Joule's Law
The heat produced by a current I flowing
through a resistance R for a time t, is proportional to l^{2}Rt or, Heat = (I^{2} 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 corkscrew rule
This also gives the direction of the magnetic
field around a current carrying conductor. Imagine that a righthanded 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 fluxlinkage
Lenz's Law
The electromagnetically induced
current always flows in such a direction as to oppose the very cause which produces
it.
Fleming's Lefthand 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 Righthand 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. 10^{1}
Fourth
band  Gold = ±5%
tolerance
Therefore the value of resistance = 47 x 10^{1} ±
5% = 470 ohm ± 5% tolerance
No comments:
Post a Comment