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Energy that flows from something warm to
something cooler
A hotter substance gives KE to a cooler one
Heat is the transfer of energy between two
objects due to temperature differences
Heat is measured in Joules (J) because it is a form
of energy
Heat
Heat
The name of the transfer process is heat. What gets transferred is energy.
Heat is NOT a substance although it is very convenient to think of it that way.
In fact, it used to be thought that heat was a substance.
There is a circular nature to the definitions used: (a) energy does work or produces heat, but(b) heat is a transfer of energy.
Temperature
Temperature is the property which determines the direction heat will flow when two objects are brought into contact
Based on the motion of the particles in a substanceFast motion = high temperatureSlower motion = lower temperature
Relatively describes how the particles collide with the surface of the thermometer Collisions transfer energy
Temperature Scales
FahrenheitCommon in the USFreezing point of water is 32ºFBoiling point of water is 212ºF
CelsiusCommon in the rest of the world (SI)Freezing point of water is 0ºCBoiling point of water is 100ºC
KelvinUsed for scienceFreezing point of water is 273KBoiling point of water is 373K
Temperature and the Phases of Matter
In the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees
The Celsius scale divides the difference between the freezing and boiling points of water into 100 degrees (instead of 180).
Temperature and the Phases of Matter
All thermometers are based on some physical property (such as color or volume) that changes with temperature.
A thermometer is a device that changes its electrical resistance as the temperature changes.
A thermocouple is another electrical sensor that measures temperature.
Heat and Thermal Energy
Temperature is NOT the same as thermal energy.
Thermal energy is energy stored in materials because of differences in temperature.
The thermal energy of an object is the total amount of random kinetic energy for all the atoms in the object.
Remember, temperature measures the random kinetic energy of each atom.
Heat and Thermal Energy
Imagine heating a cup of coffee to a temperature of 100°C.
Next think about heating up 1,000 cups of coffee to 100°C.
The final temperature is the same in both cases but the amount of energy needed is very different.
First Law of Thermodynamics
Energy loss is equal to energy gain.
Second Law of Thermodynamics
Energy and Heat flow
There Energy, in the process we call heat or heat flow, is constantly flowing into and out of all objects, including living objects.
Heat flow moves energy from a higher temperature to a lower temperature.
The bigger the difference in temperature between two objects, the faster heat flows between them.
When temperatures are the same there is no change in energy due to heat flow.
Radiation and Conduction are the two methods of heat transfer. Convection is a special type of conduction.
Heat has the units of energy; heat flow has the units of power.
Heat flow is energy moving. It has the same units as power - energy per unit time .
It means that during the given amount of time, during which heat is flowing, a certain amount of energy is transferred or moved from one place to another place
Energy and Heat flow
Three easy things to know about the way heat flows:1) There has to be a temperature difference. Energy
only flows as heat if there is a temperature difference. 2) Energy as heat flows from a higher temperature to a
lower temperature.3) The greater or larger the difference in temperature,
the faster the energy flows.
Heat flow
The science of how heat flows is called heat transfer.
There are three ways heat transfer works: conduction, convection, and radiation.
Heat flow depends on the temperature difference.
Heat flow
Heat flow
Heat Conduction
Conduction occurs when two object at different temperatures are in contact with each other.
Heat flows from the warmer to the cooler object until they are both at the same temperature.
Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object.
Heat Conduction
Conduction is the transfer of heat through materials by the direct contact of matter.
Dense metals like copper and aluminum are very good thermal conductors.
A thermal insulator is a material that conducts heat poorly.
Thermal Conductivity
The thermal conductivity of a material describes how well the material conducts heat.
Heat Conduction Equation
PH = k A (T2 -T1) L
Area of cross section (m2)
Length (m)
Thermal conductivity(watts/moC)
Heat flow (watts)
Temperaturedifference (oC)
Convection
Convection is the transfer of heat by the motion of liquids and gases.– Convection in a gas occurs
because gas expands when heated.
– Convection occurs because currents flow when hot gas rises and cool gas sink.
– Convection in liquids also occurs because of differences in density.
Convection
When the flow of gas or liquid comes from differences in density and temperature, it is called free convection.
When the flow of gas or liquid is circulated by pumps or fans it is called forced convection.
Convection
Both free and forced convection help to heat houses and cool car engines.
Convection
Convection depends on speed and surface area. Motion increases heat transfer by convection in all
fluids. If the surface contacting the fluid is increased, the
rate of heat transfer also increases.
Convection and Sea Breezes
On a smaller scale near coastlines, convection is responsible for sea breezes.
During the daytime, land is much hotter than the ocean.
A sea breeze is created when hot air over the land rises due to convection and is replaced by cooler air from the ocean.
At night the temperature reverses so a land breeze occurs.
Heat Convection Equation
PH = h A (T2 -T1)
Area contacting fluids (m2)
Heat transfer coefficient(watts/m2oC)
Heat flow (watts)
Temperaturedifference (oC)
Radiation
Radiation is heat transfer by electromagnetic waves.
Thermal radiation is electromagnetic waves (including light) produced by objects because of their temperature.
The higher the temperature of an object, the more thermal radiation it gives off.
• Radiation
Radiation
Heat from the sun is the best example of radiation.
The sun is rays reach the earth without having any material medium in between the earth and the sun.
The electromagnetic rays carry energy from the sun is surface to the earth and heat up the earth and its atmosphere.
Radiant Heat
We do not see the thermal radiation because it occurs at infrared wavelengths invisible to the human eye.
Objects glow different colors at different temperatures.
The graph of power versus wavelength for a perfect blackbody is called the blackbody spectrum.
Radiant Heat
A perfect blackbody is a surface that reflects nothing and emits pure thermal radiation.
Radiant Heat
The total power emitted as thermal radiation by a blackbody depends on temperature (T) and surface area (A).
Real surfaces usually emit less than the blackbody power, typically between 10 and 90 percent.
The Kelvin temperature scale is used in the Stefan-Boltzmann formula because thermal radiation depends on the temperature above absolute zero.
Stefan-Boltzmann formula
P = s AT4
Surface area (m2)
Stefan-Boltzmann constant5.67 x 10-8 watts/m2K4)
Power (watts)
Absolute temperature (K)
