Heat Transfer. Conduction, Convection, and Radiation. Review: Temperature

Transcription

1 Heat Transfer Conduction, Convection, and Radiation Review: Temperature! Temperature is:! The quantity that tells how hot or cold something is compared with a standard! A measure of the average kinetic energy of the molecules of a substance! Temperature IS NOT energy! If something has a higher temperature, its molecules have more energy, but temperature is not the same thing as energy

2 Review: Thermal Energy! Thermal Energy is:! The total energy (kinetic plus potential) of the particles that make up a substance.! A.K.A.: Internal Energy! Heat is: Review: Heat! The thermal energy that flows from an object at higher temperature to one at lower temperature, commonly measured in calories or Joules.! Heat is exchanged until thermal equilibrium is reached

3 A Comparison Temperature Thermal Energy Heat Measures average kinetic energy. The sum of all the kinetic energies. The flow of thermal Energy. Question 1:! If a hot object is placed next to a cold object:! Is heat exchanged?! If heat is exchanged, in what direction does heat travel?

4 Question 2:! Do warmer objects always have more thermal energy (internal energy) than cooler objects? Conduction

5 Conduction: How does this method work?! Conduction takes place within materials and between different materials that are in direct contact. Conduction: How does this method work?! Conduction is explained by collisions between atoms or molecules! Warmer atoms move more violently (they have more energy). These warmer atoms bump into nearby atoms and transfer energy, resulting in an increase in the motion of the nearby atoms.

6 Conduction: How does this method work?! Free electrons that drift through the metal jostle and transfer energy by colliding with atoms and other free electrons Conduction: What moves?! Conduction involves the transfer of energy from molecule to molecule; energy moves from one place to another but molecules do not.

7 Conduction: Heat Conductors! Materials that conduct heat well are known as heat conductors.! Metals are the best heat conductors since metals have loose outer electrons! If you touch a piece of metal that is at room temperature it often "feels cold." This is because metal is a good conductor of heat. It quickly conducts heat away from your body. Conduction: Heat Insulators! Poor conductors of heat, such as styrofoam and air, are called good insulators.! Liquids and gases are in general good insulators. Porous materials with lots of small spaces are good insulators.! A blanket on your bed does not provide your body with heat. It just slows the conduction of your body heat to the colder air.! Insulation delays heat transfer, it can not prevent it!

8 Convection Convection: How does this method work?! Convection occurs when a fluid is heated.! Fluids are both liquids and gases.

9 Convection: How does this method work?! When a fluid is heated, it expands and becomes less dense; since it is less dense, it rises.! Warmer fluid floats on top of cooler fluid.! Cooler fluid then moves to the bottom and the process continues.! Convection currents keep a fluid stirred as it is heated. Convection: What Moves! Convection involves the movement of a fluid; if there is not a fluid, convection cannot occur.

10 Convection: Example! Convection currents stir the atmosphere and produce winds.! Convection currents are produced by uneven heating of the air near the surface of the earth. Convection: Example! Land warms and cools more quickly than water. Thus convection currents are often evident at the shore in sea breezes and lands breezes.

11 Radiation Radiation: How does this method work?! Radiation is heat transfer through electromagnetic waves.! Any object that has temperature radiates heat.! Radiation is the only form of heat transfer that can occur in a vacuum.! Heat transfer by radiation takes place from everything to everything, even in empty space.

12 Radiation: What Moves! With radiation, only energy moves from the warmer object to the cooler object. Radiation: Examples! We receive lots of warmth from the sun as radiation. Go outside on a sunny day and the warmth you feel of the sun on your face is radiation.! Most of the heat we feel when near a fire is radiation.

13 Radiation: Good Absorbers are Good Emitters! Good absorbers of radiant energy are also good emitters (and poor absorbers are poor emitters.) For example, a radio antenna that is constructed to be a good emitter of radio waves will also be a good receiver of radio waves.! A black mug will allow warm liquids to cool really quickly but it will also allow cool liquids to warm really quickly. On a sunny day Earth's surface is a net absorber. At night it is a net emitter. Radiation: Wavelengths Emitted! All objects continually emit radiant energy in a mixture of wavelengths. Objects at lower temperatures emit longer waves and objects at higher temperature emit waves of shorter wavelength. Objects of everyday temperatures emit waves mostly in the longer wavelength end of the infrared region.

14 Radiation: Wavelengths Emitted! Shorter wavelength infrared waves absorbed by our skin produce the sensation of heat.! If an object is hot enough, some of the radiant energy it emits is in the form of visible light. At a temperature of about 500 C an object begins to emit the longest wavelengths we can see, red light. At about 1200 C all the different waves to which the eye is sensitive are emitted and we see an object as white hot. Radiation: Absorption and Reflection are Opposite Processes! A good absorber of radiant energy reflects very little radiant energy and appears dark. A perfect absorber reflects no radiant energy and appears perfectly black.! Good reflectors of radiant energy are poor absorbers. Light colored objects reflect more light and heat than darker colored objects. This is why wear lighter colored clothing in summer to stay cool.

15 Heat Transfer for a Mouse