Objective:
To demonstrate that heat is
produced in air, hot air goes up and cooler air from surrounding
flows towards this making convection currents.
Equipment:
A
kerosene lamp, an extra glass cover of the kerocen lamp. Introduction:
When
kerosene lamp is lighted, the surrounding air in the glass cover
gets heated. The density of hot air is less than the cold air.
The gases produced during burning are also hot. All this hot
gaseous mass rises and goes out of the lamp through the glass
cover. There are holes made in the part below the glass cover.
Fresh air from outside enters the lamp through these holes, get
heated and goes up. Thus a convection current is set up.
The
present demo makes arrangement where one can visually see the
flow paths of gaseous mass when a lamp is lighted.
Procedure:
Take a
kerosene lamp with some kerosene and wick in place. Take a
cardboard box, like a she box. Place it in a way that the
opening side is vertical and is towards you. Cut two holes on
the top cover of the box so that the glass covers of the lamp
can be fitted in these. Put a lamp inside the box below one of
the holes. One of the glass covers should go through the hole
and fit with the lamp. Let me call this Cover-1. Put another
glass cover in the other hole. I will call it cover-2. Close any
gap remaining between the glass covers and the cardboard top
cover.
Light
the lamp, put it in place and close the box. Light an incense
stick. Where is the smoke going? It goes up. That is the natural
tendency of smoke. Put the stick near the glass cover-1. The
smoke goes up. Now put the stick near the glass cover-2. Here is
the real climax. The smoke is pulled down into the glass cover-2
and it come out from the glass cover-1.
Why is
the smoke dragged into the glass cover against its natural
tendency to go up? This is because the burning wick of the lamp
produced hot gases which rose up and went out through the glass
cover-1. To fill the void, air should rush in. The only path
available for fresh air is through the glass cover-2. So
convection current is set up where air from outside goes into
cover-2, goes into the box, and then to the wick area. The hot
gases formed there go up and come out of cover-1. When the stick
is placed near the cover-2, smoke is dragged by the air current
that already exist there. The flow path of smoke is just the
path of convection current.
When to do: At class 8 level convection
currents are introduced.
Instrument cost and availability :Available
easily, cost Rs 25
