Thermal Physics lec 3-5 Flashcards
Define linear thermal expansion
is the increase in size of an object with an increase in its temperature
- is the consquence of the change in the average between atoms in an object
true or false; the amont of expansion is proportional to the initial length
true
true or false; the fractional increase in length is proportional to the change in temperature
true
What is a bimetallic strip
a strip of metal, who’s faces are made up of different metals; so that they have different average co- efficients of expansion (this can be used in thermostats)
What is the area of expansion proportional to
the change in area is proportional to the original area and to the change in temperature
What is volume of expansion proportional to
the change in volume is proportional to the orginal volume and to teh cahnge in temperature
Beta is the coefficient of volume expansion (true or false)
true
3alpha = beta for volume expansion
true
What are the three main modes of heat transfer
conduction, convection, radiation
In general what is conduction
the transfer of heat via the collision of particles (physical contact; matter does not move)
- it is an exchange of energy between microscopic particles by collisions
In general what is convection
heat is transferred via macroscopic movement of matter (fluids)
In general what is radiation
heat is transferred by electromagnetic waves that are emitted or absorbed
what happens to a particle with less energy during a collision
it will gain energy; and become more energetic
Why are metals good conductors?
- they contain large numbers of electrons taht are relatively free to move
- the can transport energy form one region to another
What does it mean by a poor conductor
doesn’t transfer energy as efficiently between microscopic particles (good insulator)
What does the rate of conduction depend on?
the characteristics of the substance
What does the temperature gradient of a material measure delta T/ delta x
it measures the RATE at which temperature varies with position
what does the proportionality constant k mean
the thermal conductivity of the substance
What is heat flow measured in (rate of work)
Watts (W)
true or false; good conductors have high k values
true
true or false; good insulators have high k values
false; they have low k values; this is because k is the proportionality constant that represents the thermal conductivity of the substance
true or false the larger the thermal conductivity of a substance the faster heat will be asbsorbed from our bodies
true
Define thermal convection
energy transferred by movement of a substance
- is the flow of heat accompanying the flow of a fluid around an object
What type of convection: when movement results from differences in density?
natural/ passive convection
What type of convection: when movement is forced by a fan or a pump
forced convection
in natural convection what causes the flow of the fluid
the flow of the heat
in forced convection, what happens to the flow of heat?
the flow of heat is accelerated by rapid movement of cool fluid across the warm object
in forced convection, what happens to the flow of heat?
the flow of heat is accelerated by rapid movement of cool fluid
What does q symbolize
the coefficient of convective heat transfer
What does q symbolize
…
What does q depend on (object)?
the object:
- shape
- orientation
What does q depend on (fluid)
fluid
- density
- specific heat capacity
- thermal conductivity
- viscosity
Define radiation
exists as electromagnetic waves ranging from long (radio waves) to short wave lengths (x rays)
What is the color of long waves
red
What is the color a short wave
violet
What is radiation (its temperature) measured in
kelvin
What is terrestrial radiation
it comes from the earths’ surface, in the form of infrared waves; below our threshold of sight
What are conditions for the emission of radiant energy
- every object above absolute zero
What type of radiation comes from the surface of the sun
solar radiation; electromagnetic radiation
At room temperature; what is the emission?
infrared
What is the color of the light above 500 C
red
What is the color of light above 600C
yellow
what is the color of light at 1500C
white
What is a net absorber
absorbs more than it emits
What is net emitter
emits more than in absorbs
true or false; the net absorption is relative to temperature of surroundings
true
a good absorber is good emitter?
true
poor absorbers are poor emitters?
true
Question: a hot pizza placed in the snow is a net emitter?
the net energy glow foes from higher to lower temperature. Since the pizza is hotter than the snow, emission is greater than absorption, so it’s a net emitter
What does reflection of radiant energy mean?
- opposite of absorption of radiant energy
Why does Darkness occur?
eg. occurs in a closed box
- darkness is often due to the reflection of light back and forth many times partially absorbing with each reflection
true or false; good reflectors are poor absorbers
true
What is an ideal absorber
- defined as an object that absorbs all of the energy of the incident on it ; this object is called a black body
what does ‘e’ stand for
emissivity; which is equal to absorptivity
What is black body
absorbs all energy incident on it
true or false; an ideal absorber is also an ideal radiator of energy
true
what is an ideal reflector e=0
an ideal reflector absorbs NONE of the energy incident on it
Which law do you use to measure the rate of energy transfer of a black body
Stefan’s Law
What happens when an object is in equilibrium with its surroundings
it radiates and absorbs at the same rate; and its temperature will not change
what is emissivity
reduces the rate of emission
What is light made of
electromagnetic waves
true or false; the wavelength at which the maximum occurs is inversely proportional to the absolute temperature
true
What is Dewar flask
- is a container designed to minimize the energy losses by conduction, convection and radiation
- it is used to store either cold or hot liquids for long periods of time
What causes the greenhouse effect
- energy absorbed from the sun, part re-radiated by the earth as longer wavelength terrestrial radiation
What does transparency depend on?
the wavelength of an object
What does it mean by terrestrial radiation
- absorbed by atmospheric gases and re-emitted as long wavelength terrestrial radiation back to Earth
Newtons’ Law of cooling
proportional to the temperature difference, delta T, between the object and its surroundings
What is an example of Newton’s Law of cooling
eg. Hot apple pie cools more each minute in a freezer than if left on the kitchen table (because their temperature difference is a lot)
true or false; Newton’s law of cooling also applies to warming
true; object cooler than its surroundings warms up at a rate proportional to change in T
