Lesson: Heat Transfer Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder

Keywords:  conduction, convection, cooling, efficien cy, energy, heat, heat transfer, heating, housing, materials, rad iation, renewable, renewable energy, solar, solar energy, sun, thermal conductivity, thermal energy

Learning Objectives  Explain in detail the three types of heat transfer, and give examples of each. Explain why certain materials are better than others for transferring heat. Apply what they have learned about heat transfer and materials to real-world problems.

Pre-Lesson Assessment Why is it important to understand the way that energy is transferred through a particular medium (such as air or water)? Ear on Desk and Knock.

Introduction 

8 minutes Every day, the sun's thermal energy travels through 91 million miles (146 million km) of empty space all the way to earth.

Energy (Earth) The sun's energy is the driving force for all life systems that exist on our planet today. Without a way to transfer this energy from the sun to the earth, we would be in big trouble. All energy comes from the sun except geothermal and tidal.

The sun's energy is radiated to us in the form of heat and visible light. And, it is the predominant source of renewable energies used throughout the world today.

The physics behind the transfer of thermal energy explains how we are able to harness renewable energy from the sun and convert it to energy that we can use in our daily lives. The transfer of heat between substances at different temperatures occurs in three different ways: conduction, convection, and radiation.

3 Types of energy transfer 1. Conduction is the transfer of heat through a specific material, such as heat moving through a metal pot being heated on the stove. 

Convection Convection is the transfer of heat through a fluid such as water or air, instead of a material. You can experience this by lying down on the floor of your house. The air down low is cooler, right? This is because warmer air rises towards the ceiling, leaving the cool air behind, low to the ground. Situations in which heat is transferred through a fluid medium (air) are examples of convection. 

Radiation Radiation is energy that is radiated or transmitted in the form of rays, waves or particles. An everyday example of this is the heat that you feel on your skin from sunlight.

Insulation and Conduction What is the importance of material selection in choosing a pipe for a hot water system in a house?

Lesson https://www.wisc- online.com/learn/natural- science/earth-science/sce304/heat- transfer--conduction--convection-- radiation

Thermal Conductivity Thermal conductivity is the quantity of heat transmitted during a specific time through a thickness of material, in a direction normal (perpendicular) to a surface of area.

Reason why some surfaces feel cold and some warm.

Evaporation When the atoms or molecules of a liquid (such as water) are exposed to a significant volume of gas, they tend to spontaneously enter the gaseous state, or evaporate. This happens because the molecules near the fluid's surface are constantly moving in all different directions at random speeds, and colliding with one another.

During these collisions, a few molecules gain enough kinetic energy to push them past the boiling point of water, causing them to evaporate and become water vapor. But, most of the molecules do not have enough energy to do this, which is why liquids do not instantly turn into vapor. Boiling water significantly increases the energy of the molecules, which, in turn, speeds up the evaporation process.

Cooling by Way of Evaporation When liquid water evaporates, heat transfers from the higher temperature of the air (through convection) to the lower temperature of the water, cooling the air. One might think that the temperature of the water would increase until thermal equilibrium between the air and water is reached, but this does not happen.

Because energy is required for the water to evaporate in the first place, the heat that is transferred to the water ends up being used to facilitate the process of evaporation. So, the temperature of the water remains below the temperature of the air. The process of evaporative cooling continues until the air is fully saturated, or at 100% relative humidity. As the relative humidity of the air increases however, the process becomes less effective as less water is able to evaporate.

Lesson Summary Assessment How can we capitalize on the principles of heat transfer to improve the way we live?

Lesson Closure  We've learned that heat can be transferred in three different ways. What are they? In what ways do we see materials being used because they are good conductors or good insulators?