2.  Consider a very hot mug of coffee on the countertop of your kitchen.  Over time the coffee’s temperature will cool.

3.  Transfer of heat occurs from the hot coffee and hot mug to the surrounding air. The fact that the coffee lowers its temperature is a sign that the average kinetic energy of its particles is decreasing. The coffee is losing energy.

4.  Heat is simply the transfer of energy from a hot object to a colder object.

5.  Now let's consider a different scenario - that of a cold can of pop placed on the same kitchen counter.  Over time, the pop and the container increase their temperature. This increase in temperature is a sign that the average kinetic energy of the particles within the pop and the container is increasing.

6.  So what is happening?  Energy is being transferred from the surroundings (countertop, air in the kitchen, etc.) in the form of heat.

7. Summary Points  An object decreases its temperature by releasing energy in the form of heat to its surroundings.  And an object increases its temperature by gaining energy in the form of heat from its surroundings.  Both the warming up and the cooling down of objects works in the same way - by heat transfer from the higher temperature object to the lower temperature object.

8. New Def’n of Temperature  Temperature is a measure of the ability of a substance, or more generally of any physical system, to transfer heat energy to another physical system.  The higher the temperature of an object is, the greater the tendency of that object to transfer heat. The lower the temperature of an object is, the greater the tendency of that object to be on the receiving end of the heat transfer.

9. Thermal Energy  Molecules have two forms of energy - - kinetic because of their motion, and potential because of the electric forces holding them together.  Thermal energy is the total of all kinetic and potential energies, or the total energy of all particles in a substance.

10. Thermal Expansion  We already know that thermal expansion provides a method of measuring temperature. At the level of atoms and molecules, thermal expansion occurs because the distance between molecules increases as their thermal energy increases.

11. Linear Expansion  Solids expand in all directions (length, width, thickness) when heated, and similarly contract when cooled. For long, thin objects the change is most noticeable only in length. The change in length in one direction is termed linear expansion.

12.  Linear expansion depends on several factors: change in temperature, original length, type of material.  1. The change in length is directly proportional to the change in temperature. eg. An iron rod length increases twice as much when the temperature change is 160°C than if it is 80°C.

13.  2. The change in length is also directly proportional to the original length of the object. eg. An aluminum rod 2 m long will expand twice as much as a 1m aluminum rod.

14.  3. Some substances expand more than others. Expansion values for different materials can be found in charts of "Coefficients of Linear/Volume Expansion". eg. Aluminum will expand twice as much as iron.

15.  Linear Expansion Formula

16.  Volume Expansion  Just as linear expansion occurs in solids, volume expansion occurs in liquids and gases. Volume expansion depends the change in temperature, original volume, and the type of substance.

17.  Volume Expansion Formula