Thermal Energy Chapter 12 Physics Principles and Problems Zitzewitz, Elliot, Haase, Harper, Herzog, Nelson, Nelson, Schuler and Zorn McGraw Hill, 2005

Molecules in a substance may have potential or kinetic energies. For example, gas molecules are constantly trading kinetic energies when they collide. Thermal Energy - the total energy of all the molecules. Temperature - the average energy per molecule.

Molecules in a hot object have great kinetic and potential energies than molecules in a cold object. Hence, a balloon that is heat will expand (the molecules are striking the sides of the balloon harder and faster), while a balloon that is cooled will shrink

Remember, temperature is the average kinetic energy of the particles in an object. Hence, if two objects have the same mass, and one object has a greater temperature, then its particles have a greater average kinetic energy. However, thermal energy is the total amount of kinetic energy of all the particles. Therefore, thermal energy is dependent upon the amount of particles in the object while temperature is not.

Thermal Equilibrium When two objects with different temperatures are placed together (i.e. a thermometer and water). The hotter object’s particles will transfer their energy to the colder object’s particles. When the flow of energy between these two objects is equal so too is their temperatures.

Heat - energy that is transferred between objects. Conduction - particles that are in direct contact with each other Convection - motion of a fluid (liquid or gas) caused by temperature differences (hot air rises, cold air sinks). Radiation - transfer of energy by electromagnetic waves (matter is not needed, such as the energy of the sun reaching the earth throughout the vacuum of space).

Specific Heat - amount of energy that must be added to a material to raise the temperature of a unit mass by one temperature unit (depends of object’s size and material it is made from).

The heat gained or lost (heat transfer) by an object as its temperature changes depends on the mass, change in temperature and the specific heat of the substance. Heat Transfer Q = mC∆T = mC(T f - T i )

In a closed system the conservation of energy dictates that thermal energy of object A plus the thermal energy of object B is constant. E A + E B = 0 (constant) Thus the changes in thermal energies is also constant. ∆E A + ∆E B = 0 (constant) Remember that ∆E = Q = mC ∆T Therefore m A C A ∆ T A + m B C B ∆ T B = 0 Using this equation you can solve for the final or initial temperatures of A or B.

Melting Point Temperature at which a substance changes from a solid to a liquid. All of the added thermal energy goes into overcoming the forces that hold the particle together in a solid form.

Boiling Point Temperature at which a substance changes from a liquid to a gas. All of the added thermal energy goes into overcoming the forces that hold the particle together in a liquid form.

This graph shows the temperature of ice being converted to steam. Notice the flat places of the graph. What is happening here?

Heat of Fusion - Amount of energy needed to melt 1-kg of a substance (change in state not temperature). Q = mH f (amount of heat needed to melt a solid = mass times the heat of fusin) Heat of Vaporization - Amount of energy needed to vaporize 1-kg of liquid (change in state not temperature). Q = mH v (amount of heat needed to vaporize a liquid = mass times the heat of vapor) Negative values of either of these indicate the amount of heat that must be removed to turn into liquid or solid.

First Law of Thermodynamics The change in thermal energy (internal) of an object is equal to the heat added to the object minus the work done by the object. ∆U = Q - W

Converting mechanical energy into thermal energy is easy (rub your hands together). A heat engine (i.e. a car engine) coverts thermal energy into mechanical energy.

Second Law of Thermodynamics Connects entropy (the measure of disorder in a system) with the first law of thermodynamics. For example, a falling ball has both potential and kinetic energy. When it collides with air molecules they absorb this energy and move off in random directions and speeds. This increase in disorder can be termed entropy. du/hbase/therm/imgthe/timarr.gif

Second Law of Thermodynamics Entropy is contained in an object. If heat is added to that object, then the entropy is increased (and vice-versa). The change in entropy in an object is equal to the heat added to the object divided by the temperature of the object. ∆S = Q / T

This law is also called the law of entropy. Because of this law heat will only flow from a hot object to a cold object. Use work is added, objects will always move towards greater entropy.