newest physics
for HKDSE
Revision Notes
1
Kendy
Contents
Part I  Heat and Gases
Temperature, Heat and Internal Energy
1
1.2
Transfer Processes
7
1.3
Change of State
12
1.4
Gases
19
Chapter 1.1
Part II  Force and Motion
Position and Movement
27
2.2
Force and Motion
40
2.3
Projectile Motion
49
2.4
Work, Energy and Power
52
2.5
Momentum
57
2.6
Uniform Circular Motion
61
2.7
Gravitation
68
Chapter 2.1
1
1.1
Temperature, Heat and
Internal Energy
1. Temperature and thermometers
„
Temperature is the degree of hotness of an object.
„
Temperature is a quantity associated with the average kinetic energy (KE) of molecules due to their
random motions.
temperature ɥ ɦ average KE ɥ
„
Average kinetic energy of molecules depends only on temperature. 1 gram of water at 90oC and 100 gram of
meat at 90oC have the same amount of average kinetic energy of molecules.
„
Thermometer must have temperature-dependent properties.
temperature ɦ volume or resistance…or length…
„
3 common temperature scales
¾
¾
¾
Celsius scale
Fahrenheit scale
Kelvin scale
„
In the calibration process of thermometer, the thermometer is put into melting ice and graduation is made on
the thermometer. This is repeated in boiling water. The range between lower and higher fixed point is
divided into 100 equal divisions. A single division is called 1 degree Celsius (1oC).
„
lower fixed point of Celsius scale
¾
„
upper fixed point of Celsius scale
¾
„
0oC (melting ice)
100oC (boiling water)
Kelvin scale is also known as absolute temperature scale. Temperature in Kelvin scale must be positive.
The lowest possible temperature is 0 K, it is the absolute zero.
temperature in Kelvin = temperature in Celsius + 273
1
1.1
Temperature, Heat and Internal Energy
What is the lower fixed point in absolute temperature scale?
A.
273 K
B.
0K
C.
273 K
D.
373 K
Ÿ
Ÿ
Ÿ
lower fixed point in Celsius scale: 0oC
temperature in Kelvin = temperature in Celsius + 273
lower fixed point in absolute temperature scale = 0 + 273 = 273 K
Answer: C
„
Degree Celsius was used for a unit increment in Kelvin scale.
o
1 K = 1 C
The temperature of a block of copper was reduced from 425oC to 100oC. Express the decrement in Kelvin scale.
A.
152 K
B.
325 K
C.
373 K
D.
698 K
Ÿ
decrement in Kelvin scale = decrement in Celsius scale = 325 K
Answer: B
2
1.1
Temperature, Heat and Internal Energy
2. Heat and internal energy
„
Heat is the energy transferred as a result of temperature difference between objects.
„
If 2 objects have the same temperature, there is no net heat flow.
„
Internal energy (IE) is the sum of the kinetic energy (KE) and potential energies (PE) of molecules. For the
same substance:
¾
¾
¾
mass IE temperature internal energy (temperature average KE of molecules )
IE in gas state > IE in liquid state > IE in solid state
(average separation between gas molecules is the largest largest average PE of molecules)
internal energy (IE) = KE of molecules + PE of molecules
„
Relationship between heat and internal energy (temperature: A > B):
¾
¾
Net heat flows from A to B
IE of A & IE of B until thermal equilibrium (same temperature) is reached.
3. Heat capacity and specific heat capacity
„
Heat capacity (C) of a substance is the energy required to raise its temperature by 1oC .
¾
¾
depends on mass and material
unit of C: JK1 / J oC1
Q = CT
¾
„
where Q is the heat
Specific heat capacity (c) of a substance is the energy required to raise the temperature of 1 kg of it
by 1 oC.
¾
¾
depends on material only
unit of c: Jkg1K1 / Jkg-1 oC 1
Q = mcT
„
Relationship between heat capacity and specific heat capacity:
C = mc
3
1.1
Temperature, Heat and Internal Energy
Practical importance of the high specific heat capacity of water
„
A substance with high specific heat capacity, such as water, a large amount of energy is needed to raise its
temperature by a little amount. Its temperature is relatively stable.
„
In contrast, a substance with low specific heat capacity, such as copper, only a little amount of energy is
needed to raise its temperature by a large amount. Its temperature is relatively unstable.
„
Specific heat capacity of water is high (c = 4200 Jkg1 oC1)
¾ coolant in motor car and central air conditioning system
¾ maintenance of body temperature even when the surrounding temperature changes sharply
¾ mild weather in coastal area
Each object is heated separately with the same heater for the same period of time. Which of the following object
will have the greatest increase in temperature?
(Given:
Specific heat capacity of copper = 400 Jkg1 oC1
Specific heat capacity of water = 4200 Jkg1 oC 1)
A.
1 kg of copper
B.
5 kg of copper
C.
1 kg of water
D.
5 kg of water
Ÿ
The question requires us to compare different objects instead of different materials.
Ÿ
i.e. heat capacity should be considered, NOT specific heat capacity.
heat capacity (J oC 1)
A.
400
B.
2000
C.
4200
D.
21000
smallest heat capacity, largest increment
largest heat capacity, smallest increment
Answer: A
4
1.1
Temperature, Heat and Internal Energy
4. Experiment
Determine the specific heat capacity of water
Schematic diagram
Equation: c
Q
m'T
Sources of error
„
Underestimate the specific heat capacity
¾ water spills out
¾ stir water vigorously
„
Overestimate the specific heat capacity
¾ heat loss to surroundings
¾ heat requires to heat up the apparatus
¾ the heater is not totally immersed in water
¾ final temperature is taken once the heater is switched off
„
Precautions
¾ to reduce heat loss, polystyrene cup with lid should be used
¾ to ensure uniform temperature, water should be stirred gently throughout the experiment
¾ to reflect the actual temperature, the bulb of thermometer should not be placed too close to heater
Determine the specific heat capacity of copper
Schematic diagram
immersion heater
Equation: c
Q
m'T
5