Chapter 16: Heat Transfer Flashcards
Object in thermal contact at different temperature tend to reach a common temperature in three ways
1) Convection
2) Conduction
3) Radiation
1) Convection
The transfer of heat through a fluid (liquid or gas) caused by molecular motion
2) Conduction
The transfer of heat or electric current from one substance to another by direct contact
3) Radiation
Energy that is radiated or transmitted in the form of rays or waves or particles
Conduction (2nd)
Transfer of internal energy by electron and molecular collisions within a substance, especially a solids
Conduction occurs when two objects of different temperatures come in
Direct contact with each other
Heat is transferred from the high temperature object toward the low temperature object from
The collisions between the particles at the contact area of the objects
Collisions continues to transfer energy until the temperature of the two objects are
Identical
A blacksmith heating up a sword in hot coals, and the heat
Transferring up through the metal
A lizard sunbathing on a warm rock to
Raise their body temperature
Swiftly are transfer
Heat
Metals are excellent conductors tend to
Have more free electrons ready to transfer energy via collisions
Fluids tend to be
Poor conductors
Poor conductors are
Great insulators
Insulators tightly held electrons
Vibrate in place and transfer energy slowly
Insulators impede the transfer of
Heat
Insulators wool, paper, feathers, and snow are
Great insulators they trap air
Air are both
1) Poor Conductor
2) Good Insulator
The rate of heat flow from the hot air to your relatively cool hand is
Low
Insulation does not prevent the flow of
Internal energy
Insulation slow the rate at which
Internal energy flows
Rock wool or fiberglass between walls slows the transfer of internal energy from a warm house to
A cool exterior in winter, and the reverse in summer
Snow patterns on the roof of a house shows areas of
Conduction and insulation
Bare patches show where heat from inside has leaked through the
Roof and melted the snow
Convection
Transfer of heat involving only bulk motion of fluids
Convection occurs when heat is transferred from
The movement of heated fluids (liquids and gases)
Convection heated fluids expand and become
Less dense
Lower density fluid rises while the higher density fluid sinks resulting in the formation of
Convection currents
Convection currents allow the heat to be
Spread throughout the fluid
Warm air rises expands, becomes less dense, and is
Buoyed upward (Archimedes)
It rises and cools until its density
Equals that of the surrounding air
Convection cooling by
Expansion that opposite to the warming that occurs when air is compressed
A combination of air expansion and mixing with
Cooler surrounding air
Although warm air rises, why are mountaintops colder and snow covered, while the valleys below are relatively warm and green?
1) Warm air cools when rising
2) There is a thick insulating blanket of air above the valley
Both of the answers
Winds result of
Unevent heating of the air near the ground
Wind’s absorption of Sun energy occurs more readily on
Different parts of Earth’s surface
Sea breeze
The ground warms more than water in the daytime
Warm air close to the ground rises and is replaced by
Cooler air from above the water
Radiation
Transfer of energy from the Sun through empty space
Radiation occurs when electromagnetic waves are
Transmitted from an origin to the space surrounding it
Unlike with conduction and convection and it does not rely upon any contact between
The heat source and the heated object
All objects carry energy in the form of
Electromagnetic waves and the hotter the object, the more it radiates
Radiant energy emitted by
All bodies, transferred through space
Radiant energy exists as
Electromagnetic waves ranging from long (radio waves) to short wavelengths (X-ray/Gamma)
Radiant energy includes visible light, ranging from
Longer waves (red) to shorter waves (violet)
Wavelength of radiation related to frequency of
Vibration of an electromagnetic wave
Wavelength of radiation are
Inversely proportional
Low frequency vibration implies
Long wavelength waves
High frequency vibration implies
Short wavelength waves
Frequency of radiation is
Proportional to the absolute temperature of the source
If an object is hot enough, some of the radiant energy it emits is
In the range of visible light
Every object above absolute zero radiates: Sun produces solar radiation including
Visible light
Earth produces terrestrial radiation in the form of
Infrared waves
Frequencies beneath the threshold for our eyes to
Detect as visible light
The surface of Earth loses energy to outer space due mostly to
Radiation
Range of temperature of radiating objects
1) Room temperature emission is in the infrared (20 to 22 degrees Celcius)
2) Temperature above (500 degrees Celcius), red light emitted, longest waves visible
3) About (600 degrees Celcius), yellow light emitted
4) At (1500 degrees Celcius), object emits white light whole range of visible light
The colors of stars
Hottest (Blue, blue white), and Mid (Yellow), and (Orange, Red) for Coldest
Why body glows with electromagnetic waves?
Sun and the Earth
The absorption of radiant energy occurs along with
Emission of radiant energy
Any material that absorbs more than it emits is
A net absorber
Any material that emits more than it absorb is
A net emitter
Net absorption or emission is
Relative to temperature of surroundings
Good absorbers are
Good emitters
Poor absorbers are
Poor emitters
If a good absorber of radiant energy were a poor emitter, its temperature compared with
Its surroundings would be higher
A hot pizza placed in a snow is
A net emitters
Reflection of radiant energy this is a opposite to
Absorption of radiant energy
Any surface that reflects very little or no radiant energy
“Looks” dark
Radiation openings appear black because
1) Light that enters them is reflected on the inside walls many times
2) Partly absorbed at each reflection are good absorbers
Good reflectors are
Poor absorbers
A black object readily
Absorbs visible light
Newton’s Law of Cooling were the rate of cooling is
Approximately proportional to the temperature difference and heat between the object and its surrounding
Newton’s Law of Cooling also applies to
Rate of warming
An object cooler than its surroundings warm up at
A rate proportional to heat proportional
Frozen food will warm faster in
A warm room than in a cold room (defrosting)
The Greenhouse effect named after a similar temperature raising effect in
Florists greenhouse
Short wavelength radiation from the Sun is
Transmitted through the glass
Long wavelength reradiated energy is
Not transmitted out through the glass and is trapped inside
All things radiate at frequencies (and therefore wavelengths) determined by
The temperature of the emitting object
Things (such as glass) can be transparent to some wavelengths but
Opaque or reflective to others
Sun’s rays are very short and pass through
The car’s windows
Absorption of Sun’s energy
Warms the car interior
Car interior radiates its own waves, which are longer and do not transmit through
The window
Car’s radiated energy remains inside, making the car’s interior
Very warm
Global warming: Energy is absorbed from
The sun and partly reradiated by Earth as longer wavelength terrestrial radiation
Global warming: Terrestrial radiation is absorbed by
Atmospheric gases and re-emitted as long wavelength terrestrial radiation back to Earth
Global warming: Reradiated energy unable to
Escape, so warming of Earth occurs
The greenhouse gases that contribute to
Global warmings absorb more infrared radiation than visible
