16.2 – Heat & Thermodynamics “Everything around us is made up of energy. To attract positive things in your life, start by giving off positive energy.” - Unknown

Learning Objectives Section 16.2 Describe conduction, convection and radiation. Classify materials as thermal conductors or thermal insulators. Apply the Law of Conservation energy to conversions between thermal energy and other forms of energy. Apply the Second Law of Thermodynamics in situations where thermal energy moves from a cooler to warmers objectives State the Third Law of Thermodynamics.

Why is conduction slower in gases than in liquid or solids? What is conduction? Conduction is the transfer of thermal energy with no overall transfer of matter. Why is conduction slower in gases than in liquid or solids? Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often.

Conduction Conduction occurs within a material or between materials that are touching. In conduction, collisions between particles transfer thermal energy, without any overall transfer of matter.

Conduction The arrows show how thermal energy is conducted away from the heat source in a metal frying pan.

Thermal Conductors Thermal Insulators A thermal conductor is a material that conducts thermal energy well. Thermal Insulators A material that conducts thermal energy poorly is called a thermal insulator. Air is a very good insulator. Wool garments and plastic foam cups use trapped air to slow down conduction

More on Conduction Metals are good thermal conductors. When a frying pan is on a hot stove, the bottom of the metal pan heats first and the metal handle last. The flames do not directly heat the handle. Tile is a better conductor than wood. A tile floor feels colder than a wooden floor when both floors are at room temperature. The tile transfers thermal energy more rapidly away from your skin.

What is convection? Convection is the transfer of thermal energy particles of a fluid move from one place to another. In what natural cycles do convection current occur? Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior

Convection Passing sandbags along a line is like transferring thermal energy by convection. The arrows show convection of air in an oven.

Convection A convection current occurs when a fluid circulates in a loop as it alternately heats up and cools down. Air at the bottom of an oven heats up, expands, and becomes less dense. The hot air rises. Rising hot air cools as it moves away from the heat source. As a result, the coolest air is at the top of the oven.

What is radiation? How does an object’s temperature affect radiation Radiation is the transfer of energy by waves moving through space. How does an object’s temperature affect radiation All objects radiate energy. As an object’s temperature increases, the rate at which it radiates energy increases.

Radiation When you stand to the side of a charcoal grill, heat reaches you without convection or conduction. The sun warms you by radiation on a clear day. The space between the sun and Earth has no air to transfer thermal energy. Heat lamps used in restaurants are another example of radiation.

Radiation The heating coil on a stove radiates thermal energy. The changing color of the red arrows indicates that the farther you are from the coil, the less radiation you receive.

3 Law of Thermodynamics The study of conversions between thermal energy and other forms of energy is called thermodynamics. James Prescott Joule (1818-1889) carefully measured the energy changes in a system. Joule's system included a falling weight that turned a paddle wheel in a container of water. Joule found that the work done by the falling weight almost exactly equaled the thermal energy gained by the water.

First Law of Thermodynamics The first law of thermodynamics states that energy is conserved. Energy cannot be created or destroyed, but it can be converted into different forms. Added energy increases the thermal energy of a system or does work on the system. In either case, energy is conserved.

Second Law of Thermodynamics The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system. Thermal energy flows spontaneously only from hotter to colder objects. A refrigerator must do work to transfer thermal energy from the cold food compartment to the warm room air. The thermal energy is released by coils at the bottom or in the back of the refrigerator

More on Thermodynamics A heat engine is any device that converts heat into work. The efficiency of a heat engine is always less than 100 percent. Thermal energy that is not converted into work is called waste heat. Waste heat is lost to the surrounding environment

Third Law of Thermodynamics The Third Law of Thermodynamics states absolute zero cannot be reached.

More on the Third Law of Thermodynamics Spontaneous changes will always make a system less orderly, unless work is done on the system. For example, if you walk long enough, your shoelaces will become untied. But the opposite won't happen; shoelaces don't tie themselves. Disorder in the universe as a whole is always increasing

More on Thermodynamics The efficiency of a heat engine increases with a greater difference between the high temperature inside and the cold temperature outside the engine. A heat engine could be 100 percent efficient if the cold outside environment were at absolute zero (0 Kelvin). This would violate the third law of thermodynamics.