Understanding Heat Transfer, Conduction, Convection and Radiation : Understanding Heat Transfer, Conduction, Convection and Radiation What is heat? : What is heat? Heat is a form of energy created by motion of atoms and molecules. What is temperature? : What is temperature? Temperature measures the average kinetic energy of random motion of atoms and molecules Heat Transfer : 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 : Question If a cup of coffee and a red popsickle 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 popsickle will melt and then the liquid will warm to room temperature. 2nd Law of Thermodynamics : 2nd Law of Thermodynamics It is impossible for a process to have as its sole result the transfer of heat from a cooler body to a hotter one.
It is impossible for a self-acting machine working in a cyclic process, to transfer from lower temperature to a body at a higher temperature without the aid of an external agency. Heat Transfer Methods : Heat Transfer Methods Heat transfers in three ways:
Radiation Conduction : 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 adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction Metals are different : Metals are different The outer e______ of metal atoms drift, and are free to move. When the metal is heated, this ‘sea of electrons’ gain k_____ energy and transfer it throughout the metal. 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. lectrons inetic ood lastic Why does metal feel colder than wood, if they are both at the same temperature? : 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. Fourier’s 1st law of heat conduction : Fourier’s 1st law of heat conduction q=Q/A is the rate of heat transfer per unit cross-sectional area, or heat flux is directly proportional to the temperature gradient. The negative sign represents that heat flows in the direction opposite to temperature gradient Convection : Convection What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. This effects fluid movement. What is a fluid? A liquid or gas Fluid movement : Fluid movement Cooler, more d____, fluids sink through w_____, less dense fluids. In effect, warmer liquids and gases r___ up. Cooler liquids and gases s___. ense armer ise ink Water movement : Water movement Hot water rises Cooler water sinks Convection current Cools at the surface Why is it windy at the seaside? : Why is it windy at the seaside? Cold air sinks : 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. Newton’s law of cooling : Newton’s law of cooling The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperature between the two bodies. Q is the rate of heat transfer by convection to the surrounding fluid.
A is the object’s exposed area.
T and TW are the fluid free-stream temperature and object temperature, respectively.
h is the convection heat transfer coefficient. The third method of heat transfer : 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 Radiation : Radiation Radiation travels in straight lines
Radiation can travel through a vacuum
Radiation requires particles to travel
Radiation travels at the speed of light
True/False emissivity : emissivity Ratio of total emissive power of a body to the total emissive power of a black body. Black body : Black body A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation. Emission experiment : Emission experiment Four containers were filled with warm water. Which container would have the warmest water after ten minutes? The __________ container would be the warmest after ten minutes because its shiny surface reflects heat _______ back into the container so less is lost. The ________ container would be the coolest because it is the best at _______ heat radiation. shiny metal radiation dull black emitting Absorption experiment : Absorption experiment Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes? The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation. dull black radiation shiny metal absorbing Convection questions : Convection questions 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. 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. Radiation questions : 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. Slide 26: 1. Which of the following is not a method of heat transfer? A. Radiation B. Insulation C. Conduction D. Convection Slide 27: 2. In which of the following are the particles closest together? A. Solid B. Liquid C. Gas D. Fluid Slide 28: 3. How does heat energy reach the Earth from the Sun? A. Radiation B. Conduction C. Convection D. Insulation Slide 29: 4. Which is the best surface for reflecting heat radiation? A. Shiny white B. Dull white C. Shiny black D. Dull black Slide 30: 5. Which is the best surface for absorbing heat radiation? A. Shiny white B. Dull white C. Shiny black D. Dull black