# AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班 - PowerPoint PPT Presentation

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Ch 18: Thermodynamics. AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班. M Sittig. Heat, Temperature and Power. Thermal energy: ( E therm ) Internal energy: The sum of all energies of all molecules in a substance (U).

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AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班

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Ch 18: Thermodynamics

## AP Physics B Summer Course 20122012年AP物理B暑假班

M Sittig

### Heat, Temperature and Power

• Thermal energy: (Etherm)

• Internal energy: The sum of all energies of all molecules in a substance (U).

• Temperature: Related to the average kinetic energy per molecule in a substance (T).

• Heat: Thermal energy (Q).

• Power: Work per time, rate of use of energy (P).

### Rate of Heat Transfer

• H=k·A·ΔT/L (= Q/t)

• Directly proportional to

• Thermal conductivity (k)

• Area

• Temperature Difference.

• Inversely proportional to

• Length

• Think of it like water flowing through a pipe.

### Thermal Expansion

• Heat makes molecules vibrate more, move apart.

• Materials expand, holes in material expand.

### Ideal Gas Law

• Just remember from Chemistry:

• PV = nRT

• Where n is moles.

• Also in the form:

• PV = NkBT

• Where N is molecules.

### Ideal Gas Law

• Remember that the ideal gas law summarizes the three smaller gas laws:

• Boyle’s Law: V ~ 1/P

• Charle’s Law: V ~ T

• Pressure Law: P ~ T

• But you don’t have to memorize these, just remember the Ideal Gas Law.

### Kinetic Theory of Gases

• Molecules move in continuous, ____ motion.

• There are a _____ number of molecules in any container of gas.

• The _____ between gas molecules is large.

• Molecules do not act on one another at a ____.

• All collisions between molecules, and of molecules on container walls, are _____.

### Kinetic Theory of Gases

• U = (3/2) nRT: most important thing is that internal energy (U) is directly proportional to (T), and only T.

• vrms = sqrt( 3kBT/m ): most important thing is that T is related to KE, so you need a square root and the mass of a molecule to find v.

### First Law of Thermodynamics

• Comes from the conservation of energy, so think about what flows in and out.

• ΔU = change in internal energy of a sample of gas.

• It can be positive (U increasing) if:

• Q = heat flows into the gas

• W = work is done on the gas

• If heat flows out or work is done by the gas, the value of Q or W is negative and U decreases.