Heat Transfer Heat Transfer

RadiationRadiation

ConductionConduction

Convection Convection

The The speed speed of a material's molecules of a material's molecules or atoms determines its heat energy. or atoms determines its heat energy.

Heat is the total kinetic energy of an Heat is the total kinetic energy of an object. object.

So, when you record the temperature So, when you record the temperature of a substance you are recording the of a substance you are recording the SPEED of the molecules or atoms.SPEED of the molecules or atoms.

RadiationRadiation

The transfer of energy through The transfer of energy through electromagnetic waves. electromagnetic waves.

Does not need a medium (solid, or Does not need a medium (solid, or gas) in order to move. It travels gas) in order to move. It travels through empty space. through empty space.

ConductionConduction

Heat energy can be transferred from Heat energy can be transferred from one substance to another when they one substance to another when they are in direct contact. are in direct contact.

EXAMPLE: frying an egg….the pan EXAMPLE: frying an egg….the pan (solid) heats the egg.(solid) heats the egg.

Since the atoms are closer together, solids Since the atoms are closer together, solids conduct heat better than liquids or gasses.conduct heat better than liquids or gasses.

Heat spreads by conduction when atoms Heat spreads by conduction when atoms increase their vibrations, and pass this energy increase their vibrations, and pass this energy on to those nearby. on to those nearby.

ConvectionConvection

Heat transfer in fluids (liquids and gases).Heat transfer in fluids (liquids and gases). Molecules of the fluid must come in contact Molecules of the fluid must come in contact

with one another. with one another. Image: As the warmer air rises (less dense), it Image: As the warmer air rises (less dense), it

passes its energy to other air molecules. passes its energy to other air molecules. When it moves away from the heat source it When it moves away from the heat source it gets cooler (more dense) and sinks. gets cooler (more dense) and sinks. CONVECTION CURRENT!CONVECTION CURRENT!

The LabThe Lab

You will be working with hot You will be working with hot water..so you must wear safety water..so you must wear safety goggles.goggles.

I will explain how you are to conduct I will explain how you are to conduct the lab. So please pay attention!the lab. So please pay attention!

Pay close attention to what happens Pay close attention to what happens when you place the cold color dye when you place the cold color dye and what happens to the warm color and what happens to the warm color dye. Remember……..always consider dye. Remember……..always consider the placement of the molecules!the placement of the molecules!

The labThe lab

What occurred when your group What occurred when your group placed the food color representing placed the food color representing the “cold water” into the tray?the “cold water” into the tray?

What you should have observed was What you should have observed was the food coloring “resting” on the the food coloring “resting” on the bottom of the tray. bottom of the tray.

WHY?WHY?

The cold water molecules are more The cold water molecules are more dense, which means they are closer dense, which means they are closer together. Since they are more dense together. Since they are more dense they have less energy and are they have less energy and are moving less.moving less.

When your group placed the food coloring When your group placed the food coloring that represented the warm water into the that represented the warm water into the tray over the heat source, the food tray over the heat source, the food coloring started to rise and spread.coloring started to rise and spread.

WHY?WHY?

The warm water molecules were less The warm water molecules were less dense and contained energy that was dense and contained energy that was transferred by the bottom of the tray transferred by the bottom of the tray (conduction).(conduction).

As those molecules start to rise they pass As those molecules start to rise they pass the energy to other water molecules by the energy to other water molecules by the process of convection. the process of convection.

As the warm water rises, it is moving away As the warm water rises, it is moving away from the heat source and starts to cool. from the heat source and starts to cool. However, warmer water is still rising below However, warmer water is still rising below the cooling water. This causes the cooler the cooling water. This causes the cooler rising water to be pushed out of the way.rising water to be pushed out of the way.

What happened when your group What happened when your group placed food coloring representing the placed food coloring representing the cold water closer to the heat source?cold water closer to the heat source?

The food coloring was moving The food coloring was moving towards the heat source. towards the heat source.

WHY? WHY?

Same concept from the “crushing the Same concept from the “crushing the can lab”. Warm water rising is less can lab”. Warm water rising is less dense that creates “more space” for dense that creates “more space” for the more dense cold water to fill.the more dense cold water to fill.

Warm = Low pressureWarm = Low pressure Cold = High pressureCold = High pressure