Chapter 12: Thermal Chapter 12: Thermal EnergyEnergy

What’s hot and what’s not…What’s hot and what’s not…

Energy Wheel

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K.E . =1

2mv 2

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P.E . = mgh

Height Energy

Motion Energy

Electrical Energy

Thermal Energy

Wave Energy

12.1 Temperature and Thermal Energy

12.1 Objectives

Define temperature and distinguish it from thermal energy.

Describe the nature of thermal energy.

Define specific heat and calculate heat transfer.

Use the Celsius and Kelvin temperature scales and convert between the two scales.

Thermodynamics

Thermodynamics is the study of heat.

Kinetic-Molecular Theory - matter is made up of tiny particles in motion. In hot objects (high energy) the particles move faster, and in cold objects (low energy) they move more slowly. The electromagnetic forces can be thought of as mass-less springs between the molecules.

Thermal Energy versus Temperature

Thermal Energy versus Temperature

Thermal Energy is the overall energy of motion of all particles making up the object. Thermal energy depends on the number of particles in the object. Twice the mass gives twice the thermal (kinetic) energy.

Temperature is the “hotness” of the object. Temperature does NOT depend on the number of particles in a body. It depends on the average kinetic energy of the particles. Twice the mass gives the same temperature.

Temperature Scales

Farenheit and Rankin

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Co =5

9(F o − 32)

Celcius and Kelvin

Temperature is a measure of an object’s “hotness”.

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Ro = F o + 459.67o

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K o =Co + 273.15o

Absolute zero is the temperature where all molecular motion stops. All thermal energy is removed.

Temperature Scales

Liquid Nitrogen

Thermal Energy Transfer

Heat flows from hot to cold.

Conduction - the process where heat is transferred when particle collide. The pan handle gets hot as the pan is heated.

Convection - the process where heat is transferred by the motion of a fluid. The air from the room heater is blown out on the ground and rises to create circulation.

Radiation - the process where heat is transferred by electromagnetic waves. The sun heats the earth from millions of miles away.

Walking on Coals

Specific Heat

The Specific Heat of a material is the amount of energy that must be added to the material to raise the temperature of a unit mass one temperature unit.

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Q = mCΔT = mC(Tfinal −Tinitial )

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Q = Joules of Energy;m = mass;C = specific heat

Specific Heat Measurement

Calorimeter - A well insulated device used to measure changes in thermal energy.

Air is a Poor Conductor

12.2 Change of State &

Laws of Thermodynamics

12.2 Objectives

Define heat of fusion and vaporization.

State the first and second laws of thermodynamics.

Define heat engine, refrigerator, and heat pump.

Define entropy.

Change of State

Four States of Matter: solid, liquid, vapor, and plasma.

At the melting point and boiling point all energy goes to the change of state. The temperature stays constant until every molecule has changed state.

Heat of Fusion

The Heat of Fusion of a material is the amount of energy that must be added to melt it. This seems counter-intuitive. It should be the heat of melting because fusion is making a solid from a liquid.

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Q f = mH f

Note Bene: There is NO change in temperature term since temperature is constant during the process.

Heat of Vaporization

The Heat of Vaporization of a material is the amount of energy that must be added to vaporize it.

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Qv = mHv

Note Bene: There is NO change in temperature term since temperature is constant during the process.

Summary of Formulas

The Heat of Vaporization of a material is the amount of energy that must be added to vaporize it.

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Qv = mHv

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Q f = mH f

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QI = mCIΔT

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QW = mCWΔT

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QV = mCVΔT

First Law of Thermodynamics

The First Law of Thermodynamics states that the total increase in thermal energy of a system is the sum of the heat added to it and the work done on it.

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ΔETHERMAL = ΔW + ΔHADDED

This is a restatement of the Law of Conservation of Energy: Energy is neither created nor destroyed but can change form.

Second Law of Thermodynamics

The Second Law of Thermodynamics states that natural processes go in a direction that maintains or increases the total entropy of the universe. Things tend to become more and more disordered.

Food color drops added to a glass of water demonstrates diffusion and the second law of thermodynamics.

Entropy is a measure of the disorder of a system.

Second Law of Thermodynamics

Engines and Pumps

Heat Engine - converts heat energy to work. Heat reservoir produces work and gives heat to cold reservoir.

Car Engine – Can you follow the motion of a piston around the cycle when gas explodes?

Engines and Pumps

Heat Pump - uses work to remove heat energy. Work removes heat from cold reservoir to a heat reservoir.

Engines and Pumps

Refrigerator – Can you follow the motion of a piston around the cycle when gas explodes?

Heat Pump - uses work to remove heat energy. Work removes heat from cold reservoir to a heat reservoir.