Warmup

1
Warm-up
Why are frying-pans and woks usually made
of metals but their handles are made of plastic?
2
Introduction

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

3
Energy transfer by conduction
Energy is transferred by conduction
from the cooker,
through the pan,
to the food.
4
Conduction (video)
Studying the heat transferred along a metal rod
insulating board
copper rod
wax
What happens to these drawing pins?
drawing pins
5
Conduction (video)
What kind of rod conducts heat?
Feel the ends of the rods to find out which rod
feels hot first.
wood
iron
glass
copper
very hot water
6
Conduction (video)
What kind of rod conducts heat fastest?

• Heat the ends of the metal rods and note which
  drawing pin at the other end falls first.

aluminium
copper
iron
drawing pins
7
Conduction (video)
Does water conduct heat?
Gently heat the top part of the water find out
if the ice melts.
boiling tube
wire gauze
water
ice
8
Conduction (video)
Does air conduct heat?
thermometer
Leave the cardboard for a while then take the
temperature readings.
heater
cardboard tube
9
Conduction (video)
Does air conduct heat?
Leave the cardboard for a while then take the
temperature readings.
10
Energy transfer by conduction
In conduction, heat (energy) is transferred from
the hot part to the cold part.
Materials conduct heat at different rates.
Metals (e.g. copper and iron) are good conductors
of heat.
Non-metals (e.g. wood, water and air) are poor
conductors (or good insulators).
11
Energy transfer by conduction

• In conduction, heat is transferred from the hot
  part to the cold part along an object. Conduction
  is efficient in conductors but not in insulators.

12
Energy transfer by conduction
Does the orientation of the rod matter?
13
Does the orientation of the rod matter?
Phoebe heats two metal rods as shown. Which will
be heated up first? Why?
Both of them are heated up at the same rate.
Energy transferred from the hot end to the cold
end by conduction is not affected by the
orientation of the rod.
14
Conduction and particle motion
particles at cold end vibrate less

• In kinetic theory,

particles in a solid are closely packed,
they vibrate to fro but can't change positions.
particles at hot end vibrate a lot
15
Conduction and particle motion
particles at cold end vibrate less

• The fast vibrating particles bump into the slower
  neighbouring particles

make them vibrate more rapidly
? energy is transferred
particles at hot end vibrate a lot
(from one particle to the next from hot to
cold end of rod)
16
Conduction and particle motion
(simulation)
17
Examples of conduction
How to keep warm?
A cotton jacket keeps warm by trapping air next
to the body.
18
Examples of conduction
How to keep warm?
Polar bears keep warm by trapping air in the fur.
19
Examples of conduction (video)
How to keep warm?
Birds keep warm by trapping air in their feathers.
20
Examples of conduction
Hot or cold?
Under the same condition, a metal block feels
colder than a wooden block even the 2 objects are
at the same temperature.
metal easily conducts energy away from your hand
?
? you feel cold
21
In conduction, heat is transferred...
In conduction, heat is transferred in which of
the following direction?
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 in metals and non-metals.
22
Which of the following can explain why a tile
floor feels colder than a wooden floor?
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.
23
Frying-pans are made of...
Frying-pans are made of metals because they are
good _________ of heat, while handles of
frying-pans are made of plastic because they are
good _________ of heat.
conductors
insulators