Thermal Energy & Thermodynamics Chapter 7 Thermal Energy & Thermodynamics

Thermal Energy The kinetic energy of the molecules in a solid, liquid or gas The more kinetic energy, the more thermal energy the object possesses Physicists also call this the internal energy of an object

Temperature A measure of how hot or cold something is Measured with a thermometer Celsius Scale sets freezing point of water at 0o and the boiling point at 100o Fahrenheit Scale sets freezing point of water at 32o and the boiling point at 212o

Temperature Scales Temperature is measured using the Fahrenheit, Celsius, and Kelvin temperature scales. The reference points are the boiling and freezing points of water.

Temperature Temperature is proportional to the average kinetic energy of the molecules in a substance. If all the atoms or molecules in a substance are the same, then we are saying

Absolute Zero There is no upper limit to temperature based on the equation. What if the average speed of an atom or molecule in a substance is zero? Must mean a temperature of zero!!! There is an absolute zero of temperature How to determine what it is?

Absolute 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 Temperatures

Heat Touch something hot and thermal energy enters your hand Touch something cold and thermal energy leaves your hand Direction of energy flow is from hot to cold Heat is the thermal energy transferred due to a temperature difference

Equilibrium Thermal energy transfers between two objects until the reach the same temperature This means until the average kinetic energy of the atoms and molecules is the same

Quantity of Heat Heat is a form of energy It is measured in joules Common usage is a heat unit called the calorie (the energy needed to raise the temperature of 1 gram of water by 1o C Chemists use the kilocalorie (1000 calories=4200 Joules) Nutritionists call it a food Calorie

First Law of Thermodynamics Whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred. A system is substance, device or well-defined group of atoms or molecules This Law is just our old friend Conservation of Energy

Second Law of Thermodynamics Heat never spontaneously flows from a cold substance to a hot substance. This really says that systems tend to a state of more disorder or greater entropy Read the discussion of entropy on page 167 in your textbook

Third Law of Thermodynamics No system can reach absolute zero.

Specific Heat Capacity Different substances require different amounts of heat to change temperature by a given amount Due to differences in internal structure of the molecules and how they are bound in the substance

Specific Heat Capacity The specific heat capacity of any substance is defined as the quantity of heat required to change the temperature of the substance by 1o Celsius. Water has a high heat capacity which is very useful It accounts for climatic conditions on the west coasts of continents in the northern hemisphere

Thermal Expansion At higher temperatures, molecules have more kinetic energy and on average move farther apart from each other This means the substance expands Amount of expansion depends on the structure of the substance

Expansion of Water Remarkably interesting case

Expansion of Water

Expansion of Water This is why lakes and ponds and rivers freeze with the ice on top If they didn’t, no aquatic life would be possible