Heat Transfer

• • •

Heat Energy

Heat is the form of energy that flows from a

hot region to a cold

region Heat energy is also called

thermal energy

Heat energy flows by one or more of the processes of:

1.

2.

3.

4.

CONDUCTION CONVECTION RADIATION EVAPORATION

Thermal Energy

• Thermal energy (or heat) is measured in

joules

• A space shuttle can reach heats of

1500 °C

, because of the

friction

between molecules in the air and the craft (which is travelling at 23,612 mph) • The small beaker boils first, because it needs

less thermal energy

to reach 100 °Celsius

The Four laws of thermodynamics

• No heat flows between object at the same temperature • All machines are inefficient • Heat transfers to colder matter until both objects are the same temperature • At absolute zero (-273°C) no reactions occur

Questions on heat transfer

• How does heat move?

• It

flows

from

hot areas to cold

areas • What units is thermal energy measured in?

•

Joules

• Which has more thermal energy: a swimming pool with the water at 30 °C or a cup of coffee at a temperature of 80 °C?

• The swimming pool because it

takes longer to heat

the water to 30 °C

Conduction

• • All

metals are conductors Copper

is an excellent conductor so it is used to make pipes in the house

Conduction by Atoms

•

Atoms vibrate more when they are heated

•

Conduction is the only way for heat to travel through solids

•

Gases

are the

worst conductors

• A very poor conductor is called an

insulator

• A

vacuum does not allow conduction

Finding the best conductor

• All the rods have the

same length

• They are all burner reaches a

wax melts

stick falls off

heated equally

at one end with the bunsen • When the other end of a rod

certain temperature the paraffin

and the match • A match stick will

fall

off the best conductor first • This should be the rod made of

Copper

Water - a poor conductor

• A boiling tube of water is

heated near the top of the water

• Water

boils at the top

• The

bottom

of the tube

remains cool

enough to hold • This shows that

water (and glass) only conduct heat relatively slowly

• Microwaves make water molecules in food move quickly making them rub against food to heat it

Insulators

• • • • Most

non-metals are insulators Air

is an excellent insulator Lightening travels at 435,000 km/hr and heats the air to

28,000 °C Trapped air helps to keep heat in!

Polystyrene Coffee Cup Double Glazed Windows Cavity Wall Insulation

Questions on Conduction

• Place these substances in order starting with the best conductor:

Air Water Copper Glass

•

Copper Glass Water Air

• Why do string vests keep people warm even though they are full of holes?

• They are able to

trap air

Questions on Conduction

• Why are Coke cans made of metal, but coffee cups made of polystyrene?

• Coke can placed in a fridge will leave the

Coke cool. Polystyrene coffee cups have trapped air

to keep coffee

warm

• • Why do trapped mountaineers dig snow holes when it gets too windy?

To trap air and stay warm

Convection

• • This is how heat travels through

liquids and gases Hot air rises

because it is less dense •

Cold air sinks

(it is more dense) • This creates

convection currents

Convection Currents

• • Molecules move

more quickly and

occupy more space when they are

heated Hotter dense

fluids are therefore than colder fluids

less

• •

Hotter fluids rise

top of colder fluids up to float on • A

convection current

is the path taken by rising hot fluids and sinking cold fluids

Convection does not occur in solids

Hot air rises

• Heated air provides lift for a

hot air balloon

• Heated land causes rising air currents called

thermals

• Thermals can be used by

gliders

to provide lift • Many

birds

also use thermals for lift

Questions on Convection

• In which of the following can convection take place?

Solids Liquids Gases •

Liquids

and

gases

• How do fire fighters enter a burning room?

• They

stay low

because it is

cooler

and there is less smoke (it rises with the hot air)

• Questions:

Why does it take so long for exam halls to heat up?

Hot air rises and then circulates to warm the whole room Why would a fridge not work if the coils were at the bottom?

Cold air sinks, so no air would circulate

Thermal radiation & temperature

• The

hotter

an object the

more radiation

it gives off • Radiation can travel through

vacuums

• All

hot objects radiate heat

• The Earth is heated by the

Sun’s radiation

Radiation

• Hot objects beam (radiate) out heat in

all directions

• This travels through

vacuums

(this is how the Sun heats the Earth despite the Space in between) • • Shiny objects reflect heat

Dark objects absorb heat

Silvered surfaces

• A metal

kettle

, a

firefighter

and a

marathon runner

make use of silvered surfaces.

Dark Surfaces

This Lamorghini Diablo has it’s

radiators exits painted black

Solar panels are

painted black

to absorb more sunlight

Questions on Radiation

• Why is Pluto colder than the Earth?

• It is

further away from the Sun

less radiation so it gets • • Which form of heat transfer makes you stand back from a bonfire?

Radiation

Evaporation

• When we are hot

sweat

is produced by our skin • As this

water evaporates it cools our skin

The Thermos Flask

insulated cap silvered inner wall vacuum between glass walls hot coffee outer casing of plastic insulated support

Conserving Heat In The Home

• http://www.sgcscience.co.uk/physics/Simulations/EnergyHouse/appl et.html

• A house retains 20% of its heat

Questions on Conserving Heat

• Why is there a vacuum in the thermos flasks?

• To stop

conduction

and

convection

• How is radiation stopped in a thermos flask?

• • By the

silvered

inner wall • How would you prevent heat loss from a house?

Double glazed windows

,

Cavity wall insulation

,

loft insulation

,

draught excluders

, carpet,

curtains