# 09 First Law of Thermodynamics

## Heat Capacity of Water

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.

## p-V diagram Sudoku time!

For the topic of Thermal Physics, JC1 students can try to make sense of p-V diagrams using the following exercise, which does not involve any calculation. They will need to fill in the cells in the table below with any of the three (+, – or 0) to indicate whether there is heat transfer, work done on the gas, or a change in internal energy.

## Crushing Can

We are usually unaware of the immense strength of the pressure due to the atmosphere around us, having taken it for granted. This demonstration will utilize atmospheric pressure to crush an aluminum can while introducing concepts such as the relationship between pressure and the amount of gas in a fixed volume.

Materials

1. Empty aluminum drink can
2. Pair of tongs
3. Stove or bunsen burner
4. Tank of water

Procedure

1. Put about a teaspoon of water into the drink can and heat it upright over the stove or Bunsen burner.
2. Prepare a tank of water and place it nearby.
3. When steam is seen to escape from the drink can, use the pair of tongs to grab the drink can, inverting it and placing it just slightly submerged into the tank so that the mouth of the can is sealed by the water.
4. You should observe the can being crushed instantaneously.

Physics Principles Explained

Two physics principles work in tandem to crush the can. The cooling of the air within the can will reduce the internal pressure of the can as the movement of the air particles will slow down with reduced temperature.

At the same time, the sudden cooling will cause the water vapour in the can that exists at just slightly above 100°C to revert to its liquid state, greatly reducing the amount of gases inside the can.

As air pressure depends on both the kinetic energies and amount of particles within the system, it is significantly reduced. Atmospheric pressure, being stronger than the internal pressure, will cause the can to implode.