Energy Transfer as Heat

S8P2 d. Describe how heat can be transferred through matter by collisions of atoms (conduction) or through space (radiation). In a liquid or gas, currents will facilitate the transfer of heat (convection).

Background All forms of matter, whether 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

Lesson Essential Question: How is energy transferred as heat?

How is energy transferred as heat? Activating Strategy: Activating Acrostic For each of the letters of the phrase on the following slide write one word that would characterize the phrase.

H - E - A - T - T - R - A - N - S - F - E - R -

The transfer of energy between the particles of two objects due to a temperature difference between the two objects is called heat.

The transfer of energy always takes place from a substance at a higher temperature to a substance at a lower temperature Temperature is an indicator of the particles’ average kinetic energy; therefore, you can use it to predict which way energy will be transferred The greater the difference in the temperatures of two objects, the more energy that will transfer as heat

Energy can be transferred as heat in three ways: Conduction Convection Radiation

Conduction

Conduction Conduction is the transfer of energy that occurs when molecules bump into one another (heat transfer from one atom to another within a substance).

Solid at Different Temperatures Conduction Molecules are always in motion, but molecules in warmer objects move faster than molecules in cooler objects. Solid at Different Temperatures

Conduction When objects are in contact, energy is transferred from warmer objects to cooler objects.

Conduction Example of conduction: A spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon Conduction is most effective in solids, but it can happen in fluids. Gases and liquids are generally poor conductors of heat because their molecules are farther apart than are the molecules in solids. Therefore, neighboring molecules in a gas or in a liquid are less affected by the increased motions of heated molecules, and consequently heat is not conducted rapidly.

Conduction Use the following website to view animations of energy transfer by conduction in solids, liquids, and gases. http://www.scienceonline.co.uk/energy/conduction2.html

Conduction Why does the lady in the pink dress drop her roasting stick when the lady in the blue dress does not?

When heat is conducted, it is led from the heat source by traveling along a physical object. The young woman in the pink dress was using a straightened out coat hanger wire to cook her marshmallow. Metal is a good conductor of heat. The heat from the fire traveled along the thick metal wire. The wire got so hot that the girl had to let go of it. The young girl in the blue dress is using a wooden rod to hold her marshmallow. Wood does not conduct heat well. The wood in her hand is still cool and the marshmallows are almost done!

Conduction Any material through which energy can be easily transferred as heat is called a conductor What determines how well a substance conducts is whether it is a gas, liquid, or solid Gases are extremely poor conductors because their particles are far apart Liquids conduct better than gases, but they are still not very effective conductors Some solids, like rubber and wood, conduct energy about as well as liquids Metals such as copper and silver conduct heat transfer very well Some solids conduct better than others In general, metals are better conductors than nonmetals

Conduction A material that is a poor conductor that is used to reduce or stop energy is called an insulator Ex. Wood

Conductivity Chart

Convection

Convection Convection is the transfer of heat by the flow of material. Convection circulates heat.

Convection Heat moves with the fluid Warmer portions are less dense and therefore rise; cooler portions fall because they are denser. When the atoms at the top cool down, they become more dense and fall and vice versa for the warmer atoms. Hence a continuous cycle The cycle of a heated fluid that rises and then cools and falls is called a convection current

Convection occurs mostly in gas or liquid.

Convection

Convection Use the following link to observe an animation of convection: http://www.geophysik.uni-frankfurt.de/~schmelin/what-is-convection.html Scroll down to the 3rd animation at the bottom of the page

Radiation

Radiation Radiation is energy that is transferred in the form of rays or electromagnetic waves. This includes infrared radiation, visible light, and ultraviolet rays.

Radiation What happens in space? There are no solids touching the sun for conduction to occur There are no fluids in space for convection to occur Therefore, heat travels by radiation

Use the animations on the website below to review conduction, convection, and radiation: http://www.bbc.co.uk/science/hottopics/cooking/chemistry.shtml Scroll down until you see the stove

Think, Pair, Ink Look at the diagram below Think, Pair, Ink Look at the diagram below. In a sentence or two explain how radiation, conduction, and convection are used or prevented.

The shiny inside reflects radiant energy back into the fluid The shiny inside reflects radiant energy back into the fluid. It also prevents heat from entering the thermos from outside. The vacuum space prevents conduction and convection from occurring. Since heat cannot easily leave or enter the thermos, fluids retain their original temperature.

Conduction, Convection, and Radiation together distribute the Sun’s heat throughout the Earth

Lesson Essential Question: How is energy transferred as heat? Summarizing Strategy: On the following slides, identify whether the statement describes cooking popcorn using conduction, convection, or radiation.

Put oil in the bottom of a pan Put oil in the bottom of a pan. Cover the bottom of the pan with popcorn kernels. Place the pan on the stove and turn on the burner to medium heat. Cover the pan with a lid. Periodically shake the pan so the kernels move around in the oil.

And the answer is… Conduction The heat is transferred by direct contact from the pan, to the oil, to the kernels of popcorn.

Microwave a bag of popcorn.

And the answer is… Radiation The kernels are heated by the radiation in the microwave.

Cooking popcorn in a popcorn popper.

And the answer is… Convection The hot air transfers the heat to the cooler kernels, and when enough hot air heats the kernels they pop.