# Temperature and Heat
## Temperature and Its Measurement
- If two objects are in contact with one another long enough, the two objects have the same temperature.
- This begins to define _temperature_ , by defining when two objects have the same temperature.
- When the physical properties are no longer changing, the objects are said to be in _thermal equilibrium_ .
- **Two or more objects in thermal equilibrium have the same temperature.**
- This is the _zeroth law of thermodynamics_ .
The first widely used temperature scale was devised by Gabriel Fahrenheit.
Another widely used scale was devised by Anders Celsius.
The Celsius degree is larger than the Fahrenheit degree: the ratio of Fahrenheit degrees to Celsius degrees is 180/100, or 9/5.
They are both equal at -40.
The zero point on the Fahrenheit scale was based on the temperature of a mixture of salt and ice in a saturated salt solution.
The zero point on the Celsius scale is the freezing point of water.
Both scales go below zero.
Is there such a thing as _absolute zero_ ?
## What is absolute zero?
If the volume of a gas is kept constant while the temperature is increased, the pressure will increase.
This can be used as a means of measuring temperature.
A constant-volume gas thermometer allows the pressure to change with temperature while the volume is held constant.
The difference in height of the two mercury columns is proportional to the pressure.
We can then plot the pressure of a gas as a function of the temperature.
The curves for different gases or amounts are all straight lines.
When these lines are extended backward to zero pressure, they all intersect at the same temperature, _273.2_ _°_ _C_.
Since negative pressure has no meaning, this suggests that the temperature can never get lower than _273.2_ *°* _C_ , or _0 K (kelvin)_.
- Can anything ever get _colder_ than 0 K?
- No.
- Can absolute zero ever be reached?
- No.
## Fun with Liquid Nitrogen
Even colder…liquid helium
## Heat and Specific Heat Capacity
- What happens when objects or fluids at different temperatures come in contact with one another?
- The colder object gets hotter, and the hotter object gets colder, until they both reach the same temperature.
- What is it that flows between the objects to account for this?
- We use the term _heat_ for this quantity.
Heat flow is a form of energy transfer between objects.
One-hundred grams of room-temperature water is more effective than 100 grams of room-temperature steel shot in cooling a hot cup of water.
Steel has a lower _specific heat capacity_ than water.
The _specific heat capacity_ of a material is the relative amount of heat needed to raise its temperature.
- The _specific heat capacity_ of a material is the quantity of heat needed to change a unit mass of the material by a unit amount in temperature.
- For example, to change 1 gram by 1 Celsius degree.
- It is a property of the material, determined by experiment.
- The specific heat capacity of water is 1 cal/gC: it takes 1 _calorie_ of heat to raise the temperature of 1 gram of water by 1C.
- We can then calculate how much heat must be absorbed by a material to change its temperature by a given amount:
_Q = mc_ _T
**where** Q = **quantity of heat**
m = **mass**
c = **specific heat capacity**
T = **change in temperature**
# James Prescott Joule
## Joule’s Experiment and the First Law of Thermodynamics
Rumford noticed that cannon barrels became hot during drilling.
Joule performed a series of experiments showing that mechanical work could raise the temperature of a system.
In one such experiment, a falling mass turns a paddle in an insulated beaker of water, producing an increase in temperature.
- Joule’s experiments led to Kelvin’s statement of the _first law of thermodynamics_ .
- Both work and heat represent transfers of energy into or out of a system.
- If energy is added to a system either as work or heat, the internal energy of the system increases accordingly.
- **The increase in the internal energy of a system is equal to the amount of heat added to a system minus the amount of work done by the system.** **** _U = Q - W
- This introduced the concept of the _internal energy_ of a system.
- An increase in internal energy may show up as an increase in temperature, or as a change in phase, or any other increase in the kinetic and/or potential energy of the atoms or molecules making up the system.
- Internal energy is a property of the system uniquely determined by the state of the system.
-**The internal energy of the system is the sum of the kinetic and potential energies of the atoms and molecules making up the system.**
# The Flow of Heat
- There are three basic processes for heat flow:
- Conduction
- Convection
- Radiation
- In _conduction_ , heat flows through a material when objects at different temperatures are placed in contact with one another.
- Conduction
- The rate of heat flow depends on the temperature difference between the objects.
- It also depends on the _thermal conductivity_ of the materials, a measure of how well the materials conduct heat.
- For example, a metal block at room temperature will feel colder than a wood block of the exact same temperature.
- The metal block is a better thermal conductor, so heat flows more readily from your hand into the metal.
- Since contact with the metal cools your hand more rapidly, the metal feels colder.
- In _convection_ , heat is transferred by the motion of a fluid containing thermal energy.
- Convection is the main method of heating a house.
- It is also the main method heat is lost from buildings.
- In _radiation_ , heat energy is transferred by electromagnetic waves.
- The electromagnetic waves involved in the transfer of heat lie primarily in the infrared portion of the spectrum.
- Unlike conduction and convection, which both require a medium to travel through, radiation can take place across a vacuum.
- For example, the evacuated space in a thermos bottle.
