Understanding Heat Transfer Conduction Convection and Radiation Heat

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 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.

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 adjacent particles vibrate, and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

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

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.

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 What A liquid isfluid aorfluid? gas. movement.

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

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

Why is it windy at the seaside?

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

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

Emission experiment Four containers were filled with warm water. Which container would have the warmest water after ten minutes? Dull metal Shiny black Dull black shiny metal container would be the warmest after ten The _____ radiation back minutes because its shiny surface reflects heat _______ dull black container into the container so less is lost. The ____ emitting heat would be the coolest because it is the best at _______ radiation.

Absorption experiment Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes? Dull metal Shiny black Dull black dull black container would be the warmest after ten The _____ radiation the best. minutes because its surface absorbs heat _______ shiny metal container would be the coolest because it is The _____ the poorest at _____ absorbing heat radiation.

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.

1. Which of the following is not a method of heat transfer? A. Radiation B. Insulation C. Conduction D. Convection

2. In which of the following are the particles closest together? A. Solid B. Liquid C. Gas D. Fluid

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

4. Which is the best surface for reflecting heat radiation? A. Shiny white B. Dull white C. Shiny black D. Dull black

5. Which is the best surface for absorbing heat radiation? A. Shiny white B. Dull white C. Shiny black D. Dull black

Newton's Law of Cooling The rate at which an object loses (gains) temperature is proportional to the difference in temperature between the object and its surroundings.

Newton's Law of Cooling The rate at which an object loses (gains) temperature is proportional to the difference in temperature between the object and its surroundings. Or,

It is winter. I am leaving the house to go to school and will be gone for eight hours. Should I a) Leave thermostat alone. (It will cost a lot of money to heat the house back up if it cools off. ) b) Turn the heat way down c) Turn it down a little bit, but not too much

FYI: Furnaces run more efficiently if they are allowed to run for a long time versus a short time.

Newton's Law of Cooling Crime Scene Per 2 A detective is called to the scene of a crime where a dead body has just been found. She arrives on the scene at 10: 23 pm and begins her investigation. Immediately, the temperature of the body is taken and is found to be 80 o F. The detective checks the programmable thermostat and finds that the room has been kept at a constant 68 o F for the past 3 days. After evidence from the crime scene is collected, the temperature of the body is taken once more and found to be 78. 5 o F. This last temperature reading was taken exactly one hour after the first one. The next day the detective is asked by another investigator, “What time did our victim die? ” Assuming that the victim’s body temperature was normal (98. 6 o F) prior to death, what is her answer to this question? Newton's Law of Cooling can be used to determine a victim's time of death.

Newton's Law of Cooling Newton’s Law of Cooling describes the cooling of a warmer object to the cooler temperature of the environment. Specifically we write this law as, T (t) = Te + (T 0 − Te ) e - kt, where T (t) is the temperature of the object at time t, Te is the constant temperature of the environment, T 0 is the initial temperature of the object, and k is a constant that depends on the material properties of the object. To organize our thinking about this problem, let’s be explicit about what we are trying to solve for. We would like to know the time at which a person died. In particular, we know the investigator arrived on the scene at 10: 23 pm, which we will call τ hours after death. At 10: 23 (i. e. τ hours after death), the temperature of the body was found to be 80 o F. One hour later, τ + 1 hours after death, the body was found to be 78. 5 o F. Our known constants for this problem are, Te = 68 o F and T 0 = 98. 6 o F. At what time did our victim die?

Methods of measuring temperature Per 1 A. Thermometer A. Mercury B. Other B. Thermocouple C. Infrared D. Boiling water / Ice in water / other phase change A. Double boiler

A measure of the resistance of building materials and structures to the flow of heat; the higher the R-value the better the substance is as thermal insulation. R = Temp diff x Time X Area / heat loss where the temperature difference is in degrees Fahrenheit, the area is in square feet, the time in hours, and the heat loss in Btus. If you know the R-value of a partition, you can use this formula to find the heat loss.