Download Ist law of thermodynamics

2015.08.31.
Thermodynamics.
Heat and the first law of thermodynamics
Dr. Yin Li
Department of Biophysics, Medical School
University of Pecs
Energy conservation
• Energy can not be created or destroyed. It can
only be transformed from one form to another
form.
Energy of the ball was transformed from kinetic energy to
potential energy when it goes up, and from potential energy
to kinetic energy when it goes down.
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Internal energy
• Internal energy (also called thermal energy) is
the energy an object or substance is due to the
kinetic and potential energies associated with
the random motions of all the particles that
make it up.
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Temperature
• Temperature is related to the kinetic energy of
the atoms inside a system. The faster these
atoms move, the higher temperature the system
has.
• Temperature and internal energy are related to each
other, but they are not the same!
Question:
Which system has a higher temperature, a cup of hot
water or an iceberg? And which system has a larger
internal energy?
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Thermal equilibrium
• Two bodies are said to be at thermal equilibrium
if they are at the same temperature. This means
there is no net exchange of thermal energy
between the two bodies.
• if two thermodynamic systems are each in thermal
equilibrium with a third, then they are in thermal
equilibrium with each other. (Zeroth law of
thermodynamics)
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Heat
• Energy that is transferred from one object or
system to another because of a temperature
difference.
• units:
• 1 joule (J) = 1 N m, One joule is the work done by a
force of one newton moving an object one meter
along the direction of the force.
• calorie (cal), Calorie (Cal; kcal) 1 cal = 4,19 J
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Heat transfer
• If two systems are not in thermo equilibrium, the
heat will spontaneously transfer from hot side to
cold side.
hot
heat
cold
• Question: Why can refrigerator transfer the heat
from cold place to hot place? Is it a spontaneous
process?
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Heat Transfer
• Conduction: Energy is transferred when two
objects are in direct contact.
• Convection: Energy is transferred from one body to
another via currents in a fluid (a gas or liquid).
• Radiation: All objects, at any temperature, radiate
electromagnetic radiation (light of visible and
invisible wavelengths). Unlike conduction &
convection, no medium (matter of any type) is
necessary for heat transfer through radiation.
Objects absorb radiation as well. At thermal
equilibrium it will absorb as much as it radiates.
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First law of thermodynamics
• The change in internal energy of a system is
equal to the heat change of the system plus the
work change of the system. It is an extension of
energy conservation.
U  Q  W
• When the heat is transferred into the system, Q is
positive; When the system transfer the heat to others,
Q is negative.
• When the system does work for others, W is negative;
when others do work for the system, W is positive.
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Four processes
• Isothermal – constant temperature
• The change in temperature is zero. The internal
energy is kept as constant. The change of Q and W
cancel out.
P V  n  R  T
U  Q  W  0
Q  W
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Four processes
• Adiabatic process
• no heat transferred indicating that Q=0
• The internal energy of the system only depends on
the work done by the system.
Q0
U  W
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Four processes
• Isobaric process– constant pressure
• Pressure is a constant which makes us easy to
calculate the work done by the system.
W  P  V
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Four processes
• Isochoric process– constant volume
• When the volume is zero, ΔV=0 resulting in W=0.
W 0
U  Q
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