PHYS2001_Ch. 12

PHYS2001_Ch. 12 - Ch. 12 Temperature and Heat 12.1...

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Ch. 12 Temperature and Heat 12.1 Temperature What is Temperature ? It is a measure of how “hot” something is. We use Thermometers to measure the temperature of something. Most rely on the fact that materials expand when heated. There are several different temperature scales. The two most commonly used are Fahrenheit and Celsius : Celsius Fahrenheit Boiling point of water 100 o C 212 o F Freezing point of water 0 o C 32 o F Between the freezing and boiling points, we have 180 o F and 100 o C. Thus, 100 o C = 180 o F. Relative to the freezing point, then: F C 1 o 5 9 o =
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9 5 o o 32) - F ( C = 32 C F o 5 9 o + = 12.2 Kelvin Temperature Scale The Kelvin temperature scale was introduced by a Scottish physicist, Lord Kelvin, in the late 1800’s. The Kelvin (K) is the SI unit of temperature. From the Ideal Gas Law at constant volume, nRT PV = P nRT V = If P increases, then T must also increase to keep V constant, and vice versa. P T Regardless of the gas, the temperature always extrapolates back to -273.15 o C at zero pressure. This defines the absolute zero point for a temperature measurement. -273.15 o C This is Absolute Zero and it = 0 K. C -273.15 K 0 o =
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Nothing can get colder than absolute zero (0 K). Converting between Kelvin and Celsius: 15 . 273 C o + = T T K Freezing Point 0 o C 273.15 K Boiling Point 100 o C 373.15 K Room Temperature 27 o C 300 K Celsius Kelvin You should read section 12.3 on Thermometers . 12.4 Thermal Expansion Most solids expand when heated: Heat a thin metal rod, and it expands. For small changes in temperature, o L L T L L o = α is the Coefficient of Linear Expansion.
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Units on α ? [1/ o C] So, T needs to be in o C is material dependent, i.e. it’s different for steel, aluminum, concrete, etc. Since the coefficient of linear expansion is different for different materials, this can lead to thermal stresses in mechanical systems. D
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When the temperature of a metal ring increases, the hole in the ring gets bigger. gets smaller. stays the same size. 33% 33% 33% 1. gets bigger. 2. gets smaller. 3. stays the same size. Clicker Question
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When the temperature of the piece of metal shown below is increased and the metal expands, what happens to the gap between the ends? It becomes narrower. It becomes wider. It remains unchanged. 33% 33% 33% Clicker Question 1. It becomes narrower. 2. It becomes wider. 3. It remains unchanged.
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Example : A steel section of the Alaskan Pipeline had a length of 65 m when it was installed at a temperature of 18 o C. What is its change in length when the temperature drops to a frigid -45 o C? ) ( o f o steel T T L - = α m -0.049 C) 18 - C 45 )( 65 )( 10 12 ( o o 6 = - × = - L T L L o = α The pipe contracts (shrinks) by 4.9 cm.
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PHYS2001_Ch. 12 - Ch. 12 Temperature and Heat 12.1...

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