convection smoke box experiment

UCSC Physics Demonstration Room

Demonstration Resources for UCSC

Equipment: 

• Chimney apparatus
• Smoke source
• Cylindrical heater and L shaped tube
• Water and ice
• Rheoscopic fluid
• Lamp with shade
• 2 Variac transformers
• Convection Box:  When a lit candle is placed under one clear chimney and a source of smoke (incense or punk) is placed above the other clear chimney, convection currents can be seen. See top photo.
• Convection in Liquids:  This setup (middle photo) is based on a small cylindrical heater attached to the tip of an L-shaped tube and immersed into a square glass tank with a mixture of water and rheoscopic fluid (bottom photo). The tank is resting on the lab jack and illuminated by the table lamp with a deep plastic shade. The heater temperature and lamp brightness can be regulated by variacs. Adding ice to the tank enhances downflow of the liquid. Switching the heater on (70-80V) forces bulk movement of the liquid up. Ice and heat together cause interesting convection patterns (watch this  video  to see demonstration).

Explanation:

Convection is heat transfer by mass motion in a fluid. When the fluid it heated and moves away from the heat source it carries heat energy with it. When the fluid heats up it expands and becomes less dense. It then moves towards the more dense and colder fluid, creating convection currents.

• Convection Box:  The candle in the right chimney causes hot air to rise. To keep the pressure equalized cold denser air flows in from the left chimney creating a suction that brings smoke with it.
• Convection in Liquids: In this demo you will be able to see convection currents in liquid. The less dense (hot) liquid will rise and the denser (cold) liquid will fall creating circular currents that you will be able to observe.

Convection Currents

Convection is an important physical process that relates to the differential heating, and therefore density differences, that give rise to movement of fluids like air and water. We can see convection happening in a pot on the kitchen stove, in the rise and fall of a hot air balloon, in the circulation of ocean currents , in the molten liquid part of the earth's core, and in the formation of thunderstorms.

Perform the following experiment for a simple demonstration of convection:

If you don't want to build your own, here's one you can buy .

• Incense sticks
• Cardboard box
• Black construction paper
• Transparency sheets
• Lighter or matches

Building the Apparatus:

• Lay the cardboard box along its long side, and cut three edges of one end, so that the end has a closeable flap. (see picture)
• Cut two circular holes in the top of the box.
• Cut a viewing window in the front of the box.
• Tape a piece of transparency sheet to the viewing window opening.
• Tape a piece of black paper to the back inside of the box.
• Roll two pieces of transparency sheet into tubes, and insert them into the two holes on top of the box.

Conducting the Experiment:

• Warning: Never leave a flame unattended. Be prepared to extinguish a fire.
• Put a lit candle on the floor of the box, under the left "chimney" tube. Do this in a well ventilated space.
• Close the access flap so that the box is sealed.
• Light one or more incense sticks and hold the lit end over the top of the second chimney.
• Observe the movement of the smoke.

• Describe the direction in which the smoke from the incense sticks moves.
• Is warm air more dense or less dense than cold air?
• Is a less dense material lighter or heavier than a more dense material?
• What happens to the air over the candle?
• What happens to the air above the right chimney when the box is closed?
• Since the ocean water retains heat better than the land does, should air over water rise or sink?
• In the morning, breezes tend to move from land to ocean and, later in the day, from ocean to land. Explain.
• An unstirred pot of spaghetti sauce on the stove will bubble and sputter as it starts to boil. Explain why this happens.
• A hot air balloon has a burner under the opening at its base. Explain how the balloon rises and falls when heat is added and removed.
• Explain how thunderstorms form.

Watch the video:

https://www.youtube.com/watch?v=O3Z-lioH2k0

Convection Current Experiment

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Convection is one of three main types of heat transfer. The other two are radiation and conduction. Convection is the transfer of heat by the  movement of heated particles into an area  of cooler  particles. You can experience convection when you light a match. The air directly above the lit match is always hotter than the air around the match.

This difference in temperature around the match is caused by the effect of heat on the density of air. Hot air is less dense than cool air and will rise leaving the cooler air below. As the warm air rises, a pattern of air movement is formed called a convection current. We can see these convection currents in the air and in water.

*This post contains affiliate links.

Related post – Weather Study Resources

The Gulf Stream off the eastern coast of the States is a convection current. It carries warm water from the tropics up the east coast north toward the cold arctic waters.

