Download Heat - SD43 Teacher Sites

Heat
What are some natural sources of heat?
Lightning, fire caused by lightning strikes, volcanoes, geothermal hot springs,
living animal bodies
What are some artificial sources of heat?
Furnaces, electric and gas heaters, campfires, blow dryers,
WHY STUDY HEAT?
o Weather – how does heat and the movement of heat energy influence
world weather patterns?
o Cooking – how does heat change the chemical and structural composition
of foods?
o Effects on materials – thermal expansion in buildings and roads
o Effects on earth’s surface – how the Earth is heated by the sun, and what
effects take place; geothermal energy; natural weathering and erosion
o Affects of heat energy on our bodies
What are some professions or jobs that depend on heat transfer?
Cook, baker, meteorologist, appliance manufacturer, launderer, welder, sheet
metal worker, machinist, etc.
DEFINITION
o Heat is a form of energy
o Heat is the flow of energy from warmer matter to cooler matter
o When you heat an object, you are transferring thermal energy to it from
another object that is at a higher temperature. Heat is the amount of
thermal energy that is transferred between the two objects due to a
temperature difference. Heat transfer between objects is done by
conduction, convection and radiation. The standard unit of heat
measurement is the calorie.
An object feels warm or hot if its temperature is higher than your skin.
Cooling an object is when you are transferring thermal energy from the object to
another object that is at a lower temperature. You could say that you are
removing thermal energy from the first object.
Review of STATES OF MATTER (solid, liquid, gas)
o LIQUID has a fixed volume but takes on the shape of the container
o SOLIDS have a fixed shape and volume; atoms are in a regular
arrangement
o GASES assume the shape and volume of its container
o CHANGE OF STATE is when the addition or removal of heat energy alters
matter
MOLECULES AND ATOMS
The heat energy of a substance is determined by how active its atoms and
molecules are. A hot object is one whose atoms and molecules are excited and
show rapid movement. A cooler object’s molecules and atoms will be less
excited and show less movement. When they are in the excited state, they take
up lots of space because they’re moving around so fast. When the atoms and
molecules settle down, or cool down, they take up less space.
If a hot high-energy atom comes into contact with a cool low-energy atom, the
excited atom will lose some of its energy to the cool atom. The two atoms will
settle into an energy level that’s between where they started out. That level is
called THERMAL EQUILIBRIUM. E.g. adding ice cubes to a cup of hot water
(use demo)
Heat also causes things to grow. This is called THERMAL EXPANSION. When
things get cold, they contract, or get smaller. Railway tracks are laid with a short
gap between the rails, in order to allow room for expansion when the steel tracks
heat up and safely expand on a hot day without derailing the trains traveling over
them. Thermometers work on the principal of thermal expansion.
Demos:
o Cold jar
o Movement of atoms when heated (PP balls in cage demo)
o Thermal expansion (balloon demo)
o Thermal equilibrium (ice in warm water demo)
TYPES OF HEAT TRANSFER
(Convection, Conduction, Radiation Lab Sheets)
How does heat travel from one object to another?
By Conduction, Convection and Radiation.
RADIATION
Radiation is when warm or hot matter emits electromagnetic radiation that is then
absorbed by an object at a distance. The absorption heats the second object.
THE SUN (Light bulb demo, Sunshine demo outside)
Our atmosphere is largely heated from below, by the earth itself. The sun’s
radiant energy is first absorbed by Earth’s water, rocks, and soil, and changed
into heat. These warmed substances then heat the layer of air closest to Earth’s
surface through “infrared radiation” (radiant heat). The temperature of the
atmosphere is warmer closer to Earth than farther away because of this heating
process. Also, air closer to Earth – which is denser, dustier, and moister than air
in upper layers – is able to absorb more of the sun’s radiation, as well as more of
the infrared radiation from Earth.
FUN FACT: If, for some reason, you want to fly to the surface of the sun one day,
it would take a commercial passenger jet about – get this – nineteen years, flying
non-stop! Traveling at the speed of light (300 000 km per second), how many
seconds does it take light from the sun to reach Earth (about 150 000 000 km?
