Heat

1. Heat AP Physics B Chapter 14 Notes

2. Heat as Energy Transfer • Heat is the transfer of energy—units calories • 1 cal is the amount of heat needed to raise 1 g of water 1 ˚C • James Joule demonstrated the mechanical equivalent of heat 4186 J = 1 kcal

3. Heat as Energy Transfer • Heat is energy transfer from one object to another due to ∆T • Recall T is a measure of the KE of molecules

4. Internal Energy • Some important definitions: Internal Energy: Total of all the energy of all the molecules (Thermal Energy) Temperature: Average KE of molecules Heat: Transfer of energy from one object to another

5. Internal Energy • Internal energy (U) is due to all the motion of molecules in a substance—for an ideal monatomic gas this translational KE • For N molecules it is the sum of average KE: U = N(1/2mv2) or U = 3/2 NkT or U = 3/2nRT Ideal gas U depends only on T and N

6. Internal Energy • For molecules with more than one atom, rotational and vibrational energy must also be included and for liquids and solids bond energy

7. Specific Heat • When heat flows into an object its temperature rises • How much depends on the type and amount of substance: Q = mc∆T where c is the specific heat of the substance (units J/kg˚C)

8. Specific Heat--Example P. 8 pg. 404 An automobile cooling system holds 16 L of water. How much heat does it absorb if its temperature rises from 20˚C to 90˚C? What would the temperature increase be if the cooling liquid were ethyl alcohol instead?

9. Calorimetry • Need to review definitions for systems (object or objects being studied) Closed: No mass in or out, but energy may flow Open: Mass and energy may enter/leave Isolated: No energy in any form may enter/leave • For an isolated system, energy may flow from one part to another, but overall it is conserved

10. Calorimetry • A calorimeter is an isolated system that allows measurement of energy flow (energy conservation) Q LOST = Q GAINED m C ΔTL = m C ΔTG Make ΔT positive for both sides when doing calcs.

11. Calorimetry—Example Prob. 14 pg. 404 A 215 g sample of a substance is heated to 330˚C and then plunged into a 105 g Al calorimeter cup containing 165 g of water and a 17 g glass thermometer at 12.5 ˚C. The final T is 35˚C. What is the specific heat of the substance?

12. Latent Heat • Latent heat refers to the energy required for a substance to undergo a phase change • Temperature is constant during phase change

13. Latent Heat Heat of Vaporization (Lf) is energy required to change liquid to gas (or vice versa) Heat of Fusion (Lf) is energy required to change solid to liquid (or vice versa) • Temperature is constant during phase change

14. Latent Heat-Example Ex. 14-8 pg. 393 At a physics class party a 0.5 kg chunk of ice at -10˚C is placed in 3 kg of “iced” tea at 20˚C. At what temperature and in what phase will the final mixture be? Consider the tea as water and ignore heat flow to the surroundings.

15. Heat Transfer-Conduction • Conduction occurs when heat increases KE of molecules in one location and they collide with adjacent molecules • Quantitatively: Where k is thermal conductivity (J/sm˚C

16. Heat Transfer-Convection • Convection occurs in fluids due to movement of mass • Hotter molecules move faster, so are further apart, so less dense • Forced versus natural

17. Heat Transfer-Radiation • Radiation occurs due to a ∆T between two bodies • Does not require medium • Function of T4 and emissivity of object (how well radiates—good absorber is good radiator, range of 0-1) • Thermal imaging