Create Your Own Convection Current

• ice cube tray
• food coloring
• clear drinking glass or jar

• Put the ice cube tray in the freezer until frozen solid.
• Fill a clear glass with warm water.
• Add one ice cube to the glass of water.
• Observe what happens.

What’s Happening?

The warm water will melt the ice cube, but the resulting water will be very cold. This cold, dense water will sink to the bottom of the glass. You can see this happening because the melted water from the ice cube will be whatever color you made your ice cube.

Try this simple experiment at home!

Convection in a box  J.L. Henshaw (aka P.F.)- experiments in 1978, Denver   Other Airwork ,

• A single 2D vortex ring - displaying apparent contradictions
• Detailed diagrams of vortex flow - the winding & wrapping of layers
• Smoke pool - with branching columns of clear air
• Very strange indeed - a clear confusion between up and down

O ne thing these experiments demonstrate is O rganized M olecular M otion , fluid states that store and release energy.   This is clearly evident from experiment 4, and then can help explain experiments 1, 2 & 3.   There is a simpler way to perform experiment 4 that usually works.   On a table in a quiet place stand a burning cigarette on its end and patiently watch.   After a while smoke will be seen pouring down the outside to form a pool of smoke around the base.   Wisps will then sometimes be seen spontaneously rising from the smoke pool, storage and release , QED.   Perhaps several repetitions will be needed for you to believe what you see.

W hat's wrong with the idea suggested by some, that the walls drain enough heat from the smoke to make it heavier than air, is that : 1) the effect increases over time, rather than slowing as the walls get hotter, and 2) the smoke would spread out over time, not compress, seeking the cooler portions of the surface 3) Plexiglas has a very low specific heat and the smoke is not in circulating contact with it.

O thers have suggested that it's just the proximity of vertical surfaces.  If the same smoke source is held close to a similar surface in quiet open air, smoke is seen easily drifting upward even within the near ('sticky' ~1/32") boundary layer where all flow is laminar.  The proximity of the surface alone is not what makes the smoke turn and go down, but proximity must have something to do with it, since proximity is a controlling variable.

A fter removing the incense stick and covering the hole at the bottom the 2D vortex ring  settles into the shape at the right, similar to an apple upside down.  This continues without  outwardly changing shape or size for a very long time.   It seems to just spiral slower and slower until the smoke particles settle to the bottom of the chamber.

• Reflections

WHAT IS THIS PROCESS? HOW DOES IT WORK?

what is Convection?

• CCEA Single Award

Heat transfer - CCEA Convection in gases

Learn about how heat transfer occurs.

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Convection in gases

A convection current in air can be demonstrated using the following apparatus.

• A burning splint or piece of rope is blown out so that it is smoky.
• When the smoky rope is held in air the smoke rises.
• The rope is then held over both chimneys in turn as shown above.
• When the rope is held above the candle the smoke rises.
• When the rope is held above the other chimney the smoke is drawn down the chimney. It then passes across the horizontal section and up through the chimney above the candle.

What is happening?

• The candle flame heats the air around it.
• The hot air expands and increases in volume.
• The density of the air decreases, and it floats upwards, rising through the chimney.
• Cooler air is sucked in through the other chimney to replace the rising, warm air – a convection current has been set up.
• The smoke from the smouldering rope shows the path of the cool air.

Applications

Early coal mines were ventilated with fresh air using a similar method.

Two shafts were dug down from ground level to the mine and a fire was lit beneath one of them.

As hot air rose upwards, fresh, clean, cool air was sucked down the other shaft and across the coal mine.

Mind you, lighting a fire in a coal mine did have some safety issues.

A convection current from a radiator

• Air close to the radiator is heated.
• It expands, becomes less dense and rises.
• It is replaced by the cooler, denser air which surrounds it.
• This air is in turn heated, expands becomes less dense and rises.
• The process continues, a convection current is set up and heat is transferred through the air and hence through the room.
• A radiator heats mainly by convection – not by radiation.

More guides on this topic

• Energy forms - CCEA
• Energy resources - CCEA
• Electricity generation - CCEA

Related links

• BBC Science
• School Science
• Revision Buddies Subscription

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Chimney Convection Box AKA: Gas Convection Apparatus

A box comprising two chimneys and a candle. Convection currents draw smoke down one chimney and up the other.