A simple hairdryer experiment to demonstrate the first law of thermodynamics

Abstract

Equipment for thermodynamics experiments and lab demonstrations can be very expensive. A common inexpensive hairdryer makes an excellent example of an open thermodynamic system, and can be used as an effective piece of lab equipment to illustrate the principles of the first law of thermodynamics. Heat, work and mass all cross the boundary. From the first law of thermodynamics, the energy into the system has to equal the energy out for steady state. From conservation of mass, the mass in has to equal the mass out for steady state. This experiment requires the student to consider all of the energy terms associated with the hairdryer. The energy going in includes the electric work, the total enthalpy of the incoming air, and the kinetic energy of the incoming air. Energy out includes the total enthalpy of the outgoing air, kinetic energy of the outgoing air, and any heat transfer from the case to the ambient. Potential energy differences between the inlet and outlet are also considered. By accounting for all of the energy terms the students begin to recognize what is most significant and what could be neglected. One of the difficulties encountered in this test arises from the fact that the air velocity and the air temperature across the nozzle exit are not constant. Students are required to take data at multiple points and treat the area around each point as a separate outlet. After flow rate calculations are made for each area, the total rate is determined by summing the individual rates. The same process is used when calculating the total enthalpy leaving at the nozzle. The energy terms usually balance within a range of about 1.5% to about 15%. The amount of error usually depends on the patience and diligence of the student group doing the experiment. The students are asked to report on any possible sources of error they recognize while doing the testing.