Basic Electrical Laws

Joule's Law

The heat produced by a current I flowing through a resistance R for a time t, is proportional to l²Rt  or,  Heat = (I² R t) / J   where J = 4.2 Joules / cal

Ampere's Rule

This rule gives the direction in which a magnetic needle deflect when placed near a current carrying conductor. Imagine a person swimming in the direction of the current, facing the magnetic needle. Then the direction of deflection of the north pole of the needle will be towards his left side.

Coulomb's Law

The force between two electric charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them.

Ampere's right hand rule

This gives the direction of magnetic field around a current carrying conductor. Imagine that the wire is gripped in the right hand with thumb pointing along the wire in the direction of current. Then the direction of the fingers will give the direction of the magnetic field.

Maxwell's cork-screw rule

This also gives the direction of the magnetic field around a current carrying conductor. Imagine that a right-handed corkscrew is driven in the direction of the current flow. Then the direction of magnetic field due to the current will be the direction of rotation of the head of the screw.

Laws of Parallel Currents

1.    Two parallel conductors attract each other if the currents through them flow in the same direction and repel each other if the currents through them flow in the opposite direction.

2.    The force between two such parallel conductors is proportional to the product of the current strengths and to the length of the conductors considered and varies inversely as the distance between them

Faraday's Laws of Electromagnetic Induction

1.    Whenever the magnetic flux linked with a circuit changes, an EMF is always induced in it.

2.    The magnitude of the induced EMF is proportional to the rate of change of flux-linkage

Lenz's Law

The electromagnetically induced current always flows in such a direction as to oppose the very cause which produces it.

Fleming's Left-hand Rule (for motor action)

This gives the direction of force on a current carrying conductor placed in a magnetic field. Hold the left hand with forefinger, middle finger and thumb at right angles to one another. If the forefinger represents the direction of the field and the middle finger that of the current, then the thumb gives the direction of motion of the conductor.

Fleming's Right-hand Rule (for generator action)

This gives the direction of induced current in a conductor, which is moved in a magnetic field. Hold the right hand with forefinger, middle finger and thumb at right angles to one another. If the forefinger represents the direction of the field and the thumb that of the conductor motion, then the middle finger gives the direction of induced current.

Kirchhoff's Laws

1.    In an electrical network, the sum of the currents entering a junction is equal to the sum of the currents leaving the junction.

2.    The algebraic sum of the potential differences around a closed circuit is zero.

Maxwell's Law

1.    Any two circuits carrying current tend so to dispose themselves as to include the largest possible number of lines of force common to the two.

2.    Every electromagnetic system tends to change its configuration so that the exciting circuit embraces the largest number of lines of force in a positive direction

Hall Effect

It states that if a magnetic field is applied perpendicular to a metal plate, which carries a current, then a transverse voltage is set up in the plate perpendicular to both the current and the magnetic field. This transverse voltage is known as Hall Voltage.

Seebeck Effect

When two dissimilar metal wires are joined at the ends to form two junctions (thermocouple) and when these junctions are kept at different temperatures, an EMF is produced in the circuit. This is called Seebeck Effect.

Peltier Effect

When an electric current is passed through a thermocouple, then heating is produced at one junction and cooling at the other junction.

Colour coding of resistors

Colour	Number	Colour	Number	Tolerance
Black	0	Green	5	Gold: ± 5% Silver: ± 10% No colour: ± 20%
Brown	1	Blue	6
Red	2	Violet	7
Orange	3	Grey	8
Yellow	4	White	9

Assessment of the value of resistance

First two bands from the end indicate the first two significant figures of resistance in ohm.

The 3rd band indicates the decimal multiplier.

The last band stands for the tolerance in percentage.

Example

First band - Yellow                 = 4

Second band - Violet             = 7

Third band - Brown                = 1       i.e. 10¹

Fourth band - Gold                = ±5% tolerance

Therefore the value of resistance   = 47 x 10¹ ± 5% = 470 ohm ± 5% tolerance