Category: 11 Thermal Physics

# 11.6.2 Cyclic Processes

This toy is a mini Sterling Engine. It demonstrates the cyclic process of a heat engine. Heat is taken in from the hot plate, partly converted into mechanical work to turn the wheel, and partly “wasted” as heat dumped to the cold plate.

It takes time for heat transfer through conduction, convection or radiation. Compressing or expanding a gas, on the other hand, can be completed in a split second.

As such, if we plunge the piston into the syringe suddenly, we get what is practically an adiabatic compression, accompanied by a dramatic increase in temperature.

Similarly, if we “pop the champagne” and let the air out suddenly, we get what is practically an adiabatic expansion., accompanied by a dramatic decrease in temperature.

# 11.4.0 First Law of Thermodynamics

This applet provides a microscopic view of the first law of thermodynamics. Click here for the applet’s url.

Basically, heating is not the only way to change the temperature of a gas. The other way is to do work on it. If the piston charges towards the gas molecules, KE is passed to the gas molecules through the collisions. Higher average KE of gas molecules means higher temperature. On the other hand, if the piston moves away from the gas molecules, the gas molecules actually lose KE through the collisions. The gas cools down since its average KE has been lowered.

# 11.3.2 Root-mean-square Speed Applet

Here is a nice applet to visualise the motion of gas molecules in a gas. (Click here for the applet’s url)

The speeds of the molecules are faster at higher temperatures. But at any one temperature, the speeds of the molecules are not uniform. In fact, the distribution of speed follows the Maxwell-Boltzman’s distribution (shown in the histogram at the bottom of the applet). The gas molecules are also colour coded according to their speeds in this applet. Did you notice that the speed of the gas molecules are unchanged when colliding with the walls? When they collide with one another, their speeds do get changed as they exchange momentum. But because they are all elastic collisions, the total (and thus average) KE of the gas remains constant.

The following video provides discussion on the root-mean-square square.

# 11.3.1 Equations of State

Most real gases at standard temperature and pressure do not deviate too far from pV=nRT . Even when they do, the ideal gas equation still provides a good model for qualitative understanding of how the state varibles p, V and T play out.

Check out the following demonstrations!

# B07 Sterling Engine

This toy is a mini Sterling Engine. It demonstrates the cyclic process of a heat engine. Heat is taken in from the hot plate, partly converted into mechanical work to turn the wheel, and partly “wasted” as heat dumped to the cold plate.

It takes time for heat transfer through conduction, convection or radiation. Compressing or expanding a gas, on the other hand, can be completed in a split second.

As such, if we plunge the piston into the syringe suddenly, we get what is practically an adiabatic compression, accompanied by a dramatic increase in temperature.

Similarly, if we “pop the champagne” and let the air out suddenly, we get what is practically an adiabatic expansion., accompanied by a dramatic decrease in temperature.

# B01 Latent Heat

We like to impress upon students that water has a high specific latent heat of fusion (334 kJ/kg), and even higher specific latent heat of vaporization (2.260 MJ/kg). That’s why it feels so cold when we get out of the swimming pool. The amount of water on our wet skin may be small, but they sure used a lot of our body heat to evaporate away. It’s also why it feels so hot just before it rains. Those rain clouds are formed by condensing water vapour dumping huge amounts of latent heat into the atmosphere in the process.

Latent heat can be used to move a lot of thermal energy in a short time. In air conditioners, the coolant is made to vaporize and condense repetitively to move heat in or out of the room. It is amazing such a small amount of coolant can move so much heat.

Unlike melting, vaporization usually involves a large change in volume. This is why volatile liquids can make very fascinating toys. Check out the hand boiler and the drinking bird!

# B00 Temperature Scales

The human perception of temperature is unreliable. For example, a metal chair feels colder than a wooden chair when indoor. But the same metal chair feels hotter than the wooden chair when they are out in the hot sun. But we know that both chairs are at the same ambient temperature!

This is why we invent thermometers. In the school lab, I found this Galileo Thermometer. It works on the principle that the density of water changes with temperature, thus making the differently weighted glass balls rise and fall. It is one of the lousiest thermometers I have seen. It takes forever to respond (thanks to its heat capacity), it cannot measure localized temperature, suffers from poor resolution, it is bulky, etc. But as a toy, it is fascinating to watch.