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HEAT TRANSFER
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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. Heat Transfer
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Heat energy travels from hotter places towards cooler places
Heat energy travels from hotter places towards cooler places. There are 3 ways that heat can travel: Conduction Convection Radiation HEAT TRANSFER
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HEAT TRANSFER
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Conduction is the transfer of heat energy from atom to atom by physical contact.
It occurs mostly in solids. CONDUCTION
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CONDUCTION
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Conduction – transfer of thermal energy by collisions with NO overall transfer of matter
Particles vibrate and pass energy to nearby particles, like bumper cars hitting each other Works best in solids, as the particles are touching. Thermal conductors – conduct thermal energy well Ex: Metals Thermal insulators – do NOT conduct thermal energy well Ex: Air and other gases CONDUCTION
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Conduction in solids is like a bucket brigade
Conduction in solids is like a bucket brigade. Particles (people) stay in one place, but the energy (water) is transferred from one position to the next. CONDUCTION
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Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often. In most solids, conduction occurs as particles vibrate in place and push on each other. In metals, conduction is faster because some of the metal’s electrons are free to move about. Conduction
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Thermal Conductors A material that conducts thermal energy well.
Example 1: A wire rack on a grill can burn you because the metal conducts thermal energy so quickly. Example 2: Pots and pans often are made of copper or aluminum because these are good conductors. Thermal Conductors
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A thermal conductor doesn't have to be hot!
Why does a tile floor feel colder than a wooden floor? Both floors are at room temperature. But the tile feels colder because it is a better conductor and transfers thermal energy rapidly away from your skin. A thermal conductor doesn't have to be hot!
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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.
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Thermal Insulator Example 1: Wood heats up slowly because it is
A thermal insulator conducts thermal energy poorly Example 1: Wood heats up slowly because it is a poor conductor of thermal energy; therefore a good insulator. Example 2: Air found in between panes of a double-paned window insulates the house in winter/keeps heat out in summer Thermal Insulator
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Examples 3 & 4: Wool Styrofoam cups Both use trapped air to SLOW DOWN conduction
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Convection is the transfer of heat energy by the movement of the atoms themselves. It occurs in fluids (gases and liquids). CONVECTION
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Convection The transfer of thermal energy when particles of a fluid move from one place to another.
When fluids heat up they expand, and thus become less dense. Fluids less dense than surroundings rise. Fluids more dense than surroundings sink. Convection
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The particles spread out and become less dense.
Convection What happens to the particles in a liquid or a gas when you heat them? Liquids and gases are called fluids The particles spread out and become less dense. . This causes expansion and fluid movement.
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Water movement Convection current Cooler water sinks Hot water rises
Cools at the surface Convection current Cooler water sinks Hot water rises
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Convection is like people (particles) carrying water (energy) to the fire. There is movement of the particles AND of the heat. Convection
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CONVECTION CURRENTS
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CONVECTION CURRENT
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Convection – transfer of thermal energy when particles of a fluid MOVE from one place to another.
Particles transfer thermal energy from hot to cold. When fluids heat up they expand, and thus become less dense. Fluids less dense than surroundings rise. Fluids more dense than surroundings sink. CONVECTION
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The third method of heat transfer
There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection. How does heat energy get from the Sun to the Earth? RADIATION ?
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RADIATION Radiation is the transfer of heat energy by waves.
NO atoms, molecules or matter are needed for radiation heat transfer. Heat can be transferred through the vacuum of outer space. RADIATION
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Radiation moves across space without being carried on particles (no buckets). It’s like a fire hose squirting water (energy). RADIATION
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Radiation – transfer of energy by electromagnetic waves moving through space
No matter required – can pass through a VACUUM (Space!) All objects radiate energy, as all objects have some heat (nothing is at 0 °K!) Radiation is often called radiant heat. What do you think happens to the rate of radiation as an object’s temperature increases? RADIATION
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Radiation experiment Four containers were filled with warm water. Which container would have the warmest water after ten minutes? Shiny metal Dull metal Dull black Shiny black The shiny metal container would be the warmest after ten minutes because its shiny surface reflects heat back into the container so less is lost. The dull black container would be the coolest because it is the best at radiating heat.
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Absorption experiment
Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes? Shiny metal Dull metal Dull black Shiny black 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
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RADIANT ENERGY What can objects “do” with radiant energy?
Absorb it – when radiation is absorbed thermal energy increases (dark surfaces) 2. Reflect it – bounce it back (shiny things) 3. Transmit it – let it though (windows in a greenhouse) RADIANT ENERGY
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RADIATION HEATS THE EARTH
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RADIATION
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Body heat is thermal energy radiated in the infrared portion of the spectrum
RADIATION
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Popcorn- How can we cook it?
1) Radiation→ put popcorn bag in microwave 2) Convection → put popcorn in air popper 3) Conduction → put popcorn and oil in a pan on the stove
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