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 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?

Conduction Heat 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.

Convection Heat is carried from one location to another by movement of a heated fluid, like air. Bulk transfer of molecules.

Radiation Heat 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.