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Why are frying-pans and woks usuallymade of metals but their handles aremade of plastic?

Warm-up

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How is energy transferred from thecooker to the pan and then to the food?

Introduction

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Energy transfer by conduction

Energy is transferredby conduction

from the cooker,

through the pan,

to the food.

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Studying the heat transferred along a metal rod

What happens to

these drawing pins?

Conduction (video)

copper rod

insulating board

wax

drawing pins

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What kind of rod conducts heat?Feel the ends of therods to find out which

rod feels hot first.

Conduction (video)

very hotwater

glass

wood

iron

copper

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Heat the ends of themetal rods and note

which drawing pin atthe other end falls first.

What kind of rod conducts heat fastest?

Conduction (video)

aluminiumcopper

iron

drawingpins

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Does water conduct heat?

Gently heat the toppart of the water & find

out if the ice melts.

Conduction (video)

water wire gauze

ice

boilingtube

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Does air conduct heat?

Leave the

cardboard for awhile & then

take the

temperaturereadings.

Conduction (video)

thermometer

cardboard tube

heater

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Leave the cardboard fora while & then take the

temperature readings.

Conduction (video)

Does air conduct heat?

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Materials conduct heat at different rates.

Metals (e.g. copper and iron) are good

conductorsof heat.Non-metals (e.g. wood, water and air)are poor conductors (or good insulators).

Energy transfer by conduction

In conduction, heat (energy) is transferred

from the part to the part.

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In conduction, heat is transferred from thehot part to the cold part along an object.Conduction is efficient in conductors but notin insulators.

Energy transfer by conduction

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Does the orientation of the rod matter?

Energy transfer by conduction

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Phoebe heats two metal rods as shown.Which will be heated up first? Why?

Does the orientation of the rod matter?

of them are heatedup at the same rate.

Energy transferred from the

hot end to the cold end byconduction is not affected bythe orientation of the rod.

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Conduction and particle motion

In kinetic theory,

particles at hotend vibrate a lot

particles atcold endvibrate lessparticles in a solid are

closely packed,

they vibrate to & fro butcan't change positions.

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Conduction and particle motion

The fast vibrating particlesbump into the slowerneighbouring particles &

particles at hotend vibrate a lot

particles at

cold endvibrate less

make them vibrate morerapidly

(from one particle to thenext & from hot to coldend of rod)

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Conduction and particle motion

(simulation)

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Examples of conduction

A cotton jacketkeeps warm bytrapping air next tothe body.

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Examples of conduction

Polar bears keepwarm by trappingair in the fur.

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Examples of conduction (video)

Birds keep warmby trapping air in

their feathers.

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Examples of conduction

Under the same condition, a metal blockfeels colder than a wooden block even the 2

objects are at the same temperature.

metal easily conducts energy away fromyour hand

you feel cold

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A From high-temperature area to low-temperature area.

B From low-temperature area to high-temperature area.

C The direction of heat transferred is different inmetals and non-metals.

In conduction, heat is transferred...

In conduction, heat is transferred in which ofthe following direction?

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A The temperature of the tile floor is lower.

B Tile is a better conductor of heat than wood.

C Wood is a better conductor of heat than tile.

D Tile is smoother than wood.

Which of the following can explain why a tilefloor feels colder than a wooden floor?

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Frying-pans are made of...

conductors

Frying-pans are made of metals becausethey are good _________ of heat, whilehandles of frying-pans are made of plasticbecause they are good _________ of heat.insulators