solid is unchanged. The sum of the kinetic (energy of motion) and potential (energy stored in the springs) energies of all this internal motion is what we call the thermal energy. 2
Thermal Energy is Transferred in Three Ways: 1. Conduction is the transfer of thermal energy that occurs when highly energetic particles collide with less energetic particles, giving them energy. For example, heating a frying pan on a hot burner. 3
Thermal Energy is Transferred in Three Ways: 2. Convection is the transfer of thermal energy caused by the movement of fluids due to their different densities at different temperatures. A lava lamp is a nice illustration of how a convection current works. 4
Thermal Energy is Transferred in Three Ways: 3. Radiation is the transfer of thermal energy in the form of electromagnetic waves. These waves do not need particles to transfer their energy and can travel through a vacuum. Solar energy from the sun is an example of radiation. Also, visible light, gamma rays, x rays, radio waves, etc.. 5
Thermal Energy is Transferred in Three Ways: 6
Temperature The physical quantity that is proportional the average kinetic energy (energy of motion) of the particles in matter is what we refer to as temperature. Temperature must be measured on some definite scale. Note that absolute zero is the temperature at which scientists believe that all atomic motion stops, and therefore the thermal energy of a substance at absolute zero would be zero. 7
Temperature Scales Founder Water Boils Body Temperat ure Water Freezes Absolute Zero 8
Working With Celsius and Kelvin Temperatures A useful relationship between Celsius and Kelvin temperatures is: T K = T C + 273.15 Examples: 9
Homework: Merrill Physics Page 247 Page 1 to 4 10
Heat and Thermal Energy 11
Heat Transfer One way to increase the thermal energy (and temperature) of an object is to place it in contact with a hotter object. Thermal energy or heat (symbol: Q) flows from hot to cold and the thermal energy of the hotter object is decreased and that of the cooler object is increased. Heat transfer will depend on three things: The temperature difference between the materials. The masses of the materials involved. The thermal properties of the materials. 12
Specific Heat Capacity The specific heat capacity, c, is defined as the amount of heat energy that must be added to a material to raise the temperature of 1 kg of that material by 1 K (or by 1 o C). This is more generally written as: where Q = mc T Q = heat energy transferred (in Joules (J)) m = mass of material (in kilograms (kg)) c = specific heat capacity (in J / (kg o C) or J / (kg K)) T = T f T i = temperature change (in o C or K) T f = final temperature T i = initial temperature 13
Example 1: A 400. g block of iron is heated from 295K to 325K. How much heat is absorbed by the iron? 14
Example 2: 10. kg of water at 90.0 o C loses 232 kj of heat. What final temperature will the water have? 15
Merrill Physics Page 248 Questions 5 to 8 Homework: 16
535,000 J of heat energy are removed from water at 80.0 o C, lowering its temperature to 20.0 o C. What is the mass of the water? 17
Calorimetry and Heat Transfer One cool judgment is worth a thousand hasty counsels. The thing to do is to supply light and not heat. Woodrow T. Wilson When you can't make them see the light, make them feel the heat. Ronald Reagan 18
The Calorimeter A calorimeter is a device used to measure changes in thermal energy. The calorimeter depends on the conservation of energy in systems that are thermally isolated. That is, a calorimeter is a device designed so that no thermal energy can enter and none can leave. As a result, if energy from one part of the system increases then energy from another part of the system must decrease by the same amount. Many calorimeters are well insulated aluminum vessels with tight sealing screw tops, however, a covered Styrofoam cup or travel mug may be used instead. 19
A Laboratory Calorimeter 20
A Coffee Cup Calorimeter 21
Energy in an Isolated System 22
Example Problem What mass of milk, c milk = 3930 J/(kg. K), at 8.0 0 C must be added to 450. g of coffee initially at 95.0 o C in order to cool the resulting mixture to a temperature of 80.0 o C? (assume coffee has the same specific heat capacity as water) 23
Example Problem A 0.500 kg sample of water is at 15.0 o C in a calorimeter. A 0.0400 kg block of zinc at 115 o C is placed in the water. Find the final temperature of the system. 24
Homework: Merril Physics, Page 252, Questions 9 to 12 25
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Copy and complete. Treat this like a quiz. 82500 J of heat energy are removed from 500. g of water at 350.0 K. What is the final temperature of the water? (answer in Kelvin and Celsius) 28
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Practice Quiz: What mass of milk, c milk = 3930 J/(kg. K), at 5.0 0 C must be added to 500. g of coffee initially at 99.0 o C in order to cool the resulting mixture to a temperature of 87.0 o C? (assume coffee has the same specific heat capacity as water) 3.00 kg of hot water at 80.0 o C is added to a bathtub filled with 50.0 kg of warm water at 30.0 o C. Assuming that no heat is lost to the surroundings calculate the final temperature of the bath water. 30
What mass of milk, c milk = 3930 J/(kg. K), at 5.0 0 C must be added to 500. g of coffee initially at 99.0 o C in order to cool the resulting mixture to a temperature of 87.0 o C? (assume coffee has the same specific heat capacity as water) 31
3.00 kg of hot water at 80.0 o C is added to a bathtub filled with 50.0 kg of warm water at 30.0 o C. Assuming that no heat is lost to the surroundings calculate the final temperature of the bath water. 32
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The three most common states of matter are solid, liquid, and gas. With enough thermal energy the particles in a solid are no longer held together in their fixed locations and eventually become free enough to move past one another. At this point, called the melting point, the substance has changed from a solid to a liquid. When a substance is melting, the added thermal energy is used to break the bonds holding the particles together. It does not increase the temperature. 37
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T ( o C) 100 0 Q=mc t Q = ml Q (J) c ice = 2060 J/kg o C c water = 4180 J/kg o C c steam = 2020 J/kg o C L f = 3.34 x 10 5 J/kg L v = 2.26 x 10 6 J/kg 40
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