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PHY 301 · LECTURE 1 · International System of Units

It is built upon seven basic units. These are meter, kilogram, second, ampere, Kelvin, mole and candela.

Basic Units

The seven quantities are known to be basic in SI unit System.

Base quantity

symbol

Length meter m

Mass kilogram kg

Time second s

Electric current ampere A

Thermodynamic temperature Kelvin K

Amount of substance mole mol

Luminous intensity candela cd

Meter

Up until 1983, the meter was defined as 1,650,763.73 wavelengths in a vacuum of the orange-red line of the spectrum of krypton-86. Since then, it is equal to the distance traveled by light in vacuum in 1/299,792,45 of a second.

Second

The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the Cesium-133 atom.

Kilogram

The standard for the kilogram is a cylinder of platinum-iridium alloy kept by International Bureau of Weights and Measures at Paris. The kilogram is the only base unit still defined by a physical object.

Kelvin

The Kelvin is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water, that is, the point at which water forms an interface of the solid, liquid and vapor. This is defined as .01oC on the Centigrade scale and 32.02oF on Fahrenheit scale. The temperature

0oK is called the “absolute zero”.

Ampere

The Ampere is defined as that current, if maintained in each of two long parallel wires separated by a distance of one meter, would produce a force, between the two wires, of 2 x 10-7 Newton, for each meter

of length.

Candela

The Candela is defined as the luminous intensity of 1/600,000 of a square meter of a cavity at a temperature of freezing platinum (2,042 ok).

Mole

The mole is defined as the amount of substance of a system that contains as many elementary entities as there are as many atoms in 0.012 kilogram of the Carbon-12.

Joule

Joule is defined as the energy consumed in moving an object of one kg through a distance of one meter. One joule is equivalent to the 0.7376 foot pound-force and .2388 calories.

Watt

Watt is defined as the “Rate of doing work.” One watt =1 joule/second One watt is equivalent to 0.7376 ft--lbf/s or equivalently 1/745.7 horsepower.

Decimal System

The SI system uses the decimal system to relate larger and smaller units to basic units. It employs prefixes to signify the various power of 10.

Decimal System (Prefixes) · Factor · Symbol · Factor · Symbol

10-24 yocto y 1024 yotta Y

10-21 zepto z 1021 zetta Z

10-18 atto a 1018 exa E

10-15 femto f 1015 peta P

10-12 pico p 1012 tera T

10-9 nano n giga G

10-6 micro μ mega M

10-3 milli m kilo k

10-2 centi c hecto h

10-1 deci d Deka da

Example 1

A laser emits light at a wavelength of 248nm. This is same as

(a) .0248 millimeter. Or (b) 2.48 micrometer. Or (c) 0.248 micrometer. Or (d) 24800 angstrom.

Answer

(c) 0.248 micrometer.

Example 2

A logic gate switches from the on state to the off state in 10 neno seconds, this corresponds to (a)0.1 micro second or (b)10 micro second or (c) .001 micro second or (d) .01 micro second

Answer

(d) .01 micro second

Sub-atomic elements

Electron: It is a negatively charged particle. Proton: It is a positively charged particle. Neutron: It is a neutral particle and carries no charge.

Electrons and protons in an Atom

An atom is the smallest particle of the basic elements which form the physical substance we know as solid, liquid and gas. Each stable combination of electrons and protons make one particular type of atom. To understand the concepts of electronics we must have the understanding what is happening at the atomic level, not why it happens. There are number of methods by which electrons and protons might be grouped. They assemble in specific atomic combination for a stable arrangement. As a result, the electron stays in its orbit around the nucleus.

In an atom that have more electrons and protons than hydrogen atom, all the protons are in nucleus, while all the electrons are in one or more ings around the nucleus. The proton in the nucleus makes it heavier and stable part of the atom because it Is 1840 times heavier than the electron.

One electron is shown as the orbital ring around the nucleus. In order to account for the atom’s stability we can consider electron spinning around the nucleus as planets revolve around the sun. The electrical force attracting the electron towards proton is balanced by the mechanical force (centrifugal force) directing it outwards. The total number of electrons in the outer rings must equal to the number of protons in the nucleus in a neutral atom. The distribution of electrons in the orbital ring determines the atoms electrical stability. Especially important are the number of electrons farthest from the nucleus. For example carbon atom illustrated in the figure, 6 protons in the nucleus and 6 electrons in two outside rings.

This outermost ring requires 8 electrons stability, except, when there is only one ring which require only 2 electrons for its stability. As another example, the copper atom in figure below has only one electron in the last ring which can include 8 electrons. Therefore the outside ring of copper is less stable than carbon.