Heat and Work
1. Heat
• Heat is the transfer of thermal energy between systems by
virtue of temperature difference
2. Work
• Work is the transfer of mechanical energy
• Both heat & work = ways of transferring energy
• “Bodies contain internal energy and not heat”
• Heat and work represent energy flows to or from a system
4/13/2025
1
Heat
▪
Heat, like work, is recognized as energy in transit
▪
We know from experience that a hot object brought into contact with a cold object
becomes cooler, whereas the cold object becomes warmer.
▪
A reasonable view is that “Something” is transferred from the hot object to the
cold one, and we call that something Heat (Q)
▪
Thus, we say that heat always flows from a higher temperature to a lower one
▪
This leads to the concept of temperature as the driving force for the transfer of
energy as heat.
▪
When no temperature difference exists, no spontaneous heat transfer occurs, a
condition of thermal equilibrium
▪
In the thermodynamic sense, heat is never regarded as being stored within a body
▪
Like work, it exists only as energy in transit from one body to another; in
thermodynamics, from system to surroundings.
4/13/2025
2
Heat…
▪
Like work, it exists only as energy in transit from one body to another; in thermodynamics,
from system to surroundings.
▪
When energy in the form of heat is added to a system, it is stored not as heat but as kinetic
and potential energy of the atoms and molecules making up the system.
▪
In spite of the transient nature of heat, it is often viewed in relation to its effect on the
system from which or to which it is transferred
▪
Until about 1930 the definitions of units of heat were based on temperature changes of a unit
mass of water
▪
Thus, the calorie was defined as that quantity of heat which, when transferred to one gram of
water, raised its temperature one degree Celsius
▪
With heat now understood to be a form of energy, its SI unit is the Joule (N.m).
▪
The SI unit of power is the watt, symbol W, defined as an energy rate of one joule per second.
▪
The specific heat of a substance is the heat required in calories to raise the temperature of 1
gram of the substance by 1 degree Celsius
4/13/2025
3
Joule’s Experiments
▪
The present-day concept of heat developed following crucial experiments carried
out in the1840s by James P. Joule
▪
In the most famous series of measurements, he placed known amounts of water, oil,
or mercury in an insulated container and agitated the fluid with a rotating stirrer
▪
The amounts of work done on the fluid by the stirrer and the resulting temperature
changes of the fluid were accurately and precisely measured
▪
Joule showed that for each fluid a fixed amount of work per unit mass was required
for each degree of temperature rise caused by the stirring, and that the original
temperature of the fluid was restored by the transfer of heat through simple contact
with a cooler object
▪
These experiments demonstrated the existence of a quantitative relationship
between work and heat, and thereby showed that heat is a form of energy
4/13/2025
4
Internal Energy
▪ In experiments like those of Joule, energy added to a fluid as work is
later transferred from the fluid as heat
▪ Where does this energy reside after its addition to, and before its
transfer from the fluid?
▪ A rational answer to this question is that it is contained within the
fluid in another form, which we call internal energy
▪ The internal energy of a substance does not include energy that it
may possess as a result of its gross position or movement as a whole
▪ Rather it refers to energy of the molecules comprising the
substance
4/13/2025
5
Internal Energy….
▪
Because of their ceaseless motion, all molecules possess kinetic energy of
translation (motion through space); except for monatomic substances, they also
possess kinetic energy of rotation and of internal vibration
▪
The addition of heat to a substance increases molecular motion, and thus causes an
increase in the internal energy of the substance
▪
The internal energy of a substance also includes the potential energy associated
with intermolecular forces
▪
Molecules attract or repel one another, and potential energy is stored through these
interactions, just as potential energy of configuration is stored in a compressed or
stretched spring
▪
On a sub-molecular scale, energy is associated with the interactions of electrons and
nuclei of atoms, which includes the energy of chemical bonds that hold atoms
together as molecules
4/13/2025
6
Internal Energy….
