Chapter 4 Mallard quiz now available (due by 6 pm next Thursday 02/27)

Finish reading Chapter 4 by next Tuesday

Thursday, February 21

Spring 2008

Heat and the Second Law of Thermodynamics

Chapter 4

Great Idea:Heat is a form of energy that flows from

warmer to cooler objects

Nature’s Direction

• First Law of Thermodynamics(no comment on direction)

• Tendency for disorder• Evolution to a more probable state

Heat and Temperature

• Heat is energy that flows from one object to another when there is a difference in temperature between the objects.

• Temperature is the quantity that indicates whether or not, and in which direction, heat will flow.

Heat (Q) flows from an object at a higher temperature to an

object at a lower temperature.

The quantity of heat transferred depends on the difference in

the objects’ temperatures, their masses, and their specific heat

capacities (c).

Specific Heat Capacity

The specific heat capacity, c, of a material is the quantity of heat needed to raise the temperature of 1 gram of mass of the

material by 1 °C.

Q = mcΔT

Sample Exercise

How much heat must be added to 2 cups (~230 g) of water to raise the temperature from 25 °C to the boiling point?

Heat and Temperature

• Heat• Energy in motion• Warm to cool

• Temperature• Atomic movement

• Temperature Scales• Fahrenheit• Celsius• Kelvin

• Absolute zero

• Most materials expand as they get warmer – they become less dense.

• For a traditional clinical thermometer, liquid rises in a glass tube as the liquid gets warmer.

• The liquid stops rising when the temperature of the thermometer is the same as whatever it is in contact with... the properties of each no longer change... they are in thermal equilibrium.

• The zeroth law of thermodynamics: Two or more objects in thermal equilibrium have the same temperature.

Temperature and Thermometers

Temperature Scales

constant-volume gas thermometer

When extended backward to P = 0, the P vs. T lines for different gases all intersect the temperature axis at the

same point: T = −273 °C

K = °C + 273

Absolute zero (0 K) is the lowest possible temperature.

Temperature Conversions

• Conversion to Fahrenheit:°F = (1.8 × °C) + 32

• Conversion to Celsius: °C = (°F - 32) / 1.8

Sample Exercise

The average body temperature of a healthy dog is 38 °C.

What is this temperature in °F?

Sample Exercise

Today’s high temperature is forecasted to be 24 °F.

What is this temperature in °C?

What is this temperature in Kelvins?

ConductionHeat flows through a material when objects at different temperatures are in contact with

one another.

Movement of heat by atomic-scale collisions

The rate of heat flow depends on the temperatures and the thermal conductivity of each object.

Good thermal conductors are poor insulators.

ConvectionHeat is carried from one location to another

by movement of a heated fluid, like air.

Bulk transfer of molecules.

RadiationHeat energy transferred by electromagnetic

waves.

Transfer of energy by radiation can occur across a vacuum.

The primary form of radiation involved in heat transfer is infrared (IR) radiation.

images of IR radiation

Radiation and Color

• Dark-colored materials absorb radiation to a greater extent than light-colored materials.

• Good absorbers of radiation are also good emitters.