Thermodynamics Notes – Day 3 Laws of Thermodynamics

First Law of Thermodynamics The first law of thermodynamics states that the total increase in the energy of a system is equal to the sum of the heat added to the system and the work performed on the system ---This is merely a restatement of the law of conservation of energy.

2 nd Law of Thermodynamics  The second law of thermodynamics states that the entropy of the universe always increases.  Entropy is the measure of the amount of disorder of a system.

Law of Heat Exchange For a closed system in which heat energy cannot enter or leave, the heat lost by objects at a higher temperature is equal to the heat gained by objects at lower temperature until thermal equilibrium is reached (at which point the final temperature of both objects is the same).

Heats of Transformation When energy is absorbed as heat by a solid or liquid, the temperature of the object does not necessarily rise. The thermal energy may cause the mass to change from one phase, or state, to another. The amount of energy per unit mass that must be transferred as heat when a mass undergoes a phase change is called the heat of transformation, L.

Phase Changes

Thermal Expansion of Solids Solids expand when heated and contract when cooled (with a few exceptions). – Heated solids increase or decrease in all dimensions (length, width, and thickness). – When a solid is heated, the increase in thermal energy increases the average distance between the atoms and molecules of the solid and it expands.

Thermal Expansion of Solids Thermal expansion can be explained on a molecular basis. Picture the interatomic forces in a solid as springs, as shown in the picture on the right. Each atom vibrates about its equilibrium position. When the temperature increases, the amplitude and associated energy of the vibration also increase.

Examples of Uses of Thermal Expansion Dental materials used for fillings must be matched in their thermal expansion properties to those of tooth enamel, otherwise consuming hot drinks or cold ice cream would be painful. In aircraft manufacturing, rivets and other fasteners are often cooled using dry ice before insertion and then allowed to expand to a tight fit.

You can loosen a tight metal jar lid by holding it under a stream of hot water. Both the metal of the lid and the glass of the jar expand as the hot water adds energy to their atoms. With the added energy, the atoms can move a bit farther from each other than usual, against the interatomic forces that hold every solid together. However, because the atoms in the metal move farther apart than those in the glass, the lid expands more than the jar and is loosened. Expansions slots are often placed in bridges to accommodate roadway expansion on hot days. This prevents buckling of the roadway. Driveways and sidewalks have expansion slots for the same reason.