1 Energy
Energy is defined as the capability to do work. Energy input always produces
some effect on the matter of the system.
Two-major types of energy are:
1. Stored Energy and Energy : The stored energy of a substance may
be in the forms of Mechanical Energy and Internal Energy (other
forms of stored energy may be Chemical Energy, Electrical Energy
and Nuclear Energy). Part of the stored energy may take the form
of either Potential Energy (which is the gravitational energy due to
height above a chosen datum line) or Kinetic Energy due to velocity.
The balance part of the energy is known as Internal Energy. In a
non-flow process usually there is no change of potential or kinetic
energy and hence change of mechanical energy will not enter the cal-
culations. In a flow process, however, there may be changes in both
potential and kinetic energy and these must be taken into account while
considering the changes of stored energy.
2. Energy in Transit: Heat and work are the forms of energy in transi-
tion. These are the only forms in which energy can cross the boundaries
of a system. Neither heat nor work can exist as stored energy.
The energy unit is a derived physical quantity having dimension :
mass × length2
energy = (1)
time2
The SI unit of energy is the Joule (J):
Kgm2
1J = 1 2 (2)
s
Energy is often measured in other units, specific to particular applica-
tions. Some Common Energy Units are,
Unit SI Equivalent
British Thermal Unit (BTU) 1,055 J
Calorie (cal) 4.184 J
Electron volt (eV) 1.602 × 10−19 J
Erg (erg) 1.000 × 10−7 J
Foot pound (ft-lb) 1.356 J
Kilowatt-hour (kwh) 3.600 × 106 J
Liter atmosphere (L-atm) 101.3 J
1
, 2 Work
Work is the transfer of mechanical energy from one object to another. Since
work is a movement of energy, it is measured in the same units as energy:
joules (J). The definition of work in a physics context is quite different from
how it is used in mechanics as follows:
Work is done when a force is applied to an object through a distance
− →
→ −
w=F · d (3)
This means that when a force is applied to an object through a distance,
the object’s total energy will be affected. The object will either speed up
or slow down, resulting in a change in its kinetic energy, or it will have an
altered potential energy if, for example, it was lifted a certain height under
the force of gravity.
Thermodynamic definition of work: Positive work is done by a sys-
tem when the sole effect external to the system could be reduced to the rise of
a weight.
Work is a transient quantity which only appears at the boundary while a
change of state is taking place within a system. Work is ’something’ which
appears at the boundary when a system changes its state due to the move-
ment of a part of the boundary under the action of a force.
2.0.1 Sign Convention
• If the work is done by the system on the surroundings, e.g. , when a
fluid expands pushing a piston outwards, the work is said to be positive.
i.e. Work output of the system = +W
• If the work is done on the system by the surroundings, e.g. when a
force is applied to a rotating handle, or to a piston to compress a fluid,
the work is said to be negative. i.e. , Work input to system = – W
2
Energy is defined as the capability to do work. Energy input always produces
some effect on the matter of the system.
Two-major types of energy are:
1. Stored Energy and Energy : The stored energy of a substance may
be in the forms of Mechanical Energy and Internal Energy (other
forms of stored energy may be Chemical Energy, Electrical Energy
and Nuclear Energy). Part of the stored energy may take the form
of either Potential Energy (which is the gravitational energy due to
height above a chosen datum line) or Kinetic Energy due to velocity.
The balance part of the energy is known as Internal Energy. In a
non-flow process usually there is no change of potential or kinetic
energy and hence change of mechanical energy will not enter the cal-
culations. In a flow process, however, there may be changes in both
potential and kinetic energy and these must be taken into account while
considering the changes of stored energy.
2. Energy in Transit: Heat and work are the forms of energy in transi-
tion. These are the only forms in which energy can cross the boundaries
of a system. Neither heat nor work can exist as stored energy.
The energy unit is a derived physical quantity having dimension :
mass × length2
energy = (1)
time2
The SI unit of energy is the Joule (J):
Kgm2
1J = 1 2 (2)
s
Energy is often measured in other units, specific to particular applica-
tions. Some Common Energy Units are,
Unit SI Equivalent
British Thermal Unit (BTU) 1,055 J
Calorie (cal) 4.184 J
Electron volt (eV) 1.602 × 10−19 J
Erg (erg) 1.000 × 10−7 J
Foot pound (ft-lb) 1.356 J
Kilowatt-hour (kwh) 3.600 × 106 J
Liter atmosphere (L-atm) 101.3 J
1
, 2 Work
Work is the transfer of mechanical energy from one object to another. Since
work is a movement of energy, it is measured in the same units as energy:
joules (J). The definition of work in a physics context is quite different from
how it is used in mechanics as follows:
Work is done when a force is applied to an object through a distance
− →
→ −
w=F · d (3)
This means that when a force is applied to an object through a distance,
the object’s total energy will be affected. The object will either speed up
or slow down, resulting in a change in its kinetic energy, or it will have an
altered potential energy if, for example, it was lifted a certain height under
the force of gravity.
Thermodynamic definition of work: Positive work is done by a sys-
tem when the sole effect external to the system could be reduced to the rise of
a weight.
Work is a transient quantity which only appears at the boundary while a
change of state is taking place within a system. Work is ’something’ which
appears at the boundary when a system changes its state due to the move-
ment of a part of the boundary under the action of a force.
2.0.1 Sign Convention
• If the work is done by the system on the surroundings, e.g. , when a
fluid expands pushing a piston outwards, the work is said to be positive.
i.e. Work output of the system = +W
• If the work is done on the system by the surroundings, e.g. when a
force is applied to a rotating handle, or to a piston to compress a fluid,
the work is said to be negative. i.e. , Work input to system = – W
2
