Download Relationships Between Heat and Work

Thermodynamics
Relationships Between Heat and
Work
Heat, Work, and Internal Energy
• As long as a substance does not change
phase, its internal energy will increase as
long as its temperature increases
• Work can transfer energy to a substance
– Results in an increase in internal energy
• Can be transferred to another substance as heat
• Energy can be transferred to a substance
as heat and from the substance as work
Heat, Work, and Internal Energy
• Heat and work are both energy transferred to or
from a system
– System – a collection of matter within a clearly
defined boundary across which no matter passes
– Environment – everything outside a system that can
affect or be affected by the system’s behavior
• Also called the surroundings
• All the parts of a system are in thermal
equilibrium with each other both before and after
a process adds or removes energy
Heat, Work, and Internal Energy
• Pressure is the force per unit area acting on an
object
• Pressure = force / area
• P=F/A
• Measured in Pascal's = N/m2
– Other units are atmospheres (atm), millimeters of
mercury (mmHg), bars (bar), pounds per square inch
(psi), technical atmospheres (at), or torr (Torr)
• Caused by particle collisions
Heat, Work, and Internal Energy
• Work done on or by a gas is the pressure
multiplied by the change in volume
• Work = force * distance
• W=Fd
• W=pressure * volume change
• W=PΔV
• Change in volume = area * distance
• ΔV = Ad
Heat, Work, and Internal Energy
• If the gas is compressed, ΔV is negative
– Work is done on the system
• If the gas expands, ΔV is positive
– Work is done by the system
• If the volume remains constant, no work is
done
Heat, Work, and Internal Energy
• An engine cylinder has a cross-sectional
area of 0.010m2. How much work can be
done by a gas in the cylinder if the gas
exerts a constant pressure of 7.5*105 Pa
on the piston and moves the piston a
distance of 0.040 m?
• A = 0.010 m2
d = 0.040m
• P = 7.5*105 Pa or 7.5*105 N/m2
• ΔV = ?
W=?
Heat, Work, and Internal Energy
• ΔV=Ad = 0.010 m2 * 0.040 m = 4.0*10-4 m3
• W=PΔV = (7.5*105 N/m2) (4.0*10-4 m3) =
3.0*102 J
Thermodynamic Processes
• Work, internal energy, and heat are all
related
– Not every one of these is present in all ideal
thermodynamic processes
• No work is done in constant-volume
processes
– Called isovolumetric – a thermodynamic
process that takes place at constant volume
so that no work is done on or by the system
• Often take place in a bomb calorimeter
Thermodynamic Processes
• Internal energy is constant in a constanttemperature process
• Isothermal process – a thermodynamic
process that takes place at constant
temperature and in which the internal
energy of a system remains unchanged
– Similar to a balloon expanding as the
pressure drops before a storm hits
• The balloon expands to keep pressure equal with
the environment
Thermodynamic Processes
• Energy is not transferred as heat during an
adiabatic process
– A thermodynamic process during which work
is done on or by the system but no energy is
transferred to or from the system as heat
– Rapid compression or expansion of gases in
insulated containers (refrigerators, internal
combustion engines)