Understanding Heat Transfer, Conduction, Convection and Radiation

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

Question If a cup of coffee and a red popsicle 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 popsicle will melt and then the liquid will warm to room temperature.

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

Conduction Conduction is transfer of heat through direct contact no matter is transferred. 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!

Thermal conductors Think of a frying pan – The bottom heats up first, but even though the handle never touches the flame it gets hot. Why? – Because the pan is made of metal which is a good thermal conductor (conducts thermal energy well) – Tile floors feel colder to bare feet then wooden floors because they conduct the heat away from your feet.

Thermal Insulators If one left a wooden spoon in a pan of sauce you would be able to pick that up, for the spoon remains cool. Why? Wood is a poor conductor, but a good thermal insulator (conducts thermal energy poorly). Air is another great insulator. Double paned windows contain air to slow down the release of heat in the winter and the input of heat in the summer. Other Examples:

Convection Transfer of thermal energy when particles of a fluid move from one place to another. What are fluids? Gases and liquids 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, the cooler molecules circulate down and take their place, and a cycle is established. This circulation is called a convection current (when a fluid circulates in a loop as it alternately heats up and cools down)

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. Convection currents are important in many naturals cycles. – Ocean currents, weather cycles, and movements of how rocks.

Why is it windy at the seaside?

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

Radiation Radiated heat energy travels as an electromagnetic wave through space. Electromagnetic waves travel at the speed of light, which is 300,000,000 meters per second. 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. Examples: Feeling the heat from a charcoal grill, heat lamps, and of course the sun. ALL OBJECTS RADIATE ENERGY. AS IT’S TEMPERATURE INCTREASES THE RATE OF RADIATION INCREASES

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

Thermodynamics The study of conversions between thermal energy and other types of energy The study of thermodynamics is about energy flow in natural systems The Laws of Thermodynamics describe what is known about energy transformations in our universe

First Law of Thermodynamics States that energy is conserved. Law of conservation of energy “Energy is neither created nor destroyed but can be transformed from one form to another” If energy is added into a system it can increase thermal energy or it can do work to the system, but energy is being conserved.

Second Law of Thermodynamics T hermal energy flows spontaneously only from hotter to colder objects T hermal energy CAN flow from colder objects to hotter objects only if WORK is done on the system

H eat engine converts heat into work-NEVER 100% efficient C reates waste heat that is put into the environment S pontaneous changes always make a system less orderly. Disorder is called entropy. E very energy transformation or transfer results in an increase in the disorder of the universe

Third Law of Thermodynamics A bsolute Zero cannot be reached –C–C omplete absence of all movement is absolute zero. –N–N ever been reached even in controlled experiments