Heat Flow Through Liquids And Gases
Topic
Heat transfer by convection
Introduction
Unlike particles in solids (see Experiment 3.01), particles in liquids and gases are
free to move. How does this affect the way in which heat energy is transmitted in
liquids and gases? In liquids and gases, heat energy can be transferred from a hot
place to a colder one by the actual movement of the hot molecules. When a
liquid or gas is heated, the kinetic energy of the molecules being heated increases
and the molecules move apart. The heated areas of the liquid or gas are therefore
less dense than the cooler areas. The hot molecules rise through the body of the
gas or liquid, cooling as they reach the colder parts. As the molecules cool, they
sink down again. This movement is called a “convection current.” Convection
currents carry heat energy through the whole liquid or gas. In this experiment,
you will investigate how convection currents heat water and how heat is
transferred through gases.
Time required
30 minutes for Part A
15 minutes for Part B
Materials
For Part A:
50 ml glass bottle or Erlenmeyer
flask
stopper with two holes (to fit glass
bottle or flask)
2 pieces of glass tube (same diameter
and length to fit holes in stopper
and as long as the depth of the
glass bottle or flask)
tap water
boiling
water
food coloring
eyedropper
large transparent container (glass or
plastic) about 6 cm taller than the
bottle or flask, and with a mouth
wide enough to insert the bottle
or flask
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cold tap water
30 cm ruler
clock
potholder or tongs
For Part B:
1-liter Pyrex™ beaker
metal sheet or cardboard covered
with aluminum foil cut to the
shape shown in diagram 2A on
page 3.02–3
candle about 6 cm tall
wood splinter
matches or lighter
stopwatch
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Safety note
Be careful when pouring and handling hot water, and when using the lighted
candle and the wood splinter. Make sure long hair is pulled back.
Procedure
Part A: How convection currents heat a body of water
1. Fit the stopper in the bottle or flask and insert the two pieces of glass tube
through the stopper so that one reaches to within 0.5 cm of the base of the
bottle and the other protrudes about 1.5 cm above the stopper.
2. Use the ruler to measure the distance from the base of the bottle to the top of
the higher tube.
3. Fill the large transparent container with cold tap water to a depth about 2 cm
greater than the height measured in step 2.
4. Remove the stopper from the bottle and carefully fill it with water that has
just boiled. Add a few drops of food coloring using the eyedropper.
5. Carefully replace the stopper and glass tubes in the bottle.
6. Holding the bottle filled with hot colored water with tongs or a potholder,
lower it into the cold water in the large container (see diagram 1 below).
Make sure that both ends of both glass tubes are under water. Note the time
on the clock.
7. Record the movement of the colored water in data table A on the next page
after 30 seconds, 2 minutes, 5 minutes and 20 minutes.
1
glass tubing
stopper
transparent
container
glass bottle
cold water
hot colored water
Flask containing hot colored water inside the container full of cold water
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DATA
TABLE
A
Movement of the colored water
After 30 seconds
After 2 minutes
After 5 minutes
After 20 minutes
Part B: Observing convection currents in air
1. Stand the candle upright on one side of the beaker as shown in diagram 2A
below. If necessary, melt the base of the candle slightly to widen its diameter
and make it more stable.
2. Light the candle and allow it to burn for a few minutes until the flame is
established.
3. Carefully insert the metal sheet into the beaker to hang from the rim in the
center as shown in diagram 2A below. Make sure there is a gap between the
bottom of the beaker and the metal sheet.
4. Light the wood splinter. Allow it to burn for two minutes and then blow it
out. It should be smoking.
5. Hold the splinter up and observe how the smoke moves. Draw the path
followed by the smoke in data table B on the next page.
6. While it is still smoking, hold the splinter in the empty side of the beaker
(see diagram 2B below).
7. Draw the path followed by the smoke in the data table B on the next page.
2
smoking splinter
metal sheet
metal sheet
side view
beaker
candle
A
B
Arrangement of candle, metal sheet, and smoking splinter
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DATA
TABLE
B
Movement of smoke
Smoking splinter held in air
Smoking splinter held inside beaker
Draw the path followed by the smoke in each case.
Analysis
Part A: How convection currents heat a body of water
1. What happens to the water in the flask?
2. What happens to the water in the container?
3. What color is the water in the container after 20 minutes?
4. What does this tell us about the way heat spreads through liquids?
Part B: Observing convection currents in air
1. How does the smoke from the smoldering splinter move when it is held up
outside the beaker in the air?
2. How does the smoke from the smoldering splinter move when it is held inside
the beaker?
3. What does this tell us about the way hot and cold air behave?
Want to know more?
Click here to view our findings.
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PHYSICS EXPERIMENTS ON FILETM
OUR FINDINGS • 10.12
3.02 Heat Flow Through Liquids And Gases
Part A: How convection currents heat a body of water
1. About 30 seconds after the flask is placed in the container, colored water from
the flask rises into the clear water of the container.
2. The colored water rises to the surface of the container and gradually sinks
around the edges of the container. Initially in separate colored drops, these
drops gradually mix with the rest of the water.
3. After about 20 minutes, the water in the container is a uniform color.
4. This shows that convection currents spread heat energy (the hot colored
water) through a body of liquid.
Part B: Observing convection currents in air
1. The smoke rises.
2. Some of the smoke from the splinter sinks to the bottom of the beaker, then
moves under the metal plate and rises above the candle flame.
3. This experiment shows that when air is heated it becomes less dense and rises;
cold air is then drawn in to replace it. This is then heated and in turn rises.
This forms a convection current. Convection currents were a way of ensuring
ventilation in early coal mines. If a mine had two airshafts and a fire was
maintained at the base of one, fresh air would enter the mine to replace the air
rising above the fire (see the diagram below).
hot air rising
cold fresh air
fire
Ventilation in an early coal mine
3.03 Heat Beaming Through Air And Space
1. The tube at position B (in front of the lamp) was brightly illuminated
(radiation from the spotlight was reaching it directly).
2. The temperature of the water in tube B rose more than the others.
3. Energy reaches B directly by radiation. The spotlight emits electromagnetic
radiation ranging from the visible spectrum to the longer wavelengths of
infrared. Infrared radiation is absorbed by the water in the test tube and
converted to heat energy.
4. Heat energy reaches position A by convection. The surface of the spotlight
becomes hot and heats up the air in contact with it. This hot air becomes less
dense and rises to heat the water in tube A.
5. Some heat also reaches position C by convection. As the hot air rises up above
the spotlight, it cools and then falls – passing the test tube at position C.
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