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# 18 Heat and the First Law of Thermodynamics Heat Capacity and Specific Heat Change of Phase and Latent Heat Joule’s Experiment and the First Law of Thermodynamics.

## Presentation on theme: "18 Heat and the First Law of Thermodynamics Heat Capacity and Specific Heat Change of Phase and Latent Heat Joule’s Experiment and the First Law of Thermodynamics."— Presentation transcript:

18 Heat and the First Law of Thermodynamics Heat Capacity and Specific Heat Change of Phase and Latent Heat Joule’s Experiment and the First Law of Thermodynamics The Internal Energy of an Ideal Gas Work and the PV Diagram for a Gas Hk: 31, 37, 47, 51.

Heat & Internal Energy heat is energy that flows due to temperature differences. SI Unit: joule internal energy is the total energy of an object in its center of mass reference frame

Specific Heat Capacity The amount of heat needed to raise 1kg by 1°C c = Q/m  T SI Unit: J/kg·°C m = mass receiving (or emitting) heat  T = temperature change Q = mc  T /

Example: 300J of heat are added to 100grams of a substance. The temperature rises by 2.20 degrees C. The specific heat of the substance is: Example: Calorimetry

Other Heat & Energy Units:

Example Conversion A Gas Range is quoted (incorrectly) as “12,000 Btu”. The equivalent power for an Electric Range in watts is:

Solid/Liquid Transformation Melting Heat in KE breaks bonds Molecules move Liquid forms Freezing Heat out Molecules lose KE Settle in one place Solid forms

8 Energy in Phase Change Energy added: solid  liquid  gas energy released: gas  liquid  solid

Liquid/Gas Transformations Evaporation Highest KE molecules escape liquid from surface Evaporative cooling, e.g. sweat evaporates cooling your body Boiling Highest KE molecules form gas bubbles throughout Hot gas bubbles rise and escape at surface Liquid has passed heat out, e.g. simmering at constant temperature

Latent Heat Energy needed to melt or vaporize 1kg of a substance. Latent Heat, L = Q/m. Q = mL. Example: 2.5kg of ice at 0.0°C melts into 2.5kg of water at 0.0°C.

Joule’s Experiment confirmed the mechanical equivalent of heat.

1 st Law Thermodynamics

18-4 Internal Energy of Ideal Gas E-internal is proportional to the absolute temperature internal energy is in the kinetic energy of the molecules of the gas /

Work and the PV Diagram

Gas Processes Isobaric = constant pressure Isometric (Isochoric) = constant volume Isothermal = constant temperature Ex.

Summary: specific heat is material dependent. latent heats; objects change phase at constant temperature. calorimetry: science of heat measurements 1 st law of thermo. is energy conservation mechanical equivalent of heat Pressure x volume = work

Example: Human vs. Gasoline Engine Lance Armstrong: 460WV8: 200hp Power Ratio = 200hp/(460/746)hp = 332 times more Lance: (460W)(21,600) = 9.9 MJ = 2370 Cal Energy Consumption (6h = 21,600s): V8: (332 times more) = 3.3 GJ = 788,000 Cal

Which of the following is a unit of heat? 1.N/m 2.N/m 2 3.N·s 4.N·m

Heat is added to a system and the change in internal energy of the system is 1/3 of the heat added. Which of the following is true? 1.Work on system is + 2.Work on system is – 3.Work on system is 0

When not colliding, molecules of an ideal gas 1.Attract 2.Repel 3.Neither attract or repel

Molecules in an ideal gas 1.Never collide 2.Sometimes collide

18-5 Work and PV Diagrams

Work and PV [PV] = (N/m 2 )(m 3 )= joule = energy Work done by an expanding gas: Work done by gas during isothermal expansion:

Internal Energy of an Ideal Gas is proportional to temperature of gas internal energy is in the kinetic energy of molecules of the gas

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