Lec13 - Today Finish Gases and Plasmas(Chapter 14(including...

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Today: Finish Gases and Plasmas (Chapter 14) (including the demos) Start “Temperature, Heat, and Expansion” (Chap 15)
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Heat (Chapter 15) How hot something feels is a measure of the kinetic energy of the constituent atoms/molecules – these are continually randomly jiggling. We’ll study concepts and relationships between temperature, heat, energy, expansion. Temperature Tells us how warm or cold an object is with respect to some standard. Proportional to the average “translational” kinetic energy of molecules i.e. motion carrying molecule from one place to another, as opposed to rotational or vibrational motion – these latter don’t directly affect temperature. Eg. Microwave oven: microwaves cause water molecules in food to oscillate with considerable rotational KE. But to get the food to cook (ie temp to rise), these molecs bounce into neighboring molecules imparting their KE.
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Many different temperature scales : (1) Celsius , or centigrade : Assign “0” to the temp at which water freezes, and “100” to the temp at which water boils (at standard atmos pressure). Divide space in between evenly in 100 - each is called a “degree”, o C. (Celsius was a Swedish astronomer in 1700’s who came up with this) (2) Fahrenheit : used in the US. Assign “32” to temp at which water freezes, and “212” to temp at which water boils. So, 1 o F is smaller than 1 o C . Will become obsolete if the US ever converts to Celsius like the rest of the world. (3) Kelvin : calibrated in terms of energy, rather than water freezing/boiling points. Assign “0” to lowest possible temperature, where there is no kinetic energy – called absolute zero = -273 o C . The unit, 1 o K = 1 o C . All temps are positive on the Kelvin scale. Thermometers work by means of expansion or contraction of a liquid. Eg the common mercury-in-glass thermometer (around 1670’s).
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Themal Equilibrium The expansion of the liquid in a thermometer depends on the liquid’s temperature. So how come we say it’s reading is the temp of the object surrounding it ?? Because of “ thermal equilibrium ”: Energy flows between two objects in contact with each other until they reach the same temperature. The thermometer must be small enough that it doesn’t affect the temp of the object you want to measure. Eg Can measure your body’s temp with thermometer but can’t measure temp of drop of water with it, since contact between the thermometer and drop can change the drop’s temp.
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Heat Heat = energy transferred from one object to another due to a temperature difference between them Not a property of the material – ie don’t say an object “contains heat”, rather heat is energy in transit . ( c.f. idea of work) Rather, an object contains “ internal energy ” – sum of translation kinetic (giving rise to temp), rotational kinetic, vibrational, and potential (from intermolecular forces). Note, temperature is
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Lec13 - Today Finish Gases and Plasmas(Chapter 14(including...

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