1 Physics 170 - Mechanics Lecture 35 Heat
2 Definition and Units of Heat Heat is a form of energy, and therefore is measured in joules. There are other units of heat, the most common one is the kilocalorie: One kilocalorie (kcal) is defined as the amount of heat needed to raise the temperature of 1 kg of water by 1 C o (from 14.5 C o to 15.5 C o ). One calorie is 0.001 kilocalorie.
3 Definition and Units of Heat Confusingly, the calories listed on nutrition labels in the U.S. are really kilocalories (sometimes called Calories). Some other labels are more accurate (left, Australia; right, Germany).
4 Definition and Units of Heat This figure illustrates the three most common units of heat.
Definition and Units of Heat So, if heat is energy, how do kilocalories convert to joules? Careful experimentation using the apparatus gives the answer, as the work done by the falling weights raises the temperature of the water. This relationship is called the mechanical equivalent of heat. 5
6 Specific Heat and Calorimetry The amount of heat needed to increase the temperature of a solid or liquid depends on the amount of the substance and the properties of the substance itself. The constant c is called the specific heat of the substance.
Specific Heat and Calorimetry 7
8 Specific Heat and Calorimetry Calorimetry is the quantitative measurement of heat exchange; it is done using a calorimeter. A calorimeter is insulated from the environment, minimizing heat exchange. Therefore, heat lost by one object in the calorimeter must be gained by another. This is one way of measuring specific heats.
9 Specific Heat and Calorimetry Specific heat can be defined for gases as well, but gases do not have constant volume or pressure. We therefore define two specific heats for gases one at constant volume (cv ), and one at constant pressure (cp ). For a particular gas, cp is always greater than cv.
10 Phase Changes and Latent Heat Three phases of matter: solid, liquid, and gas. Solid has a definite shape and the strongest intermolecular bonds. Liquid flows but is relatively incompressible, so it has a definite volume. Gas is compressible, and will expand to fill a container.
11 Phase Changes and Latent Heat Phase changes: Solid liquid: melting Liquid gas: evaporating, boiling Gas liquid: condensing Liquid solid: freezing Solid gas: sublimating Latent heat is the amount of heat absorbed or released when a substance undergoes a phase transition.
12 Phase Changes and Latent Heat During a phase transition, the heat energy goes to changing the intermolecular bonds, and the temperature does not change. The heat needed for a phase change is: Here, L is the latent heat; Lf is the latent heat of fusion (solid liquid) and Lv the latent heat of vaporization.
13 Phase Changes and Latent Heat Latent heat is a property of a particular substance.
14 Phase Changes and Latent Heat Heat added as ice becomes steam:
15 Heat Transfer Heat transfer takes place via three mechanisms: 1. Conduction 2. Convection 3. Radiation
16 Heat Transfer Conduction is the transfer of heat through a substance. If the cup that holds your hot coffee, is a good conductor of heat, you will need a sleeve to keep from burning your hand. Typically, metals are good conductors of heat they have electrons that are free to move throughout the material and nonmetals are not. Nonconductors of heat are also called insulators.
17 Heat Transfer The heat flow rate through a slab of material is proportional to its surface area and to the temperature difference, and inversely proportional to its thickness. The constant k is called the thermal conductivity.
18 Heat Transfer This diagram illustrates the geometry of heat transfer by conduction
19 Heat Transfer The thermal conductivities of substances vary widely.
20 Heat Transfer When insulating a house, we want materials whose thermal conductivity is as low as possible.
21 Heat Transfer Heat transfer in fluids is mostly by convection, which is the result of mass transfer; that is, heat is transferred as warmer fluid moves to replace cooler fluid. Convection may be spontaneous (as below) or forced.
22 Heat Transfer Many homes are heated using forced hot air; this is an example of forced convection.
23 Heat Transfer In the atmosphere, convection includes large- and small-scale rising and sinking of air masses and smaller air parcels. These vertical motions distribute heat and moisture throughout the atmospheric column and contribute to cloud and storm development (where rising motion occurs) and dissipation (where sinking motion occurs)
24 Heat Transfer Radiation is the only type of heat transfer that can take place through a vacuum. You can feel the radiation of heat when you stand near a fireplace. This radiation is in the form of electromagnetic waves, in the infrared part of the spectrum.
25 Heat Transfer The rate of energy radiation is given by Stefan s law: A is the object s surface area, T is its temperature, and e is a number between 0 and 1 called the emissivity. σ is the Stefan Boltzmann constant: A good emitter of radiation is also a good absorber.
26 Heat Transfer Here we see the three types of heat transfer:
27 Heat Transfer Passive solar heating uses the changing angle of the Sun to warm buildings in the winter but not in the summer.
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