Understanding Heat Transfer Conduction Convection and Radiation WARM

Understanding Heat Transfer, Conduction, Convection and Radiation

WARM – UP Spend 5 minutes reviewing the Quiz Trade Cards in the blue basket Take the Recovery Quiz. DO NOT WRITE ON THE QUIZ PAPER Answers go on notebook paper Please go to the next available page in your journal. Title the page “Thermal Energy Notes”

Warm – Up Feb. 2, 2012 • On next clean journal page – tape in Warm Up #21/22 • Complete these activities – use colored pencils provided • You have 8 minutes • These are GRADED

Students will be familiar with the forms and transformations of energy. Elements of Standard S 8 P 2 a. Explain energy transformation in terms of the Law of Conservation of Energy. b. Explain the relationship between potential and kinetic energy. c. Compare and contrast the different forms of energy (heat, light, electricity, mechanical motion, and sound) and their characteristics. d. Describe how heat can be transferred through matter by the collisions of atoms (conduction) or through space (radiation). In a liquid or gas, currents will facilitate the transfer of heat (convection).

Essential Question • What are three methods by which thermal energy is moved from place to place?

Thermal Energy • Thermal Energy - All matter is made up of constantly moving particles. All moving things possess kinetic energy. – Kinetic energy – vibrations and movement within and between particles – Potential energy – forces that act within or between the particles (bonds)

Thermal Energy • As the particles in a sample move faster and faster, the kinetic energy becomes greater and the temperature of the object rises. • Temperature is a measure of the average kinetic energy of the particles in a sample of matter.

Thermal Energy • Thermal energy is the total energy of the particles in a material. • Temperature is measured with a thermometer.

Thermal energy depends on: 1. Mass – the greater the mass, the larger the number of moving particles 2. Temperature – the higher the temperature, the faster the particles are moving 3.

Example – Temperature as a factor Heat is thermal energy that flows from something with a higher temperature to something with a lower temperature. In most cases, heat flows from warmer to cooler materials.

Example –Mass as a factor • However, mass will compound the effect.

REMEMBER… Thermal energy travels as heat from a material of higher temperature to a material of lower temperature. Thermal energy will flow from warmer objects to cooler ones until the temperature equalizes.

Heat Transfer • Heat always moves from a warmer place to a cooler place. • Hot objects in a cooler room will cool to room temperature. EQUALIZE • Cold objects in a warmer room will heat up to room temperature. EQUALIZE

Question • If a cup of coffee and a red popsickle were left on the table in this room what would happen to them? Why? • The cup of coffee will cool until it reaches room temperature. The popsickle will melt and then the liquid will warm to room temperature.

Heat Transfer Methods • Heat transfers in three ways: – Conduction – Convection – Radiation

Conduction When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

Conduction Method #1 Conduction – transfer of thermal energy through matter by direct contact of particles Transfer by Collisions Conduction can take place in solids, liquids and gases. Matter must be present for conduction to take place!

Conduction - Examples * Heating a pan on a stove (pan heats up by conduction) * Place a spoon in a bowl of soup (spoon heats up by conduction)

Why does metal feel colder than wood, if they are both at the same temperature? Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

Convection What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. This effects fluid movement. What is a fluid? A liquid or gas.

Convection Method #2 Convection – transfer of thermal energy by bulk movement of matter Transfer by currents .

Fluid movement Convection can take place in fluids (materials that flow). Liquids and gases are fluids. Currents carry less dense, warmer material up while cooler, denser material sinks. Examples: Ocean Currents, boiling water

Water movement Cools at the surface Cooler water sinks Convection current Hot water rises

Why is it windy at the seaside?

Cold air sinks Where is the freezer compartment put in a fridge? It is put at the top, because cool air sinks, so it cools the food on the way down. Freezer compartment It is warmer at the bottom, so this warmer air rises and a convection current is set up.

Matter must be present for convection to take place!

The third method of heat transfer How does heat energy get from the Sun to the Earth? ? There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection. RADIATION

Radiation Method #3 Radiation – the transfer of energy by electromagnetic waves, such as visible light and infrared Transfer by Waves Energy that travels by radiation is often called radiant energy. Electromagnetic energy must be converted or transformed into thermal energy. Only radiant energy that is absorbed can be converted into thermal energy.

Radiation Examples: Sun heating the Earth, Microwave heating food, sun’s energy heating a sidewalk

Radiation travels in straight lines True/False Radiation can travel through a vacuum True/False Radiation requires particles to travel True/False Radiation travels at the speed of light True/False

Radiation Different materials absorb radiant energy differently. Material Effect Shiny Reflects radiant energy Dull Absorbs radiant energy Dark colors Absorbs radiant energy Light colors Reflects radiant energy Dull metal Shiny black Dull black

Emission experiment Four containers were filled with warm water. Which container would have the warmest water after ten minutes? Dull metal Shiny black Dull black shiny metal container would be the warmest after ten The _____ radiation back minutes because its shiny surface reflects heat _______ dull black container into the container so less is lost. The ____ emitting heat would be the coolest because it is the best at _______ radiation.

Absorption experiment Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes? Dull metal Shiny black Dull black dull black container would be the warmest after ten The _____ radiation the best. minutes because its surface absorbs heat _______ shiny metal container would be the coolest because it is The _____ the poorest at _____ absorbing heat radiation.

Convection questions Why does hot air rise and cold air sink? Cool air is more dense than warm air, so the cool air ‘falls through’ the warm air. Why are boilers placed beneath hot water tanks in people’s homes? Hot water rises. So when the boiler heats the water, and the hot water rises, the water tank is filled with hot water.

Radiation questions Why are houses painted white in hot countries? White reflects heat radiation and keeps the house cooler. Why are shiny foil blankets wrapped around marathon runners at the end of a race? The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.

POP – IT!!! Popcorn Lesson (Three Methods of Heating) All forms of matter, whether a solid, liquid, or gas, are composed of atoms or molecules in constant motion. Because of this constant motion, all atoms have thermal (heat) energy. Whenever a substance is heated, the atoms move faster and faster. When a substance is cooled, the atoms move slower and slower. The "average motion" of the atoms that we sense is what we call temperature. Temperature and heat ARE NOT technically the same thing. Temperature is the average motion of atoms and molecules. Heat is the energy that flows due to temperature differences. Heat is always transferred from warmer to cooler substances

POP – IT!!! CONDUCTION - Place the pan on the stove and turn on the burner to medium heat. Cover the pan. Periodically shake the pan so the kernels move around. RADIATION - Microwave a bag of microwave popcorn. CONVECTION – Using a “hot air” popper – kernels are tossed in a current of hot air until they pop.

1. Which of the following is not a method of heat transfer? A. Radiation B. Insulation C. Conduction D. Convection

2. In which of the following are the particles closest together? A. Solid B. Liquid C. Gas D. Fluid

3. How does heat energy reach the Earth from the Sun? A. Radiation B. Conduction C. Convection D. Insulation

4. Which is the best surface for reflecting heat radiation? A. Shiny white B. Dull white C. Shiny black D. Dull black

5. Which is the best surface for absorbing heat radiation? A. Shiny white B. Dull white C. Shiny black D. Dull black