09 First Law of Thermodynamics

21 October

Idealized Stirling Cycle

Seng Kwang Tan 09 First Law of Thermodynamics, GeoGebra 0 Comments

I created a new GeoGebra app based on an ideal Stirling Cycle (A. Romanelli Alternative thermodynamic cycle for the Stirling machine, American Journal of Physics 85, 926 (2017)) which includes two isothermal and two isochoric processes. The Stirling engine is a very good example to apply the First Law of Thermodynamics to, as the amount of gas is fixed so the macro-variables are only pressure, temperature and volume. Simplifying the cycle makes it even easier for first time learners to understand how the engine works.

For those who prefer to be impressed by an actual working model, it can be bought for less than S$30 on Lazada. All you need for it to run is a little hot water or some ice. Here’s a video of the one I bought:

The parts of the Stirling engine are labelled here:
How a Stirling Engine works

My simulation may not look identical to the engine shown but it does have the same power piston (to do work on the flywheel) and displacer piston (to shunt the air to and fro for more efficient heat exchange).

Geogebra link: https://www.geogebra.org/m/pbnw2yas

22 August

PV Diagram for an Ideal Gas

Seng Kwang Tan 09 First Law of Thermodynamics, GeoGebra 0 Comments

GeoGebra link: https://www.geogebra.org/m/xyqhfvyw

Applying the 1st Law of Thermodynamics to 4 simple changes on an ideal gas, students can check their understanding using this Geogebra app. When is work done positive? Which processes bring about an increase in internal energy or temperature? Which processes require heat input?

17 February

Stirling Engine

Seng Kwang Tan 09 First Law of Thermodynamics 0 Comments

I bought a simple beta Stirling engine online at dx.com recently and it came in the mail today. It works well with a cup of hot water placed under it, although it might take a little push to get it started due to the initial static friction. However, once it starts spinning, the wheel goes on and on for a very long time.

From the video, you can observe the expansion of the air within the main piston cylinder as the heat below raises the temperature and pressure. This forms the power stroke. When the piston rises, it pushes air into a secondary piston, which also helps to provide torque to the wheel. When the air in both pistons expand, it cools down. An understanding of the 1st law of Thermodynamics (JC syllabus) is necessary to appreciate why that happens. Upon cooling, pressure decreases and the pistons fall. The cycle repeats itself.

 

29 May

Can a Liquid Freeze and Boil Simultaneously?

Seng Kwang Tan 09 First Law of Thermodynamics 0 Comments

Witness the freezing and boiling of a liquid take place at the same time. A thorough explanation for this observation is found here.

Cyclohexane can both freeze and boil simultaneously under specific conditions when it is in a state known as triple point.

The triple point of a substance is the unique combination of temperature and pressure where all three phases (solid, liquid, and gas) coexist in equilibrium. For cyclohexane, at its triple point:

  • Temperature: Around 6.5°C
  • Pressure: Around 0.053 atm (40 mm Hg)

At this point, cyclohexane can freeze (turn from liquid to solid) and boil (turn from liquid to gas) simultaneously. This occurs because the conditions allow the substance to transition between all three phases at the same time. The energy added or removed from the system can cause some molecules to leave the liquid phase as a gas (boiling), while others enter the solid phase (freezing).

23 May

Adiabatic Process Demonstrations

Seng Kwang Tan 09 First Law of Thermodynamics, Demonstrations, Featured 0 Comments

Here are some interesting lecture demonstrations on adiabatic thermodynamic processes you can carry out. In an adiabatic process, there is no heat transfer between the system and other systems (including its environment.) According to the First Law of Thermodynamics ($$\Delta U=Q+W$$), where Q = 0, a compression of a gas which is associated with work being done on the gas will cause the internal energy and hence, the temperature of the gas to rise. On the other hand, when an expansion of a gas takes place, the gas will cool down.

1. Adiabatic compression using a fire syringe

(available from Funlearners for $30 in Singapore)

2. Adiabatic expansion using a fire extinguisher

26 February

Heat Capacity of Water

Seng Kwang Tan 09 First Law of Thermodynamics, Demonstrations, IP4 18 Thermal Properties of Matter

Water has a high specific heat capacity of about 4200 J kg-1 K-1. When a little bit of water is placed in a balloon, it is able to absorb a significant amount of heat from a candle flame and hence prevent the balloon from bursting.
Materials
  1. Two balloons
  2. Two candles
  3. Lighter
Procedure
  1. In this demonstration, one balloons is filled with about 3 tablespoons of water and then inflated.
  2. Another balloon is inflated to the same size as the first to serve as a control.
  3. Both balloons are then placed vertically over two identical candles. Adjust the balloons such that the distance from balloon to candle is the same for both setups. You can use retort stands to clamp the balloons in place if you have them.
  4. Light the candles with the balloons temporarily removed. The flames will have to touch the bottom of the balloon when they are placed back over the candles.
  5. Observe the balloon without water burst first.
  6. The air gushing out from the exploding balloon may put out the other candle.
  7. If you like, you can keep the balloon with water over the flame for a longer duration. The balloon still will not burst until a long time later.