If you put a hot cup of coffee into a refrigerator, If you put a hot cup of coffee into a refrigerator, would “cold” transfer from the fridge to the coffee, would “cold” transfer from the fridge to the coffee,

or would “hot” transfer from the coffee to the or would “hot” transfer from the coffee to the fridge?fridge?

Hmmmm….Hmmmm….

Here is how it works:Here is how it works: HeatHeat EnergyEnergy is what makes the coffee hot.is what makes the coffee hot. Since the inside of refrigerator has a lower Since the inside of refrigerator has a lower

temperature than the coffee, heat energy temperature than the coffee, heat energy travels from the coffee to the refrigerator, travels from the coffee to the refrigerator, following the following the temperature gradienttemperature gradient..

The temperature gradient is kind of like The temperature gradient is kind of like the slope of a hill --- the steeper the hill, the slope of a hill --- the steeper the hill, the faster a ball will roll down it….the faster a ball will roll down it….

In a similar way, heat will leave the coffee In a similar way, heat will leave the coffee faster in the beginning and at a slower faster in the beginning and at a slower rate as the coffee cools.rate as the coffee cools.

Now that we know which way Now that we know which way heat energy travels (from hot heat energy travels (from hot to cold), to cold), how doeshow does it actually it actually

travel?travel?

There are three types of heat transfer:There are three types of heat transfer:

• CONDUCTIONCONDUCTION• CONVECTIONCONVECTION• RADIATIONRADIATION

ConductionConduction

Conduction is transfer through Conduction is transfer through direct contact.direct contact.

On a molecular level, hotter On a molecular level, hotter molecules are vibrating faster molecules are vibrating faster than cooler ones.than cooler ones.

When they come in contact, When they come in contact, the faster moving molecules the faster moving molecules “bump into” the slower moving “bump into” the slower moving molecules and heat is molecules and heat is transferred!transferred!

This is how heat is transferred This is how heat is transferred to your finger if you touch a to your finger if you touch a hot stove!hot stove!

ConvectionConvection When fluids are heated, currents When fluids are heated, currents

are created.are created. This is because the individual This is because the individual

molecules that come in contact with molecules that come in contact with a hot surface expand, become less a hot surface expand, become less dense, and rise. (this is how hot air dense, and rise. (this is how hot air balloons work!)balloons work!)

When this happens, other molecules When this happens, other molecules circulate down and take their place, circulate down and take their place, and a cycle is established.and a cycle is established.

An example of this can be observed An example of this can be observed in the air currents that are created in the air currents that are created in a room with a radiator against in a room with a radiator against one wall.one wall.

The air in contact with the radiator The air in contact with the radiator rises, moves across the ceiling to rises, moves across the ceiling to the far wall, sinks, and then comes the far wall, sinks, and then comes back to the radiator across the floor.back to the radiator across the floor.

RadiationRadiation Radiated heat energy travels as Radiated heat energy travels as

an electromagnetic wave.an electromagnetic wave. Sometimes these waves are in Sometimes these waves are in

the visible part of the spectrum, the visible part of the spectrum, like when something is “red like when something is “red hot.” You can hot.” You can seesee how hot it is, how hot it is, but you can also but you can also feelfeel it from a it from a distance, as your skin absorbs distance, as your skin absorbs the energy.the energy.

QuestionQuestion: Do you think that : Do you think that radiated heat needs a medium radiated heat needs a medium to travel through like heat to travel through like heat transferred by convection does?transferred by convection does?

NO! NO!

Electromagnetic Electromagnetic waves do not need waves do not need a medium to travel a medium to travel through, although through, although they can travel they can travel through many through many substances.substances.

If this wasn’t true, If this wasn’t true, we wouldn’t feel the we wouldn’t feel the heat of the Sun heat of the Sun here on Earth!here on Earth!

Q.) When you put a teapot Q.) When you put a teapot on the stove to boil water, on the stove to boil water, which of the three kinds of which of the three kinds of heat transfer can be heat transfer can be observed?observed?

First, there is First, there is conductionconduction between the burner and the teapot, and then between the burner and the teapot, and then conduction between the teapot and the water molecules that are in direct conduction between the teapot and the water molecules that are in direct contact with the teapot.contact with the teapot.

Next, there is Next, there is convectionconvection in the water as the heated molecules of water in the water as the heated molecules of water from the bottom of the teapot rise and spread their heat energy to the from the bottom of the teapot rise and spread their heat energy to the cooler molecules above them through direct contact. This convection cooler molecules above them through direct contact. This convection current circulates and pushes cooler molecules of water down to the current circulates and pushes cooler molecules of water down to the bottom where they come in contact with the heated bottom of the teapot.bottom where they come in contact with the heated bottom of the teapot.

While all of this is occurring, heat energy is being While all of this is occurring, heat energy is being radiatedradiated in all directions in all directions from the from the burner and is absorbed by other objects. You can feel from the from the burner and is absorbed by other objects. You can feel this energy if you stand too close!this energy if you stand too close!

A.) A.) Actually, all Actually, all three!three!

Conduction, Convection and Conduction, Convection and RadiationRadiation