5. temperature and heat
TRANSCRIPT
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Unit 3: Temperature and Heat
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OBJECTIVES
Define heat.
Describe how cooling and heating affect
materials.Explain the anomalous expansion of
water. Identify the three forms of heat
transfer.
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HEAT
All matter on earth contains Energy
External energy total potential energy and kinetic
energy of an object.
Internal energy - total kinetic and potential energy ofthe molecules of an object.
Example:
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HEAT
HEAT - a measure of the internal energy that has beenabsorbed or transferred from one body to another.
It is also the energy that transfers from a body of
higher temperature to another body of lowertemperature.
Particles move about more and take up more room if
heated
this is why things expand if heated It is also why substances change from:
solids liquids gaseswhen heated
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HEAT
When you touch ice, heat is transferred
from
1) your hand to the ice
2) the ice to your hand
When you drink a hot cup of coffee, heat
is transferred from1) your mouth to the coffee
2) the coffee to your mouth
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HEAT
When you touch ice, heat is transferred
from
1) your hand to the ice
When you drink a hot cup of coffee, heat
is transferred from
2) the coffee to your mouth
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HEAT
How does cooling and heating affectmaterials?
Heating and cooling describes the direction ofenergy flow from a region of higher energy to one ofa lower energy.
Heating
the process of increasing the internalenergy.
Coolingthe process of decreasing internalenergy.
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HEAT
Thermal Expansionrefers to changes in the dimensions ofbody that is associated with changes in temperature.
Matter tends to expand when heated and contract when
cooled. As the thermal energy of a substance increases, its particles
spread out and the substance expands.
Example
Q: When you have a difficulty in opening a tight bottlecap (metal), what would you do to open it easily?
Answer: Put it in a hot water so that the metal cap
expands a little.
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HEAT
Expansionthe increase in the size of material.
Contractionthe decrease in the size of material
Invar 0.1 mm
Pyrex 0.3 mm
Platinum alloy 0.9 mm
Glass 0.9 mm
Concrete 1 mm
Steel 1 mm
Brass 2 mm
Aluminum 3 mm
Expansion of 1 meter bar, heated at 100C
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HEAT
Some applications of thermal expansion:
Choosing of materials to be used as dental fillings
Thermal expansion joints in concrete roads and railroad
tracks
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HEAT
Some applications of thermal expansion:
Thermostats an automaticswitching mechanism that depends
on a pre-set desired temperature.
Bending of bimetallic strip is used tooperate the switch in a thermostat of
an oven.
Temperature cools below the setting
of the thermostat bimetallic strip
bends toward the brass side
heating coil is ON.
Temperature warms bimetallic
strip bends in other direction
heating coil OFF.
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HEAT
Almost all materials take up some more space (expand) when
heated and less space (contract) when cooled.
EXCEPT:
Water expands when it freezes [that is why ice (less dense) floats in water(denser)]. Contracts when cooled.
warmer water stays at the bottom.
Example:
When you open a container with frozen water inside,you probably have a difficulty in opening it.Why?
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HEAT
THREE FORMS OF HEAT TRANSFER
Conduction
Convection
Radiation
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HEATCONDUCTION
Heat energy travels when two objects at
different temperatures are in direct contact with
each other.
Example:
When you left a spoon in a bowl of hot soup, you
will found the spoon too hot to hold.
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HEAT
CONDUCTION
Heat is transferred through a
material by being passed from one
particle to the nextParticles at the warm end move
faster and this then causes the next
particles to move faster and so on.
In this way heat in an object
travels from:
the HOT end the cold end
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HEAT
CONDUCTION
Occurs by the particles hitting
each other and so energy is
transferred.Can happen in solids, liquids and
gases.
Happens best in solids-particles
very close together.
Conduction does not occur very
quickly in liquids or gases
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HEAT
CONDUCTION
CONDUCTORS
Materials that conduct heat quickly are
called conductors.
All metals are good conductors of heat.
Copper is a very good conductor of heat.
Pans for cooking are usually made with a
copper or aluminium bottom and plastic
handles.
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HEAT
CONDUCTION
INSULATORS
Materials that conduct heat slowly or poorly are
called insulators
Contains many small air spaces (poor
conductors because molecules are far apart).
Wood, plastic, glass, Styrofoam, cloth areexamples of insulators.
Water and gases are poor conductors.
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HEAT
CONVECTION
The heat in fluids is transferred to cooler
regions by currents (caused by warmer and
less dense pockets of fluids rising in cooler
and more dense surroundings).
Archimedesprinciple- states that less dense
materials float on denser materials.
The cooler fluid moves to the bottom, heats
up and rises again, form a cycle.
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HEAT
CONVECTION
Takes place in material where particles can move
around inside the material, i.e. liquid or gas
Occur because an area with warm particles
expands and becomes less dense than the coolerareas nearby. The warm area rises.
Cooler particles fall into the space left by the warm
particles and convection current is set up
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HEAT
RADIATION
Heat energy travels as electromagnetic waves
in the same manner and speed as light.
can transfer heat from a source to another
object even if there is a vacuum between
them.Example: heat from the sun travels through
empty space and warm the earth.
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HEAT
RADIATION
EMITTERS
Hotterobjectsemit(give out) heat.
Different surfaces emit heat at different speeds.
A dull blacksurfaceslosesenergy more quickly it
is agood radiator.
A bright shinyorwhitesurface is apoor radiator.Marathon runners need to keep warm at the end of
races, covering in shiny blankets reduces radiation
and therefore heat loss.
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mitters of heat
Bright shiny can
Poor radiator
Dull black can
Good Radiator
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HEAT
RADIATION
ABSORBERS
Cooler objects absorb (take in) heat.
Substances absorb heat at different speeds.
Dull, black surfaces absorb heat quickly.
Bright, shiny surfaces absorb heat slowly.
In hot countries, people wear bright white clothes and painttheir houses white to reduce absorption of energy from thesun.
Petrol storage tanks sprayed silver to reflect sunsrays
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Absorbers
Shiny, bright can
Poor absorber
Dull black can
Good absorber