- Slides: 26
Heat Transfer: Conduction, Convection and Radiation
Heat • Heat – Energy that flows due to temperature difference • 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 warm cup of coffee and a red popsicle were left on the table in this room what would happen to them? Why?
Answer • The cup of coffee will cool until it reaches room temperature. The popsicle will melt and then the liquid will warm to room temperature. – Why? • Heat always moves from a warmer place to a cooler place.
Learning Check A. When you touch ice, heat is transferred from 1) your hand to the ice 2) the ice to your hand B. When you drink a hot cup of coffee, heat is transferred from 1) your mouth to the coffee 2) the coffee to your mouth Lecture. PLUS Timberlake 99 5
Solution A. When you touch ice, heat is transferred from 1) your hand to the ice B. When you drink a hot cup of coffee, heat is transferred from 2) the coffee to your mouth Lecture. PLUS Timberlake 99 6
Temperature • How would you define temperature in your own words?
Temperature • Temperature – a measure of the average motion of atoms in a substance • Units of temperature are Celsius or Kelvin. • Absolute zero (0 K) is theoretical temperature where the atoms in a substance stop moving all together
Learning Check When you heat 200 g of water for 1 minute, the water temperature rises from 10°C to 18°C. 400 g 200 g If you heat 400 g of water at 10°C in the same pan with the same amount of heat for 1 minute, what would you expect the final temperature to be? 1) 10 °C 2) 14°C 3) 18°C Lecture. PLUS Timberlake 99 9
Solution 2)14°C Heating twice the mass of water using the same amount of heat will raise the temperature only half as much. 400 g 200 g Lecture. PLUS Timberlake 99 10
Heat Transfer Methods • Heat transfers in three ways: – Conduction – Convection – Radiation
Conduction is heat transfer through matter of direct contact. 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.
Convection is heat transfer by the movement of mass from one place to another. 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 is the only way heat is transferred through the emptiness of space. Radiation travels in waves 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
Heat Measurement • Heat is measured in joules or calories • A calorie is the amount of heat it takes to raise 1 gram of water 1 degree Celsius. • This is why foods rich in calories, like Cheetos, burn so well!
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 (the same distance) 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.
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 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 from getting cold.