Heat Energy
- Review questions -
1. Arrange the following temperatures descending order: 75°C,
100°C, -25°C and 0°C 100°C, 75°C, 0°C, -25°C
2. Copy and complete these sentences
a. Temperature is a measure of how hot something is.
b. Temperature is measured with a thermometer.
c. On the Celsius scale, water boils at 100°C and freezes at
0°C.
d. Metals are good conductors of heat.
e. Plastics are good insulators because they prevent heat
from passing through them.
f. Heat can travel through solids because the particles in
them bump into each other.
g. As an object gets hotter, its particles move faster.
3. Why are you more likely to burn your lips drinking from a
metal cup than you are drinking from a plastic cup? You are
more likely to burn your lips drinking from an iron cup
because unlike the plastic cup that is a good insulator, the
metal cup conducts heat. The heat can travel easily from the
liquid through the cup and burn your lips.
4. You get into a bath containing water at 45°C.
a. What will happen to your body temperature? Your body
temperature would increase, trying to match the
temperature of the water.
b. What will happen to the temperature of the water? The
temperature of the water would decrease, trying to match
the temperature of your body.
c. Does heat flow from the water into your body or from your
body into the water? Explain. As the water has more heat
energy than your bogy holds, the heat from the water would
transfer to your body, so that your body and the water are
at equilibrium.
5. You leave the door of your heated house open on a cold night.
Does the cold come in or does the heat go out? Explain. Hot
air will leave, and enter into the cooler area through
conduction; the temperature wants to reach symmetry with the
surrounding air.
6.
a. Which is hotter – a cup of water at 50°C or a bath full of
water at 50°C? Temperature is a measure of how fast the
atoms move, and as the atoms are moving at exactly the
same speed, they are both the same temperature. However,
since there are more molecules in the bathtub that contain
heat energy, the water in the bathtub holds more heat
energy then that of the cup.
b. Which contains more heat energy – the cup of water or the
bath full of water? It depends on the amount of kinetic
energy that the molecules hold. Technically speaking, the
bathtub is more likely to hold more heat energy, as there
are more molecules.
7. Your friend Kate says that the iron in fence feels hotter than
the wooden posts. How could you convince her that the iron and
the wood are really at the same temperature, and explain to
her why the iron feels hotter? Your sense of an object’s
temperature has to do with the ability of that object to
conduct its heat from itself to your skin. The iron is a
really good conductor, and so you can feel the heat to touch
really easily. As your hand is cooler than the iron, heat
naturally flows from the iron to your hand as to reach a point
of equilibrium. The wood on the other hand, is a poor
conductor, and so the heat does not transfer as easily, and it
feels cooler to touch. The wood would not be able to quickly
transfer its heat energy to your hand like the iron did, and
so it feels cooler to touch.
8. Use what you have learnt to explain each of the following:
a. Containers (e.g. a saucepan) generally have plastic
handles. Saucepans would more often than not have a
plastic handle. This is because plastic is a good
insulator, and the metal is a good conductor. To stop your
hand fro getting burnt from direct contact with the metal,
you cover the metal with something that will stop or slow
down heat transfer between the handle and your hand. So
you cover the metal with plastic and you are less likely
to be burnt.
b. Putting a metal skewer into a potato will decrease the
time needed to bake it. By placing the metal skewer in the
potato when you bake it allows the heat to travel through
the conductor easily, the conductor being the skewer, and
directly to the middle of the potato, allowing heat to
penetrate the potato, and cook it on the outside and the
inside at the same time.
c. It is warmer to wear to thin jumpers than one thick one.
It is warmer to wear thin jumpers because layering your
insulators traps stagnant air and heat energy better,
keeping it close to your skin and not allowing to escape.
d. Drink coolers are made from foam plastic rather than solid
plastic. Styrofoam is a better insulator than polystyrene
Styrofoam is expanded polystyrene, it has pockets of
stagnant air trapped within it. Air, and gases in general,
are very poor conductors as the transfer of heat energy
relies on friction and the particles in gases are further
apart then that of solids and liquids, making it harder
for heat energy to bump into and transfer. The Styrofoam
cup is very resistant to heat transfer. The winding chain
of molecules on the inside of the cup, as a result from
blowing the polystyrene up into a foam, makes it harder
for heat to just go through smoothly, it has to follow a
winding set of molecular structures.
9. What is convection? Convection is the travel of heat energy as
it goes with the flow of another substance ie. Liquid or gas.
10.
Suggest why the air in a heated room is usually cooler
near the floor than near the ceiling. Hot air rises, because
the molecules are further apart and vibrate more rapidly than
that of cool air, allowing the hot air to work its way to the
top of the room. In a heated room, the floor will be cooler
and the roof warmer because of this principle.
11.
a. Where is the cooling element in a refrigerator? Why? The
cooling system in a refrigerator is at the top. It is
always placed at the top, because hot air naturally rises
and the cool air that it is pumping out will naturally
fall to the bottom of the fridge. The hot warmer air will
rise, but because the cooling system is at the top of the
fridge, it pumps out more cool air, making the top the
most effective place to put the cooling system.
b. What happens to the air inside a fridge when you open the
door? Why? Heat energy travels via convection and makes
the cool air in the fridge warmer. This is because heat
energy naturally travels to cooler places, not warmer.
12.
Draw a diagram showing how a hot cup of coffee cools by
convection.
13.
If you are cooking a dish that needs a very high
temperature, the recipe may tell you to put it on the top
shelf of the oven. Explain.
The recipe may tell you to put it on the top shelf of the oven
because hot air rises, and it will be exposed to more heat on the
top shelf then on other shelves.
14.
Glider pilots look for thermals. What are they and why do
gliders need them? Thermals are convection currents, where
heat travels along. Gliders use them because they can catch
them, and it’s hot air rising, and it’ll allow them to stay
suspended in the air for longer with minimal effort.
15.
Suppose you are camping and you want to boil a billy on a
fire. Would you place the billy on the ground and build the
fire around it, or would you hang the billy from above the
fire? Explain your answer.
I would put the billy on top of the fire, because hot air
rises and the kettle would be hit with the full heat of the
fire.
16.
Suppose you have two surfaces, one white and one black.
a. Which is the better radiator of heat? Black
b. Which is the better reflector? White
c. Which is the better absorber? Black
17.
The diagram below shows an experiment to measure how well
six different materials conduct heat. Use the results to help
you answer the following questions:
a. Which material is the best conductor? How do you know
this?
The copper is the best conductor, and we can tell because
it is the one with the least wax.
b. Which material is the best insulator?
The wax is the best insulator.
c. Would food cook more quickly in a glass or an iron
casserole dish?
Food would cook faster in an iron casserole dish.
d. Why do some saucepans have copper bases?
Because copper conducts heat really well, and it would
allow heat energy to be transferred from the heat source
to the food faster.
e. Is water more likely to freeze in a plastic or a copper
pipe? Explain.
It is more likely to freeze in a plastic pipe, because the
iron would conduct heat to the water, and plastic would
stop it from getting through as easily.