- The radiation is reduced to a minimum by silvering the facing walls of the evacuated space.
**What process makes a car’s interior heat up when parked in the sun?**
**radiation**
**Why are houses insulated with material in the walls instead of just empty space?**
**To reduce convection and conduction**
**Why is this insulated material often foil-backed?**
**To limit radiation**
**Is a light-colored roof or a dark-colored roof more energy efficient?**
**Light colored roofs keep a house cooler reducing cooling costs.**
# What heat-flow processes are involved in the greenhouse effect?
# Leidenfrost Effect
# Heat Engines and the Second Law of Thermodynamics
# The laws of thermodynamics
**...are critical to making intelligent choices about energy in today’s global economy.**
# Advances in Solar Energy
# Heat Engines
- A gasoline engine is a form of a _ heat engine_ .
- Gasoline is mixed with air.
- A spark ignites the mixture, which burns rapidly.
- Heat is released from the fuel as it burns.
- The heat causes the gases in the cylinder to expand, doing work on the piston.
- The work done on the piston is transferred to the drive shaft and wheels.
- The wheels push against the road.
- According to Newton’s third law, the road exerts a force on the tires, allowing the car to move forward.
- Not all the heat from burning fuel is converted to work done in moving the car.
- The exhaust gases emerging from the tailpipe release heat into the environment.
- Unused heat is a general feature of heat engines.
# Heat Engine Example
# Heat Engines
- All heat engines share these main features of operation:
- Thermal energy (heat) is introduced into the engine.
- Some of this energy is converted to mechanical work.
- Some heat (waste heat) is released into the environment at a temperature lower than the input temperature.
# Efficiency
Efficiency is the ratio of the net work done by the engine to the amount of heat that must be supplied to accomplish this work.
# A heat engine takes in 1200 J of heat from the high-temperature heat source in each cycle, and does 400 J of work in each cycle. What is the efficiency of this engine?
33%
40%
66%
_Q_ _H_ _ _ = 1200 J
_ W _ = 400 J
_ e_ = _W _ / _ Q_ _H _
_ _ = (400 J) / (1200 J)
= 1/3 = 0.33
= <span style="color:#fa4f9f">33%</span>
# Carnot Engine
- The efficiency of a typical automobile engine is less than 30%.
- This seems to be wasting a lot of energy.
- What is the best efficiency we could achieve?
- What factors determine efficiency?
- In analogy to water wheels, Carnot reasoned that the greatest efficiency of a heat engine would be obtained by taking all the input heat at a single high temperature and releasing all the unused heat at a single low temperature.
# Carnot Engine and Carnot Cycle
- Carnot also reasoned that the processes should occur without undue turbulence.
- The engine is completely _reversible_ : it can be turned around and run the other way at any point in the cycle, because it is always near equilibrium.
- This is Carnot’s ideal engine.
- The cycle devised by Carnot that an ideal engine would have to follow is called a _Carnot cycle_ .
- An (ideal, not real) engine following this cycle is called a _Carnot engine_ .
# Carnot Efficiency
The efficiency of Carnot’s ideal engine is called the _Carnot efficiency_ and is given by:
This is the _maximum efficiency possible_ for _any_ engine taking in heat from a reservoir at absolute temperature _T_ _H_ and releasing heat to a reservoir at temperature _T_ _C_ .
Even Carnot’s ideal engine is less than 100% efficient.
# A steam turbine takes in steam at a temperature of 400C and releases steam to the condenser at a temperature of 120C. What is the Carnot efficiency for this engine?
30%
41.6%
58.4%
70%
_T_ _H_ _ _ = 400C = 673 K
_ T_ _C_ _ _ = 120C = 393 K
_ e_ _C_ = _(T_ _H_ _ - T_ _C _ _) _ / _ T_ _H _
_ _ = (673 K - 393 K) / (673 K)
= 280 K / 673 K
= <span style="color:#fa4f9f">0.416 = 41.6%</span>
# Second Law of Thermodynamics
**No engine, working in a continuous cycle, can take heat from a reservoir at a single temperature and convert that heat completely to work.**
Therefore, no engine can have a greater efficiency than a Carnot engine operating between the same two temperatures.
# Refrigerators, Heat Pumps, and Entropy
If a heat engine is run in reverse, then work _W _ is done _on _ the engine as heat _Q_ _C_ is removed from the lower-temperature reservoir and a _greater_ quantity of heat _Q_ _H_ is released to the higher-temperature reservoir.
A device that moves heat from a cooler reservoir to a warmer reservoir by means of work supplied from some external source is called a _ heat pump_ .
The first law of thermodynamics requires that, for a complete cycle, the heat released at the higher temperature must equal the energy put into the engine in the form of both heat _and work_ .
The engine returns to its
initial condition at the end
of each cycle; the internal
energy of the engine does
not change.
More heat is released at
the higher temperature
than was taken in at the
lower temperature.
# Refrigerators and Heat Pumps
A _refrigerator_ is also a form of a heat pump.
It also moves heat from a cooler reservoir to a warmer reservoir by means of work supplied from some external source.
