The Laws of Thermodynamics Applied to Refrigerators and Heat Engines

Heat Work and Internal Energy  Heat can be transferred to water and thus be converted to steam which can inflate a balloon. The steam does work on the balloon. The steam does work on the balloon. The internal energy of the steam decreases as predicted by the law of conservation of energy. The internal energy of the steam decreases as predicted by the law of conservation of energy. Remember Work = Force x Distance Remember Work = Force x Distance For heated gases pressure exerts the force and the change in volume is the change in distance. For heated gases pressure exerts the force and the change in volume is the change in distance.  Internal Energy The sum of the energy a substance has due to the motion of it’s particles.The sum of the energy a substance has due to the motion of it’s particles. ΔU is the symbol for a change in internal energy and it is measure in Joules.ΔU is the symbol for a change in internal energy and it is measure in Joules. If the temperature of a substance increases so does its internal energy because temperature is a measure of the average kinetic energy in a substance.If the temperature of a substance increases so does its internal energy because temperature is a measure of the average kinetic energy in a substance.

Example  An engine cylinder has a cross sectional area of 0.01 m 2. How much work can be done by a gas in a cylinder if the gas exerts a constant pressure of 7.5 x 10 5 Pa on the piston and the piston moves a distance of 0.04 m?  Work = PΔV W =(0.01 m 2 )(0.04 m)(7.5 x 10 5 Pa) = 300 J work done by the gas W =(0.01 m 2 )(0.04 m)(7.5 x 10 5 Pa) = 300 J work done by the gas P = pressure P = pressure V = volume V = volume (remember work = force x distance)(remember work = force x distance)

Isothermic Systems  There is no temperature change.  When ice melts all of the energy is used to rearrange the crystals and there is no energy left over to raise the temperature.  ΔU = Q – W or Q = ΔU +W The internal energy (ΔU) is related to a change in temperature. The internal energy (ΔU) is related to a change in temperature. Q represents the energy transferred to and from the system. Q represents the energy transferred to and from the system. W represents the amount of work done on or by the system. W represents the amount of work done on or by the system. ΔU = 0 because the temperature remains constant. ΔU = 0 because the temperature remains constant. So, Q = -WSo, Q = -W

Isovolumetric  There is no change in volume.  Heat is added to the system but the gas does not expand there for no work is done on or by the system. Pressure Cooker. Pressure Cooker.  ΔU = Q – W or Q = ΔU +W W = 0 W = 0 So Q = ΔU All of the energy added to the system increases its energy.So Q = ΔU All of the energy added to the system increases its energy.

Adiabatic Systems  There is no energy transferred into or out of the system.  Place a balloon on top of a compressed canister of air. The air will be released and expand the balloon rapidly. The work done on the balloon is done by the expanding air.  The internal energy stored in the canister is used to do work on the balloon.  ΔU = Q – W or Q = ΔU +W Q = 0 Q = 0 So ΔU = - W So ΔU = - W

Volume, Pressure and Internal Energy  No Work is Done at a Constant Volume and Pressure.  If you compress a gas in a container, you do work on the gas in the container and as a result the internal energy increases.  However, if you heat a rigid container and there is no change in volume, the internal energy increase but no work is done because there is no change in volume. This is known as an isovolumetric system. This is known as an isovolumetric system. Iso meaning not changing. Iso meaning not changing. A system is a group of substances that are in thermal contact with one another and are in thermal equilibrium. A system is a group of substances that are in thermal contact with one another and are in thermal equilibrium. When the change in volume is + the gas does work on the object When the change in volume is + the gas does work on the object When change in volume is – the object does work on the gas When change in volume is – the object does work on the gas No change in volume no work done. (isovolumetric system) No change in volume no work done. (isovolumetric system)

Isovolumetric  Gas undergoes a change in temperature but not a change in volume. Colorimeter – small amount of substance burned but the thick walls of the vessel prevent expansion. Colorimeter – small amount of substance burned but the thick walls of the vessel prevent expansion. Increase in temperatureIncrease in temperature Increase in pressureIncrease in pressure No change in volumeNo change in volume Energy transferred to the container as heat.Energy transferred to the container as heat.

Isothermic  A balloon will respond to atmospheric pressure. As the atmospheric pressure increases the balloon will temporarily have an increase in internal energy (a rise in temperature) which will cause the balloon to expand which then lowers the temperature. In the end the temperature remains constant. As the atmospheric pressure increases the balloon will temporarily have an increase in internal energy (a rise in temperature) which will cause the balloon to expand which then lowers the temperature. In the end the temperature remains constant. Increase in volume Increase in volume Decrease in pressure Decrease in pressure No change in temperature No change in temperature Energy transferred as work and the internal energy remains constant. Energy transferred as work and the internal energy remains constant. Any transfer of energy into or out of the system is due to heat or work. There are no phase changes( water to steam) in isothermal systems because there is no temperature change. There are no phase changes( water to steam) in isothermal systems because there is no temperature change. Work = Heat Work = Heat

Adiabatic Systems  When a energy is not transferred to or from a system as heat the process is called adiabatic. A tank of compressed gas inflates a balloon so quickly that that the expansion of the balloon is due to the pressure of the gas as opposed to thermal expansion. If the tank and balloon are thermally insulated no energy is transferred in the form of heat. A tank of compressed gas inflates a balloon so quickly that that the expansion of the balloon is due to the pressure of the gas as opposed to thermal expansion. If the tank and balloon are thermally insulated no energy is transferred in the form of heat. Internal energy decreases and energy leaves the system as work done against the outside air. Internal energy decreases and energy leaves the system as work done against the outside air.

Change in Internal Energy Equals Heat Transferred to  ΔU = Q – W  Change in internal energy = heat transferred into or out of the system minus the work done to the system.