By Moizul Hasan Assistant Professor FLOW OF HEAT By Moizul Hasan Assistant Professor

3 WAYS TO TRANSFER HEAT RADIATION CONDUCTION CONVECTION DIRECTION OF HEAT FLOW 3 WAYS TO TRANSFER HEAT RADIATION CONDUCTION CONVECTION

RADIATION DEFINITION: THE TRANSFER OF HEAT ENERGY BY ELECTROMAGNETIC RADIATION. EXAMPLE: FIREPLACE INCLUDES: VISIBLE LIGHT, IR..ULTRAVIOLET..ETC.

RADIATION MOST OF THE RADIATION FROM THE SUN IS VISIBLE LIGHT. MOST OF THIS RADIATION IS ABSORBED BY THE ATMOSPHERE. MOST OF WHAT REACHES THE SURFACE IS REFLECTED BACK INTO THE ATMOSHPERE.

RADIATION DARK COLORED OBJECTS SUCH AS ASPHALT WILL ABSORB MORE RADIATION THAN LIGHTER COLORED OBJECTS. DARK OBJECTS ALSO RERADIATE THEIR ENERGY FASTER THAN LIGHTER COLOR OBJECTS.

CONDUCTION DEFINITION: THE TRANSFER OF HEAT ENERGY FROM ONE SUBSTANCE TO ANOTHER OR WITHIN A SUBSTANCE. EXAMPLE: A POT ON THE STOVE. HEAT IS TRANSFERRED FROM MOLECULE TO MOLECULE. AIR IS A POOR CONDUCTOR.

CONVECTION DEFINITION: THE TRANSFER OF HEAT ENERGY IN A FLUID. EXAMPLE: BOILING WATER. ATMOSPHERE IS SIMILAR TO A FLUID. WHEN THE SURFACE IS HEATED, A BUBBLE OF WARM AIR FORMS AND RISES.

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 When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the nearby particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

Conduction. Conduction is heat transfer that occurs when particles are in contact with each other. For example, if a hot saucepan is placed on a bench, heat will flow from the hot surface of the saucepan to the colder bench. This heat flow from a hot object to a cold object will continue until the objects reach the same temperature.                                               

Metals are different The outer e______ of metal atoms drift, and are free to move. lectrons When the metal is heated, this ‘sea of electrons’ gain k_____ energy and transfer it throughout the metal. inetic Insulators, such as w___ and p____, do not have this ‘sea of electrons’ which is why they do not conduct heat as well as metals. ood lastic

Why does metal feel colder than wood, if they are both at the same temperature? Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

The particles spread out and become less dense. Convection What happens to the particles in a liquid or a gas when you heat them? Convection is the flow or movement of thermal energy through a liquid or gas current. The particles spread out and become less dense. This effects fluid movement.

Fluid movement Cooler, more d____, fluids sink through w_____, less dense fluids. ense armer In effect, warmer liquids and gases r___ up. ise Cooler liquids and gases s___. ink

For example, when a pot of water is boiled, the water molecules closest to the bottom of the pot are heated the most. As they heat, they become less dense. This causes them to rise to the top of the pot. As the hot molecules rise to the top, they take the heat with them. The cooler particles then drop to the bottom. The heat transfer is occurring because of the movement of the warm particles.

Water movement Cools at the surface Convection current Hot water rises Cooler water sinks

Why is it windy at the seaside?

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

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 ?

Through the process of radiation, heat flows in the form of waves Through the process of radiation, heat flows in the form of waves. The radiation waves flow from hot objects and are absorbed by cooler objects. The cooler objects heat up as they absorb the waves.

In summary: All three types of heat transfer involve a flow of heat from a warm object to a cold object. As the cold object absorbs the heat, it becomes hotter. The warm object is losing the heat that the cold object is absorbing, and so it is becoming cooler. This flow from warm object to cool object continues until both objects reach equilibrium. This is the point where both objects are the same temperature.

Radiation Radiation travels in straight lines True/False Radiation can travel through a vacuum Radiation requires particles to travel Radiation travels at the speed of light

Convection questions Why does hot air rise and cold air sink? Cool air is more dense than warm air, so the cool air ‘falls through’ the warm air. Why are boilers placed beneath hot water tanks in people’s homes? Hot water rises. So when the boiler heats the water, and the hot water rises, the water tank is filled with hot water.

Radiation questions Why are houses painted white in hot countries? White reflects heat radiation and keeps the house cooler. Why are shiny foil blankets wrapped around marathon runners at the end of a race? The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.

3. How does heat energy reach the Earth from the Sun? A. Radiation B. Conduction C. Convection D. Insulation

Through which of the following ways can heat be transferred?     A. by direct or indirect contact between objects at different temperatures B. only by direct contact between objects at different temperatures C. only by indirect contact between objects at different temperatures D. Heat can never be transferred between objects.

A Answer and Explantion There are three main forms of heat transfer: Conduction involves the transfer of heat by the direct contact of a warm object with a cooler object. Convection involves the transfer of heat in liquids and gases as molecules move in currents. Radiation involves the transfer of heat through space. Both convection and radiation transfer heat without the objects being in direct contact with one another. Heat, therefore, can be transferred by direct or indirect contact between objects at different temperatures. A

4. Which of the following is NOT an example of heat being transferred by radiation? A. a fire warming a group of campers B. the Sun heating the Earth C. an electric heater warming a room D. a metal spoon in a cup of hot water

Answer and Explanation A metal spoon in a cup of hot water is not an example of heat being transferred by radiation. Since the spoon is directly touching the hot water, heat is transferred by conduction. All of the other examples transfer heat by radiation. That is, heat is transferred through space. D