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 constant-temperature 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)