SPH3U: Energy
Thermal Energy Transfer
Task: Working with your group members, answer the following questions using section 6.2
1. What does the word heat describe in physics?
Heat refers to the transfer of thermal energy.
2. What are the three main ways that thermal energy can be transferred?
conduction, convection, and radiation
3. Describe how thermal energy is transferred when a warmer substance is in contact with
a cooler substance.
This is conduction: the particles of the warmer substance have more kinetic energy
(whether it is in the form of vibration or movement). When the materials are in contact,
the warmer particles bump into the cooler particles and during the collision the warmer
(higher energy) particles lose some kinetic energy which the cooler particles absorb. This
speeds up the cooler particles so they now have more kinetic energy and thus a higher
temperature. At the same time the hotter particles slow down so they have less kinetic
energy and a lower temperature than before. This is also how thermal energy spreads
from hotter areas to cooler areas within the same substance.
4. Describe the process of convection.
When thermal energy is added to the lower area of a liquid or gas, it heats up the lower
region. As this material heats it expands and becomes less dense than the cooler
material above it. Since less dense substances float on top of more dense substances, the
less dense matter will rise and the cooler material will sink. In this way the warmer
particles move upward and carry the thermal energy to the upper part of the liquid or
gas. The cooler material that has sunk to the bottom then heats up and the process
repeats which produces circulating areas of the fluid called convection cells.
5. Which method of thermal energy transfer doesn’t involve the movement of particles?
radiation doesn’t involve the movement of particles
6. Why would we place heaters near the floor of a room instead of near the ceiling?
We do this to take advantage of convection to move the warm air around the room. If
the heaters were placed near the ceiling, we would have to wait for conduction to warm
up the air at the lower levels, which would take a lot longer (and be very dangerous
since, by the time it is warm at ground level, the ceiling would be extremely hot).
7. Sometimes, heaters are actually placed overhead, but you can still feel heat from them.
What type of thermal energy transfer is being used in this case? [Hint – in these
situations the heater will be glowing red-hot]
These heaters use radiation (in the form of infrared radiation) to transfer thermal energy
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8. Why would your metal locker feel cool, even though it is at the same temperature as
the air around it?
Metal feels cool because it is a good conductor. Since it is a good conductor, it allows
thermal energy to transfer from your hand into the locker quickly. Your hand senses this
movement of energy out of the skin as coolness. The air, on the other hand, is an
insulator so it doesn’t allow the energy to leave your body so it doesn’t seems cold.
(However, the bigger the difference is between your body temperature and the cold air
temperature, the quicker the thermal energy can leave your body and your skin senses
this flow of energy as coolness if the air is cold enough)
9. Why might handles of pots used on stoves be made of wood?
Wood is an insulator, so thermal energy won’t transfer from the pot to your hand.
10. Is air a good conductor or an insulator?
Air is an insulator (poor conductor)
11. What allows fiberglass or foam insulation in houses to work well as insulators?
Small pockets of air trapped within the structure of the fibreglass or foam allow it to be
an insulator
12. If you walk from a carpet onto a tiled floor in your bare feet you might feel as though
the carpet is warm, but the tiles are cold. If you measured their temperatures, they
would be the same. Why do they feel different? What allows the carpet to be a poor
conductor?
They feel different because the tile is is a good conductor which allows energy to flow
from your feet into the floor fairly quickly, which we sense as coolness. The carpet, on
the other hand, is a good insulator so heat doesn’t leave your feet and they feel warm
(see question 7).
Air trapped in the carpet fibres makes it a good insulator.
13. Why would a potato cook quicker in a conventional oven (not microwave) if a metal
skewer (spike) was inserted into it?
The potato cooks by conduction as thermal energy enters the potato from the hot air in
the oven. The metal spike would be able to conduct thermal energy to the centre of the
potato so it will heat from the inside outwards as well as from the outside inwards,
reducing the time it takes to cook. This should NOT be done in a microwave oven as
metal should not be used in microwave ovens.
14. What is the best insulator?
A vacuum (empty space) is the best insulator.
15. Water is a fairly good conductor of heat. Use this fact to explain why it is much more
dangerous to fall into 4°C water than it is to stand outside when the air is at 4°C.
Since water is a good conductor it allows heat to leave your body quickly and if you are
immersed in water at 4°C a lot of heat can leave in a short time which can quickly reduce
your body temperature by a dangerous amount. Since air is a good insulator, it will take
much, much longer for your body temperature to drop, so it’s not as dangerous.
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16. Are the following materials used because they are a good thermal conductor, or
because they are a good thermal insulator? Explain why you would want a good
conductor or insulator in each case.
a. cooper pot for cooking
copper is used for pots because it is a good conductor and we want to be able to
transfer energy to the food
b. wooden spoon
wood is used for spoons because it a good insulator and we want to prevent thermal
energy for travelling up the spoon to our hand
c. down-filled sleeping bag (down is very small bird feathers)
we don’t want our body heat to leave a sleeping bag so we use down (feathers)
because it is a good insulator(because of the air trapped between the fibres of the
feathers)
d. ceramic mug
the ceramic is a good insulator so it will reduce heat transfer to our hands if there is
a hot liquid in the mug
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