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Phy 102: Fundamentals of Physics II

Phy 102: Fundamentals of Physics II. Chapter 15 Heat & Temperature Lecture Notes. Joseph Black (1728-1799). English chemist (re-)discovered carbon dioxide (“fixed air”) Founder of calorimetry technique Proposed a theory of heat that did not reference phlogiston, including:

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Phy 102: Fundamentals of Physics II

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

  2. Joseph Black (1728-1799) • 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."

  3. 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 (oF) • Metric: Celsius (oC) • SI: Kelvin (K)

  4. 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 (DT) • Specific heat capacity (s) (a physical property unique to the substance) • To calculate heat (Q): Q = s . m .DT • When produced by friction, heat is mechanical energy that is irretrievably removed from a system

  5. Specific Heat Capacity (c) • The amount of heat energy (in J) required to increase the temperature of 1 kg of a substance by 1oC (or 1K) • SI units are J/kgoC (more useful units are cal/goC or J/goC) • 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 (DT): Q = cmDT

  6. 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 (DT) the greater the degree of thermal expansion (DV/Vo) or DV/Vo ~ DT where DV/Vo is the fractional change in volume of the substance (DV is the change in volume & Vo 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

  7. 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 oC • As water is further cooled below 4 oC • 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 0oC • Note: the water at 4oC sinks to the bottom (due to high density) and water of colder or warmer temperature rise upward

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