How does Heat Energy transfer from one substance to another?

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

Here is how it works: Heat Energy is what makes the coffee hot. Since the inside of refrigerator has a lower temperature than the coffee, heat energy travels from the coffee to the refrigerator, following the temperature gradient. The temperature gradient is kind of like the slope of a hill --- the steeper the hill, the faster a ball will roll down it…. In a similar way, heat will leave the coffee faster in the beginning and at a slower rate as the coffee cools.

Now that we know which way heat energy travels (from hot to cold), how does it actually travel? There are three types of heat transfer: CONDUCTION CONVECTION RADIATION

Conduction Conduction is transfer through direct contact. On a molecular level, hotter molecules are vibrating faster than cooler ones. When they come in contact, the faster moving molecules “bump into” the slower moving molecules and heat is transferred! This is how heat is transferred to your finger if you touch a hot stove!

Convection When fluids are heated, currents are created. This is because the individual molecules that come in contact with a hot surface expand, become less dense, and rise. (this is how hot air balloons work!) When this happens, other molecules circulate down and take their place, and a cycle is established. An example of this can be observed in the air currents that are created in a room with a radiator against one wall. The air in contact with the radiator rises, moves across the ceiling to the far wall, sinks, and then comes back to the radiator across the floor.

Radiation Radiated heat energy travels as an electromagnetic wave. Sometimes these waves are in the visible part of the spectrum, like when something is “red hot.” You can see how hot it is, but you can also feel it from a distance, as your skin absorbs the energy. Question: Do you think that radiated heat needs a medium to travel through like heat transferred by convection does?

NO! Electromagnetic waves do not need a medium to travel through, although they can travel through many substances. If this wasn’t true, we wouldn’t feel the heat of the Sun here on Earth!

Q.) When you put a teapot on the stove to boil water, which of the three kinds of heat transfer can be observed? A.) Actually, all three! First, there is conduction between the burner and the teapot, and then conduction between the teapot and the water molecules that are in direct contact with the teapot. Next, there is convection in the water as the heated molecules of water from the bottom of the teapot rise and spread their heat energy to the cooler molecules above them through direct contact. This convection current circulates and pushes cooler molecules of water down to the bottom where they come in contact with the heated bottom of the teapot. While all of this is occurring, heat energy is being radiated in all directions from the from the burner and is absorbed by other objects. You can feel this energy if you stand too close!

Conduction, Convection and Radiation