FIRST LAW OF THERMODYNAMICS
FIRST LAW OF THERMODYNAMICS
The First Law of Thermodynamics states that work and heat are mutually
convertible. The present tendency is to include all forms of energy. The First
Law can be stated in many ways:
1. Energy can neither be created nor destroyed; it is always conserved.
However, it can change from one form to another.
2. All energy that goes into a system comes out in some form or the other.
Energy does not vanish and has the ability to be converted into any other form
of energy.
3. If the system is carried through a cycle, the summation of work delivered to
the surroundings is equal to summation of heat taken from the surroundings.
4. No machine can produce energy without corresponding expenditure of
energy.
5. Total energy of an isolated system in all its forms, remain constant.
The first law of thermodynamics cannot be proved mathematically.
FIRST LAW FOR A CLOSED SYSTEM UNDERGOING A CYCLE
According to first law, when a closed system undergoes a thermodynamic cycle,
the net heat transfer is equal to the net work transfer. The cyclic integral of heat
transfer is equal to cyclic integral of work transfer.
where stands for cyclic integral (integral around complete cycle), dQ and dW
are small elements of heat and work transfer and have same units.
FIRST LAW FOR A CLOSED SYSTEM UNDERGOING A CHANGE OF
STATE
According to first law, when a system undergoes a thermodynamic process
(change of state) both heat and work transfer take place. The net energy transfer is
stored within the system and is called stored energy or total energy of the system.
When a process is executed by a system the change in stored energy of the system
is numerically equal to the net heat interaction minus the net work interaction
during the process.
dE = dQ - dW ----------------(1)
E2 – E1 = Q1-2 – W1-2
1
, Where E is an extensive property and represents the total energy of the system at a
given state, i.e.,
E = Total energy
dE = dPE + dKE + dU
If there is no change in PE and KE then, PE = KE = 0
dE = dU, putting in equation (1), we get
dU = dQ – dW
or dQ = dU + dW
This is the first law of thermodynamics for closed system.
Where,
dU = Change in Internal Energy
dW = Work Transfer = PdV
dQ = Heat Transfer = mcdT
{Heat added to the system taken as positive and heat rejected/removal by the
system taken as negative}
For a cycle dU = 0; dQ = dW
COROLLARIES OF FIRST LAW OF THERMODYNAMICS
Corollary 1 : (First Law for a process).
There exists a property of a closed system, the change in the value of this
property during a process is given by the difference between heat supplied and
work done.
dE = dQ - dW
where E is the property of the system and is called total energy which includes
internal energy (U), kinetic energy (KE), potential energy (PE), electrical
energy, chemical energy, magnetic energy, etc.
Corollary 2: (Isolated System).
For an isolated system, both heat and work interactions are absent (d Q = 0, d W
= 0) and E = constant.
Energy can neither be created nor destroyed, however, it can be converted from
one form to another.
Corollary 3 : (PMM - 1).
A perpetual motion machine of the first kind is impossible.
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FIRST LAW OF THERMODYNAMICS
The First Law of Thermodynamics states that work and heat are mutually
convertible. The present tendency is to include all forms of energy. The First
Law can be stated in many ways:
1. Energy can neither be created nor destroyed; it is always conserved.
However, it can change from one form to another.
2. All energy that goes into a system comes out in some form or the other.
Energy does not vanish and has the ability to be converted into any other form
of energy.
3. If the system is carried through a cycle, the summation of work delivered to
the surroundings is equal to summation of heat taken from the surroundings.
4. No machine can produce energy without corresponding expenditure of
energy.
5. Total energy of an isolated system in all its forms, remain constant.
The first law of thermodynamics cannot be proved mathematically.
FIRST LAW FOR A CLOSED SYSTEM UNDERGOING A CYCLE
According to first law, when a closed system undergoes a thermodynamic cycle,
the net heat transfer is equal to the net work transfer. The cyclic integral of heat
transfer is equal to cyclic integral of work transfer.
where stands for cyclic integral (integral around complete cycle), dQ and dW
are small elements of heat and work transfer and have same units.
FIRST LAW FOR A CLOSED SYSTEM UNDERGOING A CHANGE OF
STATE
According to first law, when a system undergoes a thermodynamic process
(change of state) both heat and work transfer take place. The net energy transfer is
stored within the system and is called stored energy or total energy of the system.
When a process is executed by a system the change in stored energy of the system
is numerically equal to the net heat interaction minus the net work interaction
during the process.
dE = dQ - dW ----------------(1)
E2 – E1 = Q1-2 – W1-2
1
, Where E is an extensive property and represents the total energy of the system at a
given state, i.e.,
E = Total energy
dE = dPE + dKE + dU
If there is no change in PE and KE then, PE = KE = 0
dE = dU, putting in equation (1), we get
dU = dQ – dW
or dQ = dU + dW
This is the first law of thermodynamics for closed system.
Where,
dU = Change in Internal Energy
dW = Work Transfer = PdV
dQ = Heat Transfer = mcdT
{Heat added to the system taken as positive and heat rejected/removal by the
system taken as negative}
For a cycle dU = 0; dQ = dW
COROLLARIES OF FIRST LAW OF THERMODYNAMICS
Corollary 1 : (First Law for a process).
There exists a property of a closed system, the change in the value of this
property during a process is given by the difference between heat supplied and
work done.
dE = dQ - dW
where E is the property of the system and is called total energy which includes
internal energy (U), kinetic energy (KE), potential energy (PE), electrical
energy, chemical energy, magnetic energy, etc.
Corollary 2: (Isolated System).
For an isolated system, both heat and work interactions are absent (d Q = 0, d W
= 0) and E = constant.
Energy can neither be created nor destroyed, however, it can be converted from
one form to another.
Corollary 3 : (PMM - 1).
A perpetual motion machine of the first kind is impossible.
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