It keeps food cold by pumping heat out of the cooler interior of the refrigerator into the warmer room.
An electric motor or gas-powered engine does the necessary work.
# Another Statement of The Second Law of Thermodynamics
Heat has a natural tendency to flow from hotter objects to colder objects. This can be expressed in a second statement of the second law of thermodynamics:
**Heat will not flow from a colder body to a hotter body unless some other process is also involved.**
For example, work can be used to pump heat against its usual direction of flow.
# Entropy
Heat can be removed from a high-temperature source either by spontaneous flow to a low-temperature reservoir or by being used to run a heat engine.</span>
If the engine is a Carnot engine, the process is completely</span> _reversible_ and the maximum possible work has been obtained from the available heat.</span>
- If the heat simply flows from hot to cold spontaneously, the process is _irreversible_ and the energy is not converted to useful work.
- In the irreversible process, *we lose some ability to do useful work*.
- _Entropy_ is the quantity that describes the extent of this loss.
- Entropy is sometimes defined as _a measure of the disorder of the system*.
- The entropy of a system increases any time the disorder or randomness of the system increases.
## A Third Statement of The Second Law of Thermodynamics
- Entropy remains constant in reversible processes but increases in irreversible processes.
- The entropy of a system decreases only if it interacts with some other system whose entropy is increased in the process.
- This happens, for example, in the growth and development of biological organisms.
- **The entropy of the universe or of an isolated system can only increase or remain constant. Its entropy can never decrease.**
The thermal energy of a gas consists of the kinetic energy of the molecules.
The velocities of these molecules are randomly directed. Only some of them move in the proper direction to push the piston to produce work.
- The basic disorganization of heat energy is responsible for the limitations encompassed by the three statements of the second law.
- The natural tendency of entropy to increase can only be countered by introducing energy into the system:
- For example, solar energy to support biological processes on earth.
- For example, work done to organize a room’s clutter.
# Perpetual Motion and Energy Fluids
- A _perpetual-motion machine of the first kind_ is a _proposed_ engine of machine that would violate the first law of thermodynamics.
- **It puts out more energy as work or heat than it takes in.**
- Buyer beware:
- Where is the energy coming from?
- How can the machine put more energy out than went in?
- A _perpetual-motion machine of the second kind_ does not put more energy out than goes in.
- Instead, it may claim to be able to take heat from a reservoir at a single temperature and convert it completely to work.
- This violates the second law of thermodynamics: some heat must be released into a lower-temperature reservoir.
- Or, it may simply claim a higher efficiency than the Carnot efficiency (for the available temperature difference). Such claims may be put forward by well-intentioned but misguided persons. Some are promoted by charlatans.
### Is it possible for a heat engine to operate as shown in the following diagram?
*Yes*
*No, not ever*
*Only if it is a Carnot engine*
*It depends*
This is a perpetual-motion machine of the second kind. According to the second law of thermodynamics, heat can never be completely converted to work without some waste heat released.
*Yes*
*No, not ever*
*Only if it is a Carnot engine*
*It depends*
This is a perpetual-motion machine of the first kind. According to the first law of thermodynamics, you can never get out more energy in work than you put in as heat.
*Yes*
*No, not ever*
*Only if it is a Carnot engine*
*It depends*
*This diagram is correct!!!*
More heat is transferred to the hot reservoir than is taken in from the cold one, with work being done.
## Thermal Power Plants and Energy Resources
The most common way of producing electric power in this country is a _thermal power plant_ that uses some form of heat engine, whether fueled by <span style="color:#ffbf8a">coal</span> , <span style="color:#ffbf8a">oil</span> , or <span style="color:#ffbf8a">natural gas</span> (fossil fuels).
Thermodynamics plays an extremely important role in any discussion of the use of energy.
As worldwide economic development continues and fossil-fuel resources are depleted, questions of the optimal uses of energy resources become critical.
_Wise decisions depend on the participation of an informed, scientifically literate citizenry._
## Thermal-Electric Power Plant
Fossil fuel (coal, oil, natural gas) is burned to release heat that causes the temperature of water and steam to increase.
Hot steam is run through a turbine (a heat engine) that turns a shaft connected to an electric generator.
Electricity is transmitted through power lines to consumers.
In practice, only about half the thermal energy released in burning coal or oil is converted to mechanical work or electrical energy.
The rest must be released into the environment at temperatures too low for running heat engines. Cooling towers transfer this waste heat into the atmosphere, or heated water is released into a river.
# Alternatives to Fossil Fuels
- **Nuclear power plants**
- _Lower thermal efficiencies_
- _More heat released into environment_
- _No carbon dioxide and other greenhouse gases released_
- _Nuclear waste must be processed and disposed of_
- **Geothermal Energy**
- _Heat from the interior of the earth, such as hot springs and geysers._
- _Water temperature not hot enough to yield a high efficiency_
Geysers Power Plant in California
- **Sun as energy source**
- _Warm ocean currents_
- _Solar power_
Mirrors focus sunlight to heat a central boiler at a solar-thermal power plant