▪ This energy is named internal to distinguish it from the kinetic and potential
energy associated with a substance because of its macroscopic position,
configuration, or motion, which can be thought of as external forms of energy
▪ It cannot be directly measured; there are no internal-energy meters. As a result,
absolute values are unknown
▪ However, this is not a disadvantage in thermodynamic analysis because only
changes in internal energy are required
▪ Internal energy is a principal property of the thermodynamic state, while heat
and work are modes of energy transfer by which a process may change this
state
▪ A change of internal energy of a system may be achieved by any combination
of heat added or removed and work performed on or by the system
4/13/2025
7
Task
• Read about:
1. Joule’s Experiments
2. Molar volume
3. Specific volume
4. Moles
5. Molar mass
6. Kinetic energy
7. Potential energy
4/13/2025
8
Phase and Phase diagrams
▪ A phase is a quantity of matter that is homogeneous
throughout
▪ This refers to the state of the substance such as solid, liquid or
gas
▪ Phase relations are commonly described graphically in terms
of phase diagrams
▪ Each point within the diagram indicates a particular
combination of pressure and temperature, as well as the phase
or phases that exist stably at this pressure and temperature
4/13/2025
9
Phase Diagrams
▪ A phase diagram is a graphical representation of the various
phases of a substance or mixture of substances that coexist in
thermodynamic equilibrium, and undergo phase changes under
different working conditions, such as temperature, pressure, or
volume
▪ A phase diagram is a type of chart used to show conditions at
which thermodynamically distinct phases occur and coexist at
equilibrium
▪ Phase diagrams also referred to as Thermodynamic Property
Diagrams
4/13/2025
10
Phase Diagrams…
4/13/2025
11
Phase Diagrams…
4/13/2025
12
Phase Diagrams…
4/13/2025
13
Phase Diagrams…
4/13/2025
14
Phase Diagrams…
Terms used in Phase Diagrams
1) Critical pressure (PC)
▪ This is the vapor pressure of a fluid at the critical temperature above which
distinct liquid and gas phases do not exist
▪ As the critical temperature is approached, the properties of the gas and
liquid phases become the same, resulting in only one phase
2) Critical temperature (TC)
▪ The highest possible temperature value at which a substance can exist as a
liquid
▪ The critical temperature for a pure substance is the temperature above
which the gas cannot become liquid, regardless of the applied pressure
4/13/2025
15
Phase Diagrams…
3) Saturated liquid
▪ A liquid that is about to vaporize
▪ At 1 atm and 20°C, water exists in the liquid phase (compressed liquid)
▪ At 1 atm pressure and 100°C, water exists as a liquid that is ready to
vaporize (saturated liquid)
4) Saturated vapor
▪ A vapor that is about to condense
5) Compressed liquid (subcooled liquid)
▪ A liquid that it is not about to vaporize
4/13/2025
16
Phase Diagrams…
6) Saturated liquid–vapor mixture
• The state at which the liquid and vapor phases coexist in equilibrium
7) Superheated vapor
▪ A vapor that is not about to condense (i.e., not a saturated vapor)
8) Saturation temperature
▪ The temperature at which a pure substance changes phase at a given
pressure e.g. the temperature at which water starts to boil, and it depends on
the pressure; therefore, if the pressure is fixed, so is the boiling
temperature. Water boils at 100oC at 1 atm pressure
9) Saturation pressure (Psat)
▪ The pressure at which a pure substance changes phase at a given
temperature
4/13/2025
17
Phase Diagrams…
10) Latent heat
▪ The amount of energy absorbed or released during a phase-change process
11) Latent heat of fusion
▪ The amount of energy absorbed during melting
▪ It is equivalent to the amount of energy released during freezing
12) Latent heat of vaporization
▪ The amount of energy absorbed during vaporization and it is equivalent to
the energy released during condensation
▪ The magnitudes of the latent heats depend on the temperature or pressure at
which the phase change occurs
4/13/2025
18
Equation of state (EOS)
▪ An equation of state is a relation between state variables
pressure, volume and temperature (PVT)
▪ It's particularly useful when you want to know the effect of a
change in one of the variables of state
▪ More specifically, an equation of state is a thermodynamic
equation describing the state of matter under a given set of
physical conditions
▪ The simplest and best-known equation of state for substances
in the gas phase is the ideal-gas equation of state
4/13/2025
19
Ideal-Gas Equation of State
An Ideal Gas
▪ This is like the idea of a frictionless surface: it doesn’t exist in
nature, but it is a very handy approximation to some real systems,
and makes problems much easier to handle mathematically
▪ An ideal gas is one which follows the ideal gas equation of state,
namely
PV = nRT
P = pressure, V= volume, n = moles, R= universal gas constant, T
= temperature
4/13/2025
20
Ideal-Gas Equation of State
▪ An ideal gas is a theoretical gas that obeys the ideal gas laws
(EOS)
▪ The ideal gas is composed of randomly moving point particles
whose only interactions are perfectly elastic collisions
▪ The particles occupy negligible volume compared to the bulk
volume of the gas
4/13/2025
21
Ideal-Gas
▪ In thermodynamics, we use the concept of the ideal gas
because:
1. It satisfactorily models the behaviour of real gases
2. It obeys simple gas laws
▪ Real gases (O2, N2 etc) fail to obey the ideal gas model fully
because:
1. Gas molecules occupy finite volume and liquefy at low T and
high P
2. Intermolecular attractions and collisions are not elastic
4/13/2025
22
Ideal-Gas….
▪ Consider an ideal gas enclosed in a container with a movable
piston
▪ The behaviour of the ideal gas depends on T, P and V and
obeys the ideal gas laws:
▪ The basis of ideal gas law is the Boyle’s law, pressure law,
Charles’ law and Avogadro's law
4/13/2025
23
Ideal-Gas….
4/13/2025
24
Ideal-Gas….
4/13/2025
25
Ideal-Gas….
4/13/2025
26
Ideal-Gas….
▪ Real gases behave like ideal gas only at high T and low P
where there is less intermolecular attractions
▪ At low T or high P, real gases condense to liquid (due to an
increase in intermolecular forces) and fail to obey the ideal gas
model
4/13/2025
27