Phy 102: Fundamentals of Physics II Chapter 15 Heat & Temperature Lecture Notes

Joseph Black ( ) English chemist (re-)discovered carbon dioxide (“fixed air”) Founder of calorimetry technique Proposed a theory of heat that did not reference phlogiston, including: –The heat equation –Specific heat capacity & Latent heat Widely renowned as a great teacher: "Many were induced, by the report of his students, to attend his courses, without having any particular relish for chemical knowledge."

Temperature The measure of how “hot” or “cold” something is Related to the average kinetic energy of the atoms/molecules of an object Units –English: Fahrenheit ( o F) –Metric: Celsius ( o C) –SI: Kelvin (K)

Heat Energy that flows from higher temperature to lower temperature Energy that is transferred due to “collisions” between atoms/molecules of different kinetic energy The heat energy that is absorbed by an object is proportional to: –The mass of the object (m) –The change in temperature the object undergoes (  T) –Specific heat capacity (s) (a physical property unique to the substance) To calculate heat (Q): Q = s. m.  T When produced by friction, heat is mechanical energy that is irretrievably removed from a system

Specific Heat Capacity (c) The amount of heat energy (in J) required to increase the temperature of 1 kg of a substance by 1 o C (or 1K) SI units are J/kg o C (more useful units are cal/g o C or J/g o C) A unique physical property of different substances –Metals have low specific heat capacity –Non-metals have higher specific heat capacity –Water has an unusually large specific heat capacity Relationship between heat gained (Q) and temperature change (  T): Q = cm  T

Thermal Expansion A temperature of a substance determines its size: –As substances cool they contract –As substances warm they expand This is called thermal expansion In general: –The greater the temperature change (  T) the greater the degree of thermal expansion (  V/V o ) or  V/V o ~  T where  V/V o is the fractional change in volume of the substance (  V is the change in volume & V o is original volume) –Liquids expand more readily than solids –Gases expand more readily than liquids –All substances expand/contract due to temperature changes in a unique, characteristic way

Thermal Properties of Water Water requires unusually large amounts of energy to change temperature (high specific capacity) As water (liquid) cools it contracts (as do most substances) as the molecules move closer together. When water gets near its freezing point: –The density of the water reaches its greatest value at 4 o C As water is further cooled below 4 o C –Small ice crystals begin to form (ice is an ionic solid) –Ice crystals have a lower density than liquid water (due to the bent shape of the molecules) –The net density of the water increases as the temperature approaches 0 o C Note: the water at 4oC sinks to the bottom (due to high density) and water of colder or warmer temperature rise upward