Which other things radiate heat besides the sun?
Light bulbs, radiating space heaters, running motors and engines
CONVECTION
*Convection is the movement of thermal energy from one area to another in a
liquid or gas.
(Paper motor demo
(Convection current in water tank demo)
Describe how a furnace works – could use a blow dryer as well
CONDUCTION
*Conduction is when materials are in physical contact and kinetic energy is
transferred through collisions of their particles.
e.g. heat being transferred from a stove top burner into a metal cooking pot, and
then into the water inside the pot.
Conduction is the main way for heat to travel through solid materials. All solid
materials conduct heat, but some do a better job than others. Generally, metals
are good conductors while porous materials are not. Styrofoam is a good
example of a poor conductor, but it’s a good insulator. Good conductors of
electricity are usually good conductors of heat.
Conductors
Types of materials that conduct heat (two surfaces demo, boil water in paper cup
demo, spoons in cup demo, cold metal demo, battery and steel wool demo)
*Insulation (oven mitts, cooler, Styrofoam)
SOURCES OF HEAT
Sun (see above)
Geothermal (from the earth’s core)
As you go deeper into the earth, the temperature becomes hotter (positive
correlation!). Molten or partially molten rock at temperatures between 650 to
1200 degrees Celsius is thought to exist everywhere beneath the Earth’s crust at
depths of 80 to 100 kilometres. It is believed that much of this heat is created by
the decay of naturally radioactive elements, and that it accounts for 45 to 90% of
heat escaping from the Earth. Other ideas include the heat caused by the falling
of meteorites when the Earth was formed, the affects of the Earth’s magnetic
field, and the affects of the tidal force causing stress on the land, making it
compress and distort, generating heat. Geothermal heat at the surface is highly
concentrated where magma is close to the surface, e.g. volcanoes and spreading
ridge areas. Some of this heat is caused by the friction between moving plates in
the subduction zones as moving tectonic plates slide beneath the continental
crust; this frictional heat melts rock and creates magma chambers whose
contents eventually rise to the surface and is expelled as lava.
Natural geysers form when underground chambers fill with water and are heated
geothermically. When the water is heated to the boiling point the geyser erupts,
spewing its contents, and the cycle starts again. (cyclic hot water demo).
FIRE
Fire is nothing more that oxygen atoms combining with atoms in the substance
being burned (like a piece of wood). Atoms have to get excited before they’ll
burn. The initial excitement is provided by external heat from an external source,
such as a match. The atoms on the surface then start to vibrate as they heat up.
They vibrate more and more until they start to move apart. The atoms of oxygen,
carbon and hydrogen in the wood begin flying apart in all directions. AS this
happens, oxygen in the air combines with the loose atoms and molecules. The
wood is on fire. As the wood burns, many new forms of matter result – water,
carbon dioxide, methane, pentane, hexane, and octane – plus light, heat and
sound. A piece of burning wood is like a chemical factory. You can make a fire
using a fire bow because of a series of steps involving matter and energy. You
start by putting energy into moving the bow. The mechanical energy is then
converted to heat energy through friction. A chemical reaction is started by the
heat, and the result is fire.
Ignition point: The minimum temperature at which combustion of a solid or fluid
can occur.
Effects of heat
Cooking (toaster demo)
Weather
Earth’s surface
MEASUREMENT OF HEAT
Thermometer – liquid changes its volume relative to its temperature. Liquids
take up more space when they are warm and less space when they are cold.
The change in volume is fairly small, so all bulb thermometers use a fairly large
bulb and a narrow tube to make the change in volume more obvious. As the
temperature rises, the liquid (usually mercury) increases in volume and is pushed
further up into the tube, where the temperature may be read by the
corresponding numbers adjacent to the tube. The position of the liquid in the
tube and the markings are calibrated with the correct temperature.
(make a thermometer activity
What are the problems with using this type of bulb thermometer?