AP Physics B Temperature & Heat
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."
Fahrenheit & Celsius Celsius & Fahrenheit The Common Temperature Scales
Calculating Linear Thermal Expansion • The extent to which something expands/contracts with temperature (DL) depends on:
Example The Eiffel Tower, constructed in 1889 by Alexandre Eiffel, is an impressive latticework structure made of iron. If the tower is 301 m high on a 22 ºC day, how much does its height decrease when the temperature cools to 0.0 ºC?
Area Expansion • Since the length of an object changes with temperature, it follows that its area changes as well…
Volume Thermal Expansion • Objects will expand in volume as well as in length • Volume thermal expansion is analogous to linear thermal expansion:
Example A copper flask with volume of 150 cm3 is filled to the brim with olive oil. If the temperature of the system is increased from 6.0 ºC to 31 ºC, how much oil spills from the flask?
Heat & Internal Energy • Heat is energy that flows from a _____________ object to a _________________object • When something absorbs heat its internal energy (or the energy of its atoms/molecules) increases • When something releases heat its internal energy decreases • The SI units of heat are ____________. • Two things occur when an object absorbs or releases heat energy (Q): • The temperature will change (which is why they expand/contract, due to changes in molecular motion) • The object (or part of it) will change phase (solid, liquid, gas)
The Mechanical Equivalent of Heat • Before it was established that heat was indeed energy, heat was traditionally measured in units of calories (cal): • 1 calorie = amount of heat required to raise the temperature of 1 g of water by 1 oC • James Prescott Joule established that heat energy is the equivalent of mechanical energy: 1 cal = 4.186 J • In nutrition we often invoke the kilocalorie (or nutritional calorie): 1 kcal = 1 Cal = 1000 cal = 4186 J • The “typical” person needs to eat ~2000 Cal daily (or 2x106 cal of food energy)
Conduction – the process whereby heat is transferred by direct contact through a material, any bulk motion of the material playing no role in the transfer.
Conduction (cont’d) • Amount of heat (Q) conducted through a metal bar depends on: • These proportionalities can be expressed together : Q/t can be defined as “rate of heat transfer”, H this is the form on the AP equation sheet
Example One of the windows in a house has the shape of a square 1.0 m on a side. The glass in the window is 0.50 cm thick. How much heat is lost through this window in one day if the temperature in the house is 21 °C and the temperature outside is 0.0 °C? (The thermal conductivity of glass is 0.84 W/(m·K).)
Example: Two 0.525 m rods, one lead and the other copper, are connected between metal plates help at 2.00 °C and 106 °C. The rods have a square cross section, 1.50 cm on a side. How much heat flows through the two rods in 1.00 s? Assume no heat is exchanged between the rods and the surroundings, except at the ends. (The thermal conductivity of lead is 34.3 W/(m·K) and the thermal conductivity for copper is 395 W/(m·K).)
Convection – the process in which heat is carried from place to place by the bulk movement of fluid
Radiation – process in which energy is transferred by means of electromagnetic waves