Heat Beaming Through Air And Space
Topic
Heat transfer by radiation
Introduction
You can feel the heat given off by the Sun. But how can heat energy travel
93 million miles through the vacuum of space where there are no particles to
transmit it by conduction or convection? The Earth is heated because the sun
emits electromagnetic radiation called infrared radiation. This is similar to visible
light but has a longer wavelength. Like light, this radiation travels in straight
lines and obeys the same laws of reflection and refraction (see Experiments 4.02
and 4.03). Radiation is the only way heat energy can be transferred in a vacuum
and directly through air. Infrared rays do not carry heat but energy. When an
object absorbs infrared rays, these rays increase the kinetic energy of its
molecules. This produces a heating effect in the object. Although you cannot see
this radiation, you will study how it is absorbed in this experiment.
Time required
1 hour
Materials
spotlight (at least 60 watt) supported on a stand
3 Pyrex™ test tubes
3 support stands with clamps
tap water
3 thermometers (0 – 100°C)
3 rubber stoppers to fit the test tubes, each with a hole large enough to
fit a thermometer
clock or watch
marker pen
30 cm ruler
Safety note
Be very careful not to spill water on the spotlight and not to drip any water near
the electricity supply to the spotlight. Do not touch the hot spotlight.
Procedure
1. Fill the three test tubes with water from the cold tap to about 2 cm from
the top.
2. Carefully push the thermometers through the stoppers and insert the stoppers
in the test tubes (see diagram 1 on the next page).
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3. Support each test tube in a clamp and stand. Label them A – C.
4. Position the test tubes 10 cm away from the spotlight and in the following
positions (see diagram 1 below):
tube A above the spotlight;
tube B in front of the spotlight;
tube C to the side of the spotlight.
1
Pyrex TM test tube
A
stopper
thermometer
B
C
clamp
water
stand
Test tubes positioned around the spotlight
5. Observe the temperature of the cold water in each test tube and record in the
data table on the next page. When reading the thermometer, make sure your
eyes are level with the liquid level in the thermometer or you may get an
inaccurate reading.
6. Turn on the spotlight.
7. Observe the temperature of the water in the tubes every five minutes for 30
minutes. Record the values in the data table on the next page. Turn off the
spotlight.
Analysis
1. What did you observe when the light was turned on?
2. Which of the three test tubes recorded the highest temperature increase?
3. Can you explain how the heat energy reaches tube B?
4. How do you think heat energy reaches the tube at position A?
5. How does the heat energy get to tube C?
6. Compare the temperature increases in the three test tubes.
Want to know more?
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DATA
Temperature
Test tube A
TABLE
Test tube B
Test tube C
Initial
After five minutes
After 10 minutes
After 15 minutes
After 20 minutes
After 25 minutes
After 30 minutes
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or transmittal is copyright protected by the publisher.
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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10.13 • OUR FINDINGS
PHYSICS EXPERIMENTS ON FILETM
6. The temperature of the water increases most in tube B. This test tube is heated
directly by radiation from the spotlight. The other two test tubes are heated by
convection currents caused by the hot surface of the lamp, which heats up the
air in contact with it. The test tube at position C rises in temperature less than
the test tube at position A. At position A, the air is hot and rising, while at
position C, the air is falling and cooler.
3.04 Why Do Some Objects Cool Down Faster Than Others?
Part A: The effect of volume on heat loss
1. The water in the small beaker cooled down the fastest.
2. If there is a number of objects of the same shape but different sizes at the same
temperature, the largest object will stay hot for longest.
Objects lose heat by conduction from their surfaces. Larger bodies have a smaller
surface area in proportion to their volume than smaller bodies. You can check
this by performing a calculation for a beaker, which is cylindrical in shape.
The volume of a cylinder is given by the formula πr2h (r is the radius of the
beaker and h is its height). The surface area of the sides of a beaker is given by
the formula 2πrh and the area of its top surface by πr2. The table below shows
an approximate calculation for a 100 ml and a 1,000 ml beaker.
100 ml beaker
1,000 ml beaker
Radius (cm)
4.5
10
Height (cm)
7.0
14
Surface area = 2πrh + πr2 (cm2)
261.54
1193.81
Volume = πr2h (cm3)
445.32
4398.23
Surface area/volume
0.59
0.27
Hot liquids in containers also lose heat through the top of an open container by
convection from their top surface. This can be prevented by fitting a lid to the
top of the container. You might like to repeat the experiment using covered
beakers.
Part B: The effect of shape on heat loss
1. Water in the beaker cooled down quicker than in the flasks.
2. The water in the beaker cooled down fastest followed by the water in the
spherical flask, then the water in the Erlenmeyer flask. Knowing that all the
containers had the same volume and that containers with larger surface areas
lose heat faster, you can deduce that the beaker must have a larger surface area
than the spherical flask, and the spherical flask a larger surface area than the
Erlenmeyer flask.
3.05 How Do We Keep Hot Objects Hot?
1. The feathers keep the contents of the flask hot for longest.
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