5. temperature and heat

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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

5. temperature and heat

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