Chapter 7Chapter 7

Thermal Energy &

Thermodynamics

Thermal EnergyThermal Energy

The kinetic energy of the molecules The kinetic energy of the molecules in a solid, liquid or gasin a solid, liquid or gas

The more kinetic energy, the more The more kinetic energy, the more thermal energy the object possessesthermal energy the object possesses

Physicists also call this the internal Physicists also call this the internal energy of an objectenergy of an object

TemperatureTemperature

A measure of how hot or cold A measure of how hot or cold something issomething is

Measured with a thermometerMeasured with a thermometer Celsius Scale sets freezing point of Celsius Scale sets freezing point of

water at 0water at 0oo and the boiling point at and the boiling point at 100100oo

Fahrenheit Scale sets freezing point Fahrenheit Scale sets freezing point of water at 32of water at 32oo and the boiling point and the boiling point at 212at 212oo

Temperature ScalesTemperature Scales

Temperature is Temperature is measured using measured using the Fahrenheit, the Fahrenheit, Celsius, and Kelvin Celsius, and Kelvin temperature temperature scales.scales.

The reference The reference points are the points are the boiling and boiling and freezing points of freezing points of water.water.

TemperatureTemperature

Temperature is proportional to the Temperature is proportional to the average kinetic energy of the average kinetic energy of the molecules in a substance.molecules in a substance.

If all the atoms or molecules in a If all the atoms or molecules in a substance are the same, then we are substance are the same, then we are sayingsaying

2

2

1averagemvT

Absolute ZeroAbsolute Zero

There is no upper limit to There is no upper limit to temperature based on the equation.temperature based on the equation.

What if the average speed of an atom What if the average speed of an atom or molecule in a substance is zero?or molecule in a substance is zero?

Must mean a temperature of zero!!!Must mean a temperature of zero!!! There is an absolute zero of There is an absolute zero of

temperaturetemperature How to determine what it is?How to determine what it is?

Absolute ZeroAbsolute Zero

We define a new temperature scale shifted from Celsius by 273o degrees. This is called the Kelvin scale. Water freezes at 273o K and boils at 373o K.

Interesting Kelvin Interesting Kelvin TemperaturesTemperatures

HeatHeat

Touch something hot and thermal Touch something hot and thermal energy enters your handenergy enters your hand

Touch something cold and thermal Touch something cold and thermal energy leaves your handenergy leaves your hand

Direction of energy flow is from hot to Direction of energy flow is from hot to coldcold

Heat is the thermal energy Heat is the thermal energy transferred due to a temperature transferred due to a temperature differencedifference

EquilibriumEquilibrium

Thermal energy transfers between Thermal energy transfers between two objects until the reach the same two objects until the reach the same temperaturetemperature

This means until the average kinetic This means until the average kinetic energy of the atoms and molecules is energy of the atoms and molecules is the samethe same

Quantity of HeatQuantity of Heat

Heat is a form of energyHeat is a form of energy It is measured in joulesIt is measured in joules Common usage is a heat unit called Common usage is a heat unit called

the calorie (the energy needed to the calorie (the energy needed to raise the temperature of 1 gram of raise the temperature of 1 gram of water by 1water by 1oo C C

Chemists use the kilocalorie (1000 Chemists use the kilocalorie (1000 calories=4200 Joules)calories=4200 Joules)

Nutritionists call it a food CalorieNutritionists call it a food Calorie

First Law of First Law of ThermodynamicsThermodynamics

Whenever heat flows into or out of a Whenever heat flows into or out of a system, the gain or loss of thermal system, the gain or loss of thermal energy equals the amount of heat energy equals the amount of heat transferred.transferred.

A system is substance, device or A system is substance, device or well-defined group of atoms or well-defined group of atoms or moleculesmolecules

This Law is just our old friend This Law is just our old friend Conservation of EnergyConservation of Energy

Second Law of Second Law of ThermodynamicsThermodynamics

Heat never spontaneously flows from Heat never spontaneously flows from a cold substance to a hot substance.a cold substance to a hot substance.

This really says that systems tend to This really says that systems tend to a state of more disorder or greater a state of more disorder or greater entropyentropy

Read the discussion of entropy on Read the discussion of entropy on page 167 in your textbookpage 167 in your textbook

Third Law of Third Law of ThermodynamicsThermodynamics

No system can No system can reach absolute reach absolute zerozero..

Specific Heat CapacitySpecific Heat Capacity

Different substances require Different substances require different amounts of heat to change different amounts of heat to change temperature by a given amounttemperature by a given amount

Due to differences in internal Due to differences in internal structure of the molecules and how structure of the molecules and how they are bound in the substancethey are bound in the substance

Specific Heat CapacitySpecific Heat Capacity

The specific heat capacity of The specific heat capacity of any substance is defined as the any substance is defined as the quantity of heat required to quantity of heat required to change the temperature of the change the temperature of the substance by 1substance by 1oo Celsius Celsius..

Water has a high heat capacity which is Water has a high heat capacity which is very usefulvery useful

It accounts for climatic conditions on It accounts for climatic conditions on the west coasts of continents in the the west coasts of continents in the northern hemispherenorthern hemisphere

Thermal ExpansionThermal Expansion

At higher temperatures, molecules At higher temperatures, molecules have more kinetic energy and on have more kinetic energy and on average move farther apart from average move farther apart from each othereach other

This means the substance expandsThis means the substance expands Amount of expansion depends on the Amount of expansion depends on the

structure of the substancestructure of the substance

Expansion of WaterExpansion of Water Remarkably interesting caseRemarkably interesting case

Expansion of WaterExpansion of Water

Expansion of WaterExpansion of Water

This is why lakes and ponds and This is why lakes and ponds and rivers freeze with the ice on toprivers freeze with the ice on top

If they didn’t, no aquatic life would If they didn’t, no aquatic life would be possiblebe possible