Chapter 5.4 Notes Thermal Energy

Thermal Energy is the total kinetic energy of the motion of atoms in an object. Molecules in an object are constantly moving in a random motion.

All molecules have 3 types of motion: Translational movement – forward or backward movement

Rotational movement – spinning motion

Vibration – small, fast movements back and forth

Temperature is decided by how much a molecule or atom bounces around a container and hit another molecule and atom. Which type of movement is this? Translational Temperature of an object is dependent upon translational movement only.

The total energy of an object is called the internal energy. As atoms collide, the Potential energy and Kinetic energy is constantly being transferred. However, the total energy or the internal energy of the object always stays the same.

A body’s internal energy depends on: Material Composition Mass Starting Temperature Physical State – Solid, liquid, gas, or plasma

Internal energy can be transferred from one object to another with a temperature difference or by heat. Another way to transfer energy is by doing work to produce friction. Rubbing hands together.

Other forms of energy can also be converted into internal energy. Example – Electric stove uses resistance to convert electrical energy to thermal energy

The science dealing with relationships between internal energy, heat, and work is called thermodynamics. There are 3 laws of thermodynamics.

The law of conservation of energy says that energy cannot be created or destroyed but can change from one form of energy to another form. This law is also called the 1 st Law of thermodynamics.1 st Law of thermodynamics

Internal energy is represented by what letter? U Heat is represented by what letter? Q Equation for 1st law of thermodynamics = Internal energy = heat – work U = Q - W

Heat is positive if it enters the system and negative if it leaves the system. Work is positive when the system does work and work is negative when work is done on the system.

A system that has 50 units of heat entering the system and 40 units of work are done on the system, what is the internal energy? Heat is positive; work is negative U = Q - W U = 50 –(-40) = U = 90

A system that has 50 units of heat entering the system and the system does 40 units of work, what is the internal energy? Heat is positive; work is positive U = Q - W U = 50 – (+40) = U = 10

A system that has 50 units of heat leaving the system and 40 units of work are done on the system, what is the internal energy? Heat is negative; work is negative U = Q - W U = -50 – (-40) = U = -10

A system that has 50 units of heat leaving the system and the system does 40 units of work, what is the internal energy? Heat is negative; work is positive U = Q - W U = -50 – (+40) = -50 – 40 U = -90

A process in which there is no heat transfer to or from the system is called an adiabatic process. 2 ways to create an adiabatic process Isolate the system with insulation Do work quickly so there is no time for heat transfer

A device that converts thermal energy into mechanical energy is called a heat engine.heat engine

Example of heat engine that burns gas – internal combustion engine

Example of heat engine that uses steam - Train

Example of heat engine that uses chemical reactions - rocket

Absorb thermal energy from a high-temperature source Convert some of the thermal energy to work Discard the remaining thermal energy into a low- temperature reservoir Every heat engine must:

Reservoirs are usually the Earth, the Earth’s Atmosphere, or bodies of water. Thermal Pollution

The 2 nd Law of Thermodynamics says that heat will flow from a body with a higher temperature to a body at a lower temperature.2 nd Law of Thermodynamics