^Advantages of SI system
Following are the advantages of SI system:
Coherent: SI system of units is coherent. i.e. any unit can be derived by simply dividing & multiplying from any seven fundamental units & two basic units.
Rational: For one physical quantity only one unit is used.
Absolute: Use of ‘g’ is avoided.
Metric: The use of multiples & submultiples is allowed. Accepted universally: As SI system can be easily reproduced, compared & time invariant.
*Value in S.I units
*Some SI Derived units with special names
| Quantity | SI unit | Symbol |
| Force | newton | N |
| Work | joule | J |
| Pressure, Stress | pascal | Pa |
| watt | W | |
| Angle | radian | rad |
| Frequency | hertz | Hz |
| Viscosity | poiseuille | Pl |
| Torque | newton meter | Nm |
| Electric charge | coulomb | C |
| Electrical resistance | ohm | W |
| Voltage | volt | V |
| Flux (Elec. or mag) | weber | Wb |
| Inductance | henry | H |
| Capacitance | farad | F |
| Magnetic field | tesla | T |
| Illuminance | lux | Lx |
| Luminous flux | Lumen | lm |
| Illuminance | lux | lx |
| Activity (radioactive) | Becquerel | Bq |
| Absorbed dose index | Gray | Gy |
*One steradian
One steradian (sr)is the solid angle subtended at the centre of a sphere by that surface of the sphere which is equal in area to square of the radius of the sphere.
^One radian
One radian (rad) is the angle subtended at the centre of a circle by an arc of length equal to the radius of circle.
*The Seven Fundamental Quantities
*Fundamental quantities
Seven physical quantities have been chosen as fundamental or base quantities these are length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Units of base quantities are called base units or fundamental units. Fundamental or base quantities are also known as the seven dimensions of the world.
^Numerical & unit
The magnitude of a quantity doesn’t change with the change of unit. i.e. numerical & unit of a physical quantity are inversely proportional.
Gaussian surface
Gauss’s law
1. According to Gauss’s law the electric flux through any arbitrary closed surface in a medium is equal to the total charge enclosed by the that surface divided by the permittivity of that medium. i.e. 
2. If medium is air or vacuum, then 
3. Gauss’s law is based on the inverse square dependence on distance contained in the Coulomb’s law. Any violation of Gauss’s law will indicate departure from the inverse square law.
4. Using Gauss law we can say no. of electric line of force originating or terminating on a charge of q coulomb is equal to q/ε0.
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