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Temperature and Thermal Energy. Chapter 12.1. Kinetic Molecular Theory. 1. Everything is made of particles 2. These particles move 3. Hot things move faster than cold things. Definitions. Temperature – a measure of average kinetic energy of an object
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Temperature and Thermal Energy Chapter 12.1
Kinetic Molecular Theory 1. Everything is made of particles 2. These particles move 3. Hot things move faster than cold things
Definitions • Temperature – a measure of average kinetic energy of an object • Thermometer - device that measures temperature (and, as a result, average KE)
Heat Flow • Always from hot to cold objects
Heat flow stops only when all objects are in equilibrium (at the same temp)
Example 1 • If A is 75 degrees and B is 25 degrees, at which temperature are they both in equilibrium? • To solve, first add up the different temperatures • Next, divide the total temp by 2 (because we have 2 objects) • This is the equilibrium temperature • Answer = 50 degrees
Absolute zero • The temp at which molecular motion stops • Is this cold or hot??
Temperature conversion • Know how to do these… • 50°C = _______ K • 325 K = ________°C
Conduction • Energy transfer when objects are in direct contact. There must be matter in order for energy to transfer by conduction.
Convection • Energy transfer by air or water currents. • Objects are not in direct contact • Look for currents • Know that hot air and water rises!! • Matter is required for heat transfer by convection
Radiation • Energy transfer thru empty space • No matter is required!! • Its how the sun heats the Earth • Sunbathers are often used as examples of heating by radiation
Heat Transfer Formula Q = CmΔT Q = heat gained or lost (J) m = mass in kg C = specific heat (see p.279) ΔT = temp change (K or °C)
Solving for a final temp when 2 objects are mixed or in contact Tf = maCaTai + mbCbTbi maCa + mbCb
Example Problem 1 How much heat is needed to raise the temperature 1.5 kg of water from 22°C to 35°C? m = 1.5 kg ΔT = 15°C C = ?? What is changing temperature? Use the specific heat for whatever is changing temp (see page 279)
Q = 4180 x 1.5 x 15 Q = 94,050 J
Example Problem 2 A 5.0 kg sample of water at 25°C is mixed with 1.5 kg of water at 75°C. What is the final temperature of the mixture? Use the Tf formula…
Tf = 5 x 4180 x 25 + 1.5 x 4180 x 75 5 x 4180 + 1.5 x 4180 Tf = 459800 + 470250 20900 + 6270 Tf = 34°C
Work Page 296 #’s 23 – 24 and 27-28