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This toy is most likely to be found in your neighborhood gift shop
it's commonly known as Dippy Bird.
You might have come across toy and would have seen the bird
repeatedly sipping water from a container.
What impels the bird to do so is a question that puzzles many people.
Often mistaken for a perpetual motion machine
the dippy bird is actually a heat engine. We will use this toy heat engine
as a practical example for understanding a few scientific phenomena.
Let us begin by understanding the components of the toy.
A glass of water is placed directly in front of the bird.
The bird's body comprises of a glass tube
that connects two hollow glass bulbs.
The bottom bulb forms the bird's bottom and the top bulb forms the bird's head.
The head and the beak are covered with felt cloth
for good absorption of water.
A red coloured volatile liquid, that is Dichloromethane
partly fills the tube as well as the bird's bottom.
The empty space inside the body is not
occupied by air, instead it is occupied by
vapour evaporated from Dichloromethane liquid.
And the body is hinged between the bird's legs.
All these components together contribute
to the working of the toy. Let us see how.
Let us begin from a point in the cycle where the bird is oscillating gently.
Here its head is wet, because it has already
dipped its beak into the water. As the liquid in the tube
rises the top part of the body becomes heavy
which makes it swing forwards.
The beak bets dipped in the water,
while the end of the tube
in the bird's bottom rises above the liquid level.
This gives way for vapour bubbles to travel through the tube
into the head draining the liquid out of the tube.
Thus the bird becomes upright
and starts oscillating again.
But what still remains unexplained is why the liquid in the tube rises!
When the beak comes in contact with the water
the water gets absorbed by the felt cloth. The water rises to the head
due to capillary action.
As the water evaporates,
it slowly cools the bird's head.
This process is known as evaporative cooling.
Let us briefly understand evaporative cooling.
Evaporation is a surface phenonmenon.
It occurs when a few molecules at the surface
gain extra energy from their neighboring molecules.
This makes them escape into the space
above the liquid. Thus the high energy molecules escape
leaving behind the low energy ones.
This decrease in energy is accompanied
by a decrease in temperature of the liquid.
In a similar way the temperature of the bird's head decreases due to evaporative cooling.
The decrease in temperature causes a
a decrease in pressure exerted by Dichloromethane vapor.
This leads to a lower pressure level in the head
as compared to the bottom. The higher pressure in the bottom
slowly pushes the liquid towards the head
thereby causing the liquid to rise through the tube.
Thus the difference in temperatures between the bird's head and
the bottom enables the bird to produce mechanical work used for its motion.
This is why the bird is considered a heat engine.
As you saw
the Dippy Bird is a good toy for learning science,
but it teaches us something beyond science as well.
It is an excellent example of how keeping a cool head
can keep us happily engaged in our work.