Friday, 8 February 2008

Prefixes to Units...

We normally write:

Average weight of a newborn baby is W = 3.5 kg
Planes fly at an altitude H = 8,000 m
An hour has 3600 s

Value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Z is written as the product of a unit [Z] and a numerical factor n: Z = n[Z]

SI syatem is the metric system used internationally in science and engineering

In the SI system, kilogram (kg), metre (m) and second (s) are the base units of mass, length and time respectively.

Other commonly used base units in the SI system are

Energy - Joule (J)
Power - Watt (W)
Electric potential - Volt (V)

Units may be combined to express complex quantities

Speed is distance travelled in unit time - so its base unit will be
metre per second or m/s or ms-1

Frequently one needs to deal with very large or very small values and it is common to use a prefix with the base unit to obtain a derived unit. For example

Radius of the Earth is 6,450,000 m or 6,450 km {1 km = 1000 m}

Size of a virus is 0.0000001 m or or 0.1 microm or 100 nm
{1 micrometre = 0.000001 m; 1 nm = 0.000000001 m or 10-9m}

Standard Prefixes are in steps of 1000 - they are listed in the following for distance measurements. Others are similar.

Large numbers:

1,000 m = 103m or 1 km
1,000,000 m = 106m or 1 Mm
1,000,000,000 m = 109m or 1 Gm
1,000,000,000,000 m = 1012m or 1 Tm
1,000,000,000,000,000 m = 1015m or 1 Pm petametre
1,000,000,000,000,000,000 m = 1018m or 1 Em
1,000,000,000,000,000,000,000 m = 1021m or 1 Zm zettametre

Small Numbers:

0.001 m = 10-3m or 1 mm millimetre
0.000,001 m = 10-6m or 1 microm micrometre
0.000,000,001 m = 10-9m or 1 nm nanometre
0.000,000,000,001 m = 10-12m or 1 pm picometre
0.000,000,000,000,001 m = 10-15m or 1 fm femtometre
0.000,000,000,000,000,001 m = 10-18m or 1 am attometre
0.000,000,000,000,000,000,001 m = 10-21m or 1 zm zeptometre

Friday, 1 February 2008

Writing numbers that are very big or very small ...

In science, you will encounter numbers that are very big or extremely small. These can be rather inconvenient to write out in the normal notation. Powers of ten is a useful shorthand method of writing very large or very small numbers. For example,

One thousand (1000) is 103 ; reads 'ten to the power 3' and is 1 followed by 3 zeros
One divided by 1000 is 0.001 or 10-3 ; reads 'ten to the power minus 3'

And that is it - the positive power on ten tells us how many zeros are after 1.
negative powers of ten tell us the position of the 1 after the decimal point.

Distance of the Sun from the Earth is 150 million km or 150,000,000 km or 15 x 107 km.
Diameter of an atom is 0.0000000002 m or 2 x 10-10 m.

Multiplication and division of powers of ten numbers is very easy...

When you multiply two numbers powers add

When you divide two numebrs powers substract.

Example: Multiply 2 million by 4 million

Longhand: 2,000,000 x 4,000,000 = 8,000,000,000,000

Powers of ten: 2 x 106 x 4 x 106 = 8 x 1012

Nuclear Talks...

The following programme is tentative. The talks will be suitable for practically everybody to attend (particularly for those who have felt that science is not for them/ I have never read science/ it is too difficult...). The talks will be presented in a visually attractive manner with no mathematics. The atmosphere is very friendly and relaxed (it is not like school!)

Please encourage your friends and neighbours to come too. The talks are free to attend.

Talk 1: Introduction to the nucleus; we shall address questions about the size, shape, what is the nucleus made of, what holds the nucleus together etc. Rutherford's discovery of the nucleus revolutionised our understanding of the structure of matter.
Talk 2: Radioactivity and its many applications like radio-isotope dating for finding ages of fossils and other objects of historic interest, nuclear medicine etc. will be discussed. Radioactivity is due to the decay of a nucleus into another by emission of energy in the form of alpha, beta and gamma radiation. We shall look at the fascinating history of the discovery of radioactive elements, the reasons why nuclei decay and the nature of the radiation that is emitted.
Talk 3: Nuclear binding energies, Nuclear fission and fusion; explain how energy is released in the fission of uranium and in fusion of hydrogen. We shall look at the particularly interesting history of the discovery of nuclear fission and the development of atomic bomb.
Talks 4 and 5: We look at global energy scene - present and future consumption trends and their impact on our climate. UK energy supply and consumption has special challenges and possibilities. We analyse aspects of nuclear power production in detail with a critical look at those of serious public concerns. Future nuclear reactor designs.