1. The Realm of Physics

2. Orders of Magnitude
When quantities are written to the nearest power of 10, the quantity is being expressed as an “order of magnitude”. Often, part of learning physics is to appreciate the order of magnitude of things – whether they are masses, times, distances, forces, or just pure numbers such as the number of times your heart will beat in your lifetime.

3. Range of Length Length/m
Distance to the edge of the observable universe
1026
Diameter of the Milky Way galaxy
1021
Distance to the nearest star outside our solar system
1016
Diameter of solar system
1013
Distance to sun
1011
Diameter of Earth
107
Radius of Earth
Size of a cell
10-5
Size of a hydrogen atom
10-10
Size of a nucleus
10-15
Size of a proton
10-17

4. Range of mass Mass/Kg The universe 1053 The Milky Way galaxy 1041
The sun
1030
The earth
1024
An apple
10-1
A raindrop
10-6
A bacterium
10-15
Smallest virus
10-21
A hydrogen atom
10-27
An electron
10-31

5. Range of time Time/s Age of the universe 1017 Age of the earth
Time of travel by light to nearest star
108
One year
107
One day
105
Period of one heartbeat
100
Period of red light
10-15
Time of passage of light across a nucleus
10-24

6. Making approximations
By using orders of magnitude, we can arrive at approximate solutions to questions like “how many times does your heart beat in your lifetime.
Ie. We live approximately 102 years, each year contains approximately 107 seconds, and our heart beats about 1 time per second. So, your heart beats about 109 times in your lifetime.

7. A similar calculation
Assuming your heart beats 1 time per second, how many times would it beat in 75 years?

8. Comparing quantities
Quantities can be compared using order of magnitude.
Ie ratio of the size of a hydrogen atom to the size of its nucleus is 105.
What is the mass of our galaxy in terms of solar mass?
How many hydrogen atoms does it take to make up the mass of the earth?

9. The SI system (metric system)
In the SI system there are seven fundamental units.
The meter (m) – the unit of distance
The kilogram (kg) – the unit of mass
The second (s) – the unit of time
The ampere (A) – the unit of electric current
The Kelvin (K) – the unit of temperature

10. Fundamental units continued
(6) The mole (mol) – the number of atoms that are present in 12 g of carbon-12. One mole of any substance contains 6.02 x 1023 “particles” of the substance.
(7) The candela (cd) – This is the unit of luminous intensity.

11. Derived units
Physical quantities other than those listed above have units that are combinations of the seven fundamental units. Such units are called derived units.
Ex. The unit of force is the Newton (N)
1N = 1 kgms-2.

12. Common metric prefixes
Power
Prefix
Symbol
10-18
atto a
10-15
femto f
10-12
pico p
10-9
nano n
10-6
micro
10-3
milli m
10-2
centi C
10-1
deci d

13. Common metric prefixes
Power
Prefix
Symbol
101
deka da
102
hecto h
103
kilo k
106
mega M
109
giga G
1012
tera T
1015
peta P
1018
exa E

14. Some questions How many meters are there in 5.3 nm?
How many meters are there in 2.4 Gm?
How many seconds are there in 24 ms?
How many seconds are there in 14 days?
What is the volume in m3 of a cube of side 2.8 cm?
What is the velocity of an electron in m/s that covers a distance of mm in ns?