Chapter 14 Lecture

Chapter 14: Temperature and Heat

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Goals for Chapter 14

•  To study temperature and temperature scales. •  To describe thermal expansion and its

applications. •  To explore and solve problems involving heat,

phase changes and calorimetry. •  To study heat transfer. •  To describe solar energy and see how

technology can lead to resource conservation.

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Temperature – Figure 14.1

•  Temperature is an attempt to measure the "hotness" or "coldness" on a scale you devise.

•  A device to do this is called a thermometer and is usually calibrated by the melting and freezing points of a substance. This is most often water with corrections for atmospheric pressure well known.

•  The thermometer is often a container filled with a substance that will expand or contract as heat flows in its surroundings.

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Thermal Equilibrium

•  If two objects are placed in contact and one has more heat energy than the other, heat will always flow from hotter to colder. This will continue until both objects are at the same temperature. This condition of stability is called thermal equilibrium.

•  When heat flow is considered, some materials, like metals, are good transmitters of heat energy. We term these materials to be thermal conductors.

•  Materials like Styrofoam are poor conductors of heat and will, in fact, severely restrict heat flow (like the one described above). Materials that conduct heat energy poorly are called insulators.

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The Zeroth Law of Thermodynamics – Figure 14.2 •  Systems A, B, and C are not

originally in thermal equilibrium.

•  Surround A, B, and C so that they are insulated from any external influence.

•  In the top figure, A and C will come to equilibrium while at the same time, B and C will also. Eventually, all three – A, B, and C – will come to equilibrium. In the lower figure, only A and B will come to equilibrium.

•  This is the essence of the zeroth law.

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Temperature Scales – Figure 14.3

•  Based on the boiling and freezing points of water, two systems developed to measure temperature.

•  In the United States, the Fahrenheit scale was developed with boiling at 212ºF and freezing at 32ºF.

•  In many other countries, the Celsius (also called Centigrade) scale was developed with water freezing at 0ºC and boiling at 100ºC.

•  Convert between them? Tf = 9/5Tc + 32º

•  Note that the interval (degree) is smaller in the Fahrenheit scale.

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An Absolute Temperature Scale – Figure 14.4 •  Scientists experimenting with gases noted a linear behavior

between pressure and temperature. Using various gases, the linear plots were all noted to converge at the same place

(−273.15ºC or 0K). → T(K) = TºC + 273.15

•  Named for its inventor, Lord Kelvin (1827−1907), the Kelvin scale took this point to be the absolute zero of all temperatures, the point at which everything is a solid and all motion ceases.

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Temperature Conversions – Figure 14.5

•  Be comfortable converting between the different temperature scales.

•  Refer to Example 1 on page 429 of your text.

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Thermal Expansion – Figure 14.6

•  The expansion is proportional to the original length.

•  The expansion is proportional to the temperature change (for reasonable ΔT).

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An Interesting Behavior – Figure 14.8

•  Different materials expand according to their coefficients of thermal expansion.

•  Refer to Table 14.1. •  Refer to Example

14.2 and Example 14.3.

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Expansion of Volume – Example 14.4

•  Refers to Figure 14.9. •  Use the coefficients of

thermal expansion in Table 14.2.

•  The graph at right is for the expansion of water from 0−10ºC.

•  This is the property that allows the mercury to rise inside a thermometer.

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Road Expansion and Contraction – Figure 14.10 •  Refer to Example

14.5 on page 434.

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The Mechanical Equivalent of Heat – Figure 14.12 •  Done by James Joule in the

1800s.

•  Potential energy stored in a raised mass was used to pull a cord wound on a rod mounted to a paddle in a water bath.

•  The measured temperature change of the water proved the equivalence of mechanical energy and heat.

•  The unit for potential energy, kinetic energy, and heat is the Joule in honor of his work.

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Calories, calories, and Joules – Figure 14.13

•  The food calorie is properly noted as a kilocalorie in SI units.

•  A calorie is 4.184 J. •  So, the Big Mac you're about to eat will cost your

diet about three and a half million joules.

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Heat Capacity – Examples 14.6 and 14.7

•  Substances have an ability to "hold heat" that goes to the atomic level.

•  One of the best reasons to spray water on a fire is that it suffocates combustion. Another reason is that water has a huge heat capacity. Stated differently, it has immense thermal inertia. In plain terms, it's good at cooling things off because it's good at holding heat.

•  Taking a copper frying pan off the stove with your bare hands is an awful idea because metals have almost no heat capacity. In plain terms, metals give heat away as fast as they can.

•  Refer to Examples 14.6 and 14.7 on page 437 of your text.

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Phase Changes – Figure 14.14 and the Snowflake •  Which is worse to touch for a burn: 100ºC water or 100ºC

steam? The steam, because it also contains the energy that it took to become a gas. In the case of water, this is 2.3 MILLION joules per kg of water.

•  The snowflake on the left needs to absorb the latent heat of fusion to become a liquid. The metal in the person's hand to the right just did that from the person's body temperature.

•  Put ice in water. You have a refreshing drink but also solid water and liquid water in equilibrium.

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Calorimetery – Examples 14.8 and 14.9

Refer to Problem Solving Strategy 14.2 on pages 441 and 442.

•  Surround a system with a known amount of fluid (therefore a known heat capacity). By measuring the change in temperature, you can solve for the heat evolved by the system. heat evolved by the system.

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Examples 14.10–14.14

shower. heated air from a furnace or heated water for a • shower. Radiation – moving heat by electromagnetic • radiation. Infrared rays from a hot burner on a stove can be felt by holding your hand over the burner. See Examples 14.13–14. stove can be felt by holding your hand over the burner. See Examples 14.13–14.

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Solar Energy and Resource Conservation

was to provide initial momentum for the one-time cost of adding such devices to existing homes.

•  The sun is ultimately the source of all energetic production on the earth but in the case of the tax credit, the plan is to harness sunlight which normally heats your roof and then escapes.

•  It will be interesting to see what happens. It will be interesting to see what happens.

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