Book 1

Book 1
Unit 2.1
Internal energy
Book 1 Unit 2.1 Internal energy
Let’s begin
Energy stored in an object
Internal energy
Checkpoint 1
Heat as a way of energy transfer
Energy transfer and power
Checkpoint 2
p.2
Let’s begin Energy stored in an object

Consider two glasses of water with the same mass
but different temperatures.
Does one of them store more energy than the

other?
Answer
p.3
Internal energy
a Microscopic model of matter
 Matter is made up of tiny atoms or molecules
constantly in motion.
 Matter exists in different states.
solid liquid gas
p.4
Internal energy
Solid Liquid Gas
 fixed volume  fixed volume  no fixed volume

 fixed shape  no fixed shape  no fixed shape
 closely packed  closely packed  far apart
 fixed in  not fixed in  not fixed in
positions positions positions
p.5
Internal energy
Solid Liquid Gas
 vibrate about a  move freely  move freely at h

fixed position igh speeds
The energy possessed by the molecules in a body is

related to the molecular movements and arrangements.
Simulation 2.1 Models of solid, liquid and gas
p.6
Internal energy
b Molecular kinetic energy
 When molecules of a body move, they possess
kinetic energy (KE).
 Temperature 
 Molecules move more rapidly.
 Molecules possess more KE.
Temperature of a body is a measure of

the average KE due to the motion of the
molecules of the body.
p.7
Internal energy
b Molecular kinetic energy
 Molecules in two bodies have same average KE.
 Two bodies have the same temperature.
 Absolute zero
 About –273 C
 No object has a temperature below absolute zero.
 Average KE of molecules has a minimum value.
p.8
Internal energy
c Internal energy
 Internal energy (IE):
total energy stored in a body
 Total KE of all the molecules is a part of the
internal energy.
 Unit of energy: Joules (J)
 Larger units of energy:
1 kilojoule (kJ) = 103 J
1 megajoule (MJ) = 106 J
p.9
Internal energy
c Internal energy
 Internal energy depends on temperature &
mass.
 Higher temperature / larger mass

 More internal energy
p.10
Internal energy
c Internal energy
Refer to Let’s begin .
hotter colder
 more energy  less energy
Hotter water stores more energy (internal energy).
p.11
Checkpoint 1
1 True or false:
When the temperature of a body increases, the
kinetic energy of its molecules increases.
T/F
2 Tommy says, ‘This object is extremely cold!’

Which of the following cannot be the
temperature of the object?
A –300 C B –30 C
C 0 C D 13 C
p.12
Checkpoint 1
3 Which of the following statements is incorrect?
A The molecules of a gas are very far
apart and move at very high speeds objects.
B When the temperature of a body rises,
the molecules of the body move faster.
C The temperature of a body is a measure
of the total KE of its molecules.
D If two bodies have the same
temperature, the molecules of these two
bodies will have the same average KE.
p.13

Hot tea in cold water bath:
Hot tea: temperature   Internal energy 

Cold water: temperature   Internal energy 
p.14

Hot tea in cold water bath:
heat
Energy is transferred from hot tea to cold water.

The energy transferred is called heat.
p.15

Heat is the energy transferred from one body to
another as a result of a temperature difference.
 Symbol of heat: Q Unit of heat: joule
 Heating: process of energy transfer due to a
temperature difference
p.16

NOT internal energy difference!
 Temperature difference determines the
direction of heat transfer (from higher T to
lower T).
 Heating / doing work:
processes to increase internal energy of a body
p.17

energy transferred Q
Power = time or P = t
 Unit: watt (W)
 1 W = 1 J s–1
 Larger units of power:

 1 kilowatt (kW) = 103 W
 1 megawatt (MW) = 106 W
p.18
Example 1 Energy transferred
A 2-kW electric kettle is used to heat

some cold water. How much energy
is transferred to the water if it is
switched on for 3 minutes?
Energy transferred Q = Pt
= 2000  180
= 360 000 J
= 360 kJ
p.19

Kilowatt-hour (kW h)
 Unit of electrical energy, and for calculating the
cost of electrical energy
 1 kilowatt-hour = the amount of energy used by

an electrical appliance of power 1 kW in 1 hour
 1 kW h = (1000 J s–1)(3600 s)
= 3 600 000 J
= 3.6  106 J
p.20

 Appliances to measure the electrical energy
supplied to an electrical appliance:
joulemeter kilowatt-hour meter

– measures electrical energy fro – measures electrical energy
m a low-voltage power supply supplied by the mains
p.21
Checkpoint 2
1 A light bulb is connected to a power supply via
a joulemeter. Which of the following is measure
d by the joulemeter?
A Heat given off by the light bulb
B Internal energy of the light bulb
C Temperature of the light bulb
D Energy supplied to the light bulb
p.22
Checkpoint 2
2 Some ice is placed in a glass
of warm water. What is being
transferred from the water to
the ice?
A Heat
B Temperature
C Mass
D Power
p.23
Checkpoint 2
3 It takes a kettle 5 minutes to transfer 540 000 J
of energy to boil some water. What is the
power of the kettle?
A 1080 W B 1800 W
C 9000 W D 108 000 W
Q
Power = t
540 000
= 5  60
= 1800 W
p.24
Checkpoint 2
4 What is the energy consumed by a 2-kW
heater in 30 minutes?
[Hint: Energy consumed by the heater = Pt = ?]
Energy consumed
= Pt
= 2000  (30  60)
= 3.6  106 J
p.25
Checkpoint 2
5 An electric water heater has a power of 2500 W.
How long does it take to transfer 1.2 kW h of
energy to water?
Q
[Hint: By P = t , t = ?]
Q
By P = t ,
Q 1.2  (3.6 
t= P= = 1728 s
2500
106)
- The End -
p.26

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Book 1

Heat as a way of energy transfer

Energy transfer and power

Let’s begin Energy stored in an object

Does one of them store more energy than the

 fixed volume  fixed volume  no fixed volume

 vibrate about a  move freely  move freely at h

The energy possessed by the molecules in a body is

Temperature of a body is a measure of

 Higher temperature / larger mass

Hotter water stores more energy (internal energy).

2 Tommy says, ‘This object is extremely cold!’

Heat as a way of energy transfer

Hot tea: temperature   Internal energy 

Heat as a way of energy transfer

Energy is transferred from hot tea to cold water.

Heat as a way of energy transfer

Heat as a way of energy transfer

Energy transfer and power

 Unit: watt (W)

 Larger units of power:

Example 1 Energy transferred

A 2-kW electric kettle is used to heat

Energy transfer and power

 1 kilowatt-hour = the amount of energy used by

Energy transfer and power

joulemeter kilowatt-hour meter

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