Theroldynamics, Electricity, and Magneticism

# Theroldynamics, Electricity, and Magneticism - Exam 1...

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Exam 1 scores <20 1.5 20-25.9 2.0 26-28.9 2.5 29-34.9 3.0 35-37.9 3.5 38-40 4.0 Remember that 15% of the grade will be from LON-CAPA and 10% from iclicker.

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Some temperatures Again, no upper limit on possible temperatures, but lower limit is absolute zero
Joule-Kelvin effect Rapidly expanding gases get colder Used for producing the cold temperatures discussed in the video

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Specific heat capacity The specific heat capacity (sometimes just called specific heat) of a substance is the quantity of heat required to change the temperature of a unit mass by 1 degree C can think of it as thermal inertia since it signifies the resistance of a substance to a change in temperature Heat an oven up to 400 o F Would you stick your hand in the oven? Would you stick your hand on the metal rack inside the oven? Both are at the same temperature But the rack has a higher specific heat (more thermal energy stored) Take a piece of apple pie straight from the oven and eat it immediately The filling will burn your tongue while the crust will not the filling has a larger specific heat Water has a high specific heat it takes 8 times as much energy to raise the temperature of a gram of water by 1 o C than it does 1 gram of iron the specific heat of water is 8 times that of iron water is very useful for the cooling systems of cars because it absorbs a large amount of heat for a small increase in temperature
Specific heat If the specific heat capacity c is known for a substance, then the heat transferred is equal to the specific heat capacity X mass X change in temperature where Q is the quantity of heat Q = cm Δ T Suppose I mix 50 grams of 20 o C water with 50 grams of 40 o C water What is the final temperature of the water? The heat gained by the cooler water = the heat lost by the warmer water conservation of energy again Since the masses are the same, the final temperature is midway We’ll end up with 100 grams of 30 o C water

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Another example Suppose I mix 100 grams of 25 o water and 50 grams of 40 o C water Show that the final temperature is 31.4 o C How would I approach this problem? Start by noting again that the heat gained by the cool water is equal to the heat lost by the hot water Now Δ T 1 does not equal Δ T 2 because of the different masses of water Let T be the final temperature T will be between 25 o and 40 o Δ T 1 =T - 25 o Δ T 2 =40 o – T Then I can write cm 1 Δ T 1 = cm 2 Δ T 2 c (100 g )( T 25) = c (75 g )(40 T )
Our friend, water Water has a much higher capacity for storing energy than almost any other substance because of the different ways that water molecules can store energy in particular, in internal degrees of freedom such as rotation or vibration of the atoms inside a water molecule Water’s high heat capacity affects the world’s climate Northern Canada and northern Europe receive about the same

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Theroldynamics, Electricity, and Magneticism - Exam 1...

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