Heat and Temperature

1. Heat and Temperature

2. Temperature • Temperature measures the “speed” of the molecules • Celsius Scale • 00C water freezes, 1000C water boils • Fahrenheit Scale • 320F water freezes, 2120F water boils • Kelvin Scale0K • 2730K water freezes, 3730K water boils • ABSOLUTE Zero 00K

3. First Law of Thermodynamics The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamics

4. Enthalpy Enthalpy is a quotient or description of thermodynamic potential of a system, which can be used to calculate the heat transfer • H = U(internal energy) + pV (pressure Volume) • The new variables often make the analysis of a system much simpler. For a gas, a useful additional state variable is the enthalpy which is defined to be the sum of the internal energy U plus the product of the pressure p and volume V. Using the symbol H for the enthalpy: .

5. Entropy • Entropy is central to the second law of thermodynamics • that over time, ignoring the effects of self-gravity, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. • Entropy is a measure of how far along this evening-out process has progressed.

6. Heat • Internal Energy of a system • “Q” amount of heat in joules • Specific Heat (c) • Cal/grams (Chemistry/Earth Science) • Joule per gram per 0C (Physics) • ( J/Kg 0C ) Formula Q = m c D Tc

7. Sample Problem • How much heat is needed to raise the temperature of 30 kilograms of liquid water from 10 0C to 20 0C ?

8. Solution to heat problem • Q = m c D Tc (Physics) = 30 Kg x 4.19 J/kg 0C x (20-10)0C = 1,257J • Q = m c D Tc ( Earth Science – Chemistry) = 30,000 x 1.0 cal/g 0C x (20-10)0C = 30,000 gm x 1.0 cal/g 0C x (20-10)0C = 300,000 cal

9. Changes in state

10. Changes in State terms Sublimation Evaporation Condensation Melting Freezing

11. HVAC systems transfer energy using phase changes to absorb and radiate HEAT Heating and Changes in Phase

12. Changes in State • Heat of Fusion (Hf) • Conversion from Solid Liquid • Heat of Vaporization (Hv) • Conversion from Liquid Gas Formula Q = m Hf or m Hv Hf = 80 cal/g or 334 J/g Hv = 540 cal/g or 2260 J/g

13. Change in State Graph 1. Copy this graph2. Identify specific heat, heat of fusion, and heat of vaporization.

14. Change in State Areas

15. Calculate Heat of Fusion from a graph Time in minutes mass = 2.0 kg rate of loss = 2.0 kJ/minute Q = mHf 8,000 J = 2,000 g Hf 4 J/kg = Hf

16. Boiling and Freezing Points What is the boiling point? 140 degrees Celsius 60 degrees Celsius What is the freezing point?

17. Sample Heat of Fusion • How much heat energy is needed to melt 5.0 kilograms of water at its melting point? Q = m Hf (Physics) = 5 Kg x 334 J/Kg = 1,670J Q = m Hf (Earth Science) =5.0 kg x 80 cal/g = 5000 g x 80 cal/g = 400,000 cal

18. Summary • Temperature is a measure of the activity of molecules. • Heat is a measure of energy • Specific Heat determines the amount of energy that a substance stores • Heat of Fusion or Vaporization is a measure of the energy stored in the physical state of matter