How Do We Keep Hot Objects Hot?
Topic
Heat insulation
Introduction
Surrounding a hot object with a poor conductor of heat (an “insulator”) allows
the temperature difference to be maintained longer. Insulators work by limiting
the heat that can be lost from an object by conduction. Air is a very
good insulator – provided it is not allowed to form convection currents
(see Experiment 3.02). Materials that trap air therefore make very good
insulators. In this experiment, you will study the insulating qualities of various
materials and consider which is most effective.
Time required
1 hour
Materials
four 500 ml round-bottomed flasks
each with a line marked at the base
of its neck
four single-hole stoppers to fit flasks
4 thermometers (0 – 100°C)
4 support stands and three-pronged
clamps
kettle containing boiling water
tap water
roll of cotton (about 350 g cotton)
polystyrene packing chips
(about 200 g)
feathers (e.g., from half an old
pillow)
4 large plastic containers
(e.g., 4-liter ice cream tubs)
4 funnels
clock
marker pen
Safety note
Be careful when pouring boiling water from the kettle into the flasks. Students
and teachers who are allergic or potentially allergic to feathers should not
handle this insulator.
Procedure
Students can either form groups of four, with one student responsible for a single
container, or alternatively the experiment can be performed by one person
repeating the experiment for the four different insulating materials.
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1. Using a stand and clamp, secure each flask so that it is supported within a
large container (see diagram 1a below). Label the flasks A – D
2. The four different insulating materials are air (A), raw cotton (B), polystyrene
packing chips (C), and feathers (D). Fill containers B – D with the appropriate
material so that the flask is surrounded (see diagrams 1b – d below).
3. Insert the thermometers into the stoppers.
4. Using the funnels fill each flask to the mark with water that has just boiled.
5. Insert a stopper into the neck of each flask (the bulb of the thermometer must
be in the water).
6. Read the temperature when the level of liquid in the thermometer has stopped
rising. Make sure your eyes are on a level with the liquid level in the
thermometer or you may get an inaccurate reading.
7. Record the initial temperature of the water in the data table on the next page
in the appropriate column.
1a
1b
clamp
stopper
raw cotton
thermometer
flask
marked line
large plastic container
1c
1d
polystyrene
packing chips
feathers
Flasks surrounded by different insulating materials
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8. Record the temperature of the water in each flask every five minutes for
30 minutes.
9. Using the data obtained from all the flasks, draw a graph of temperature
against time.
DATA
Temperature
TABLE
A
B
C
D
air (empty container)
cotton bulk
feathers
packing chips
Initial
After 5 minutes
After 10 minutes
After 15 minutes
After 20 minutes
After 25 minutes
After 30 minutes
Analysis
1. Which of the insulating materials was best at keeping heat in the flask?
2. What substance is being trapped between the different materials?
3. Why does the flask in A lose heat more quickly than the others?
Want to know more?
Click here to view our findings.
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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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PHYSICS EXPERIMENTS ON FILETM
OUR FINDINGS • 10.14
2. All the materials used (cotton, packing chips, and feathers) trap air around
the flask.
3. The flask which is surrounded by air (A) loses heat quickest because the air is
free to move in convection currents. These move the heat away from the
surface of the flask, which then cools as cooler air is drawn in to replace it.
Insulation works by trapping a non-conducting material around the container
being insulated. Air that is unable to flow freely does not conduct heat well and
thus makes a very good insulator. Flask A is surrounded by air that is able to
move. It therefore loses heat quickly to the convection currents in the air
surrounding the flask. Flasks B, C, and D are covered with materials that trap
pockets of air around the flask. The rate at which the flasks lose heat shows how
much air is trapped (and how well it is trapped) in each covering and thus its
efficiency as an insulator.
3.06 Hot Metals
Part A: Observation of expansion and contraction
1. The ball was able to pass through the ring when both were cool.
2. The hot ball did not pass through the cold ring.
3. When both the ball and ring were hot, the ball passed through the ring.
4. Both the ball and ring expand when they are hot. The cold ball passes through
the cold ring. The hot ball passes through the hot ring because both have
expanded. However, the hot ball is too large to fit through the cold ring.
Part B: Observing differential expansion of metals
1. The bar is made of two strips of metal – brass (shiny yellow) and iron (shiny
gray) – that are riveted together and supported in a wooden handle.
2. The bar bent.
3. The brass was on the outside of the bend.
4. As the outside of a bend is longer than the inside, brass must expand more
than iron on being heated.
5. The bar straightened on cooling. This shows that the metals contracted to
their original lengths when cool.
6. If the strips had not been riveted together, the metals would have expanded
and contracted independently, and there would have been be no bending.
The fact that metals expand by different lengths on being heated by the same
amount is useful in the design of heat-sensitive switches. For example, a bimetal
strip incorporated into a fire detection circuit bends in the heat if there is a fire
and completes an electrical circuit that operates sprinklers.
Gases and liquids also expand on heating – a liquid-in-glass thermometer makes
use of this phenomenon.
3.07 Changing State
1. The temperature of the stearic acid increased quickly both when it was a solid
and when it was a liquid.
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