Lect 7.ppt

who steps back and accidentally stomps on your foot with

the heel of one shoe. Would you be better off if that person were (a) a large professional basketball player wearing sneakers (b) a petite woman wearing spike-heeled shoes?

Quick Quiz 4.2 A grandfather clock depends on the period of a pendulum to keep correct time. Suppose a grandfather clock is calibrated correctly and then a mischievous child slides the bob of the pendulum downward on the oscillating rod. Does the grandfather clock run (a) slow (b) fast (c) correctly?

Law of Thermodynamics.

which involves situations in which the temperature or state (solid,

liquid, gas) of a system changes due to energy transfers. As we shall see, thermodynamics is very successful in explaining the bulk properties of matter and the correlation between these properties and the mechanics of atoms and molecules.

discussion, we assume that two objects are in thermal contact

with each other if energy can be exchanged between them by these processes due to a temperature difference.

Thermal equilibrium is a situation in which two objects would not exchange energy by heat or electromagnetic radiation if they were placed in thermal contact.

of thermodynamics. (a) and (b) If the temperatures of A

and B are measured to be the same by placing them in thermal contact with a thermometer (object C), no energy will be exchanged between them when they are placed in thermal contact with each other (c).

the zeroth law of thermodynamics (the law of equilibrium):
Temperature and

the Zeroth
Law of Thermodynamics

If objects A and B are separately in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other.

the level of the mercury in the thermometer rises as

the mercury is heated by water in the test tube.

thermometer measures the pressure of the gas contained in the

flask immersed in the bath. The volume of gas in the flask is kept constant by raising or lowering reservoir B to keep the mercury level in column A constant.

of pressure versus temperature taken with a constant-volume gas thermometer.

The two dots represent known reference temperatures (the ice and steam points of water).

for experimental trials in which gases have different pressures in

a constant-volume gas thermometer. Note that, for all three trials, the pressure extrapolates to zero at the temperature -273.15°C.

of the change in the average separation between the atoms

in an object.

cavity in a piece of material expands in the same

way as if the cavity were filled with the material.

of volume expansion.
(9.3)

strip bends as the temperature changes because the two metals

have different expansion coefficients. (b) A bimetallic strip used in a thermostat to break or make electrical contact.

density of water at atmospheric pressure with temperature. The inset

at the right shows that the maximum density of water occurs at 4°C.

the amount of gas in a given volume in terms

of the number of moles n. One mole of any substance is that amount of the substance that contains Avogadro’s number NA = 6.022 ) 1023 of constituent particles (atoms or molecules).

(9.4)

ideal gas

champagne is shaken and opened. Liquid spews out of the

opening. A common misconception is that the pressure inside the bottle is increased due to the shaking.

a system that is associated with its microscopic components—atoms and

molecules—when viewed from a reference frame at rest with respect to the center of mass of the system.

Heat is defined as the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings.

calorie (cal), which is defined as the amount of energy

transfer necessary to raise the temperature of 1 g of water from 14.5°C to 15.5°C. (Note that the “Calorie,” written with a capital “C” and used in describing the energy content of foods, is actually a kilocalorie.) The unit of energy in the U.S. customary system is the British thermal unit (Btu), which is defined as the amount of energy transfer required to raise the temperature of 1 lb of water from 63°F to 64°F.
Scientists are increasingly using the SI unit of energy, the joule, when describing thermal processes.

the mechanical equivalent of heat. The falling blocks rotate the

paddles, causing the temperature of the water to increase.

sample of a substance is defined as the amount of

energy needed to raise the temperature of that sample by 1°C.

The specific heat c of a substance is the heat capacity per unit mass.

(9.8)

(9.9)

energy Q transferred between a sample of mass m of

a material and its surroundings to a temperature change ∆T as

(9.10)

heat as our energy transfer mechanism, we find that the

energy required to change the phase of a given mass m of a pure substance is

(9.12)

Latent heat of fusion Lf is the term used when the phase change is from solid to liquid (to fuse means “to combine by melting”), and latent heat of vaporization Lv is the term used when the phase change is from liquid to gas (the liquid “vaporizes”).

on a gas contained in a cylinder at a pressure

P as the piston is pushed downward so that the gas is compressed.

(9.13)

(9.14)

compressed quasi-statically (slowly) from state i to state f. The

work done on the gas equals the negative of the area under the PV curve.

The work done on a gas in a quasi-static process that takes the gas from an initial state to a final state is the negative of the area under the curve on a PV diagram, evaluated between the initial and final states.

on a gas as it is taken from an initial

state to a final state depends on the path between these states.

work done, depends on the initial, final, and intermediate states

of the system.

Figure 9.9 (a) A gas at temperature Ti expands slowly while absorbing energy from a reservoir in order to maintain a constant temperature. (b) A gas expands rapidly into an evacuated region after a membrane is broken.

of the system,
independent of the path.
the internal energy
the change

in internal energy

The first law of thermodynamics

(9.15)

one that does not interact with its surroundings.
The internal energy

Eint of an isolated system remains constant.

system (one not isolated from its surroundings) that is taken

through a cyclic process—that is, a process that starts and ends at the same state.

In a cyclic process, the net work done on the system per cycle equals the area enclosed by the path representing the process on a PV diagram.

temperatures, are placed in thermal contact. Energy travels (a) from

the larger object to the smaller object (b) from the object with more mass to the one with less (c) from the object at higher temperature to the object at lower temperature.

pair represents two materials, one of which is twice as

hot as the other? (a) boiling water at 100°C, a glass of water at 50°C (b) boiling water at 100°C, frozen methane at -50°C (c) an ice cube at -20°C, flames from a circus fire-eater at 233°C (d) No pair represents materials one of which is twice as hot as the other