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/m 2 –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 0.010m 2. How much work can be done by a gas in the cylinder if the gas exerts a constant pressure of 7.5*10 5 Pa on the piston and moves the piston a distance of m? A = m 2 d = 0.040m P = 7.5*10 5 Pa or 7.5*10 5 N/m 2 ΔV = ?W = ?

Heat, Work, and Internal Energy ΔV=Ad = m 2 * m = 4.0*10 -4 m 3 W=PΔV = (7.5*10 5 N/m 2 ) (4.0*10 -4 m 3 ) = 3.0*10 2 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)