AP Physics B Summer Course 2012

2012年 AP物理 B暑假班

M Sittig

Ch 18: Thermodynamics

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).

Example Problem

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.

Example Problem

Thermal Expansion

Heat makes molecules vibrate more, move apart.

Materials expand, holes in material expand.

Thermal Expansion

Example Problem

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.

Example Problem

Example Problem

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.

PV Diagrams

Heat Engines and Second Law of Thermodynamics

Entropy