Chapt 10: Transfer of Thermal Energy Flashcards
Why does temperature flow?
- Thermal energy flows from a region of higher temp to a region of lower temp.
- Only occurs if there’s a difference in temperature.
What is conduction?
What are good/poor conductors of heat?
Definition:
- The flow of thermal energy though a medium without any flow of the medium.
- Good conductors of heat (Conductors): Copper, Iron, Magnesium
- Poor conductors of heat (Insulators): Water, Glass, Plastic, Wool, Air etc.
Describe how conduction occurs in metals:
- Metals conduct heat both from vibration of particles and free electron diffusion.
When one side of the metal is heated…
- The particles at the hot end will gain KE and vibrate vigorously abt their fixed positions.
- They will collide with neighboring particles, making them vibrate more vigorously. The kinetic energy of the vibrating particles at the hot end is transferred to the neighboring particles.
- The neighboring region of the rod will become hot due to the transfer of energy.
- Hence, thermal energy has been transferred without the transfer of particles.
- The particles at the cooler end of the rod will also be vibrating vigorously, and the cooler end of the rod will also become hot.
ALTERNATIVELY:
- Free electron diffusion will take place. The free electrons at the heated end absorb thermal energy, and hence gain kinetic energy.
- The free electrons that gained kinetic energy move at greater speeds, and move to the cooler regions of the rod.
- As the electrons move, they collide with the atoms in the cooler parts of the rod, making them vibrate more vigorously. Some of the KE from the moving electrons are transferred to the atoms as well.
- Hence, thermal energy is transfered via the motion of the free electrons, and the cooler end of the rod becomes hot.
Describe how conduction occurs in non-metals.
- Non-metals conduct heat through vibration of particles only.
When one side of the material is heated…
- The particles at the hot end will gain KE and vibrate vigorously abt their fixed positions.
- They will collide with neighboring particles, making them vibrate more vigorously. The kinetic energy of the vibrating particles at the hot end is transferred to the neighboring particles.
- The neighboring region of the rod will become hot due to the transfer of energy.
- Hence, thermal energy has been transferred without the transfer of particles.
- The particles at the cooler end of the rod are also set into vigorous vibration, and the cooler end of the rod becomes hot.
Describe how conduction occurs in liquids and gases.
- The process is inefficient as the particles in liquids and gases are spaced further apart than those in solids.
- The collisions between the particles are therefore less frequent, and the transfer of thermal energy via conduction is slower.
- Hence, air and water (gas and liquids) are poor conductors of heat.
What is convection?
Definition: The transfer of thermal energy by means of convection currents in a fluid (liquid of gas), due to a difference in density.
Convection does not occur in fluids as…
convection involves the bulk movement of the fluid that carries the thermal energy. In solids, the thermal energy is transferred via vibration thru particles w/o bulk movement of the particles.
Describe how convection occurs in fluids.
In liquids:
- When the water at the bottom of the flask is heated, it expands. When the water expands, it becomes less dense.
- As the expanded water is less dense than the surrounding water, it rises to the upper region of water.
- As the upper region of water is cooler, it contracts and becomes more dense, sinking to the bottom of the flask to replace the water that has risen to the top.
-The water that sinks will get heated up, and the difference in densities of water at different regions will set up a convection current.
Same thing for gases.
What is radiation?
Definition: The transfer of thermal energy in the form of electromagnetic waves such as infrared rays without the aid of a medium.
- Radiation can occur in a vacuum.
Emission of IR VS. Absorption of IR
-> Emission of IR:
~ Temp of object will fall
~ Dull and black surfaces emit IR at a faster rate than shiny and silver surfaces.
-> Absorption of IR:
~ Temp of object will rise.
~ Dull and black surfaces absorb IR at a faster rate than shiny and silver surfaces.
What are the factors affecting the rate of emission and absorption of IR?
- Color and texture of surfaces:
-> Dull and black surfaces are better emitters and absorbers of IR compared to silver and shiny surfaces - Surface temperature:
-> The higher the temp of an object surface (as compared to the surrounding temp), the higher the rate of IR. - Surface area
-> Object w larger SA will emit/absorb IR at a faster rate.
