Thursday 14 February 2013
RESISTOR
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RESISTOR
A resistor is
a component of an circuit that resists the flow of electrical current.
Or
A resistor is
a passive two-terminal electrical component that
implements electrical resistance as a circuit
element.
It has two terminals
across which electricity must pass, and it is designed to drop the voltage of
the current as it flows from one terminal to the other. Resistors are primarily
used to create and maintain known safe currents within electrical components.
Resistance is measured
in ohms, after Ohm's law.
This law states that electrical resistance is equal to the drop in voltage
across the terminals of the resistor divided by the current being applied. A
high ohm rating indicates a high resistance to current. This rating can be
written in a number of different ways — for example, 81R represents 81 ohms,
while 81K represents 81,000 ohms.
The amount of
resistance offered by a resistor is determined by its physical construction. A
carbon composition resistor has resistive carbon packed into a ceramic
cylinder, while a carbon film resistor consists of a similar ceramic tube, but
has conductive carbon film wrapped around the outside. Metal film or metal
oxide resistors are made much the same way, but with metal instead of carbon. A
wirewound resistor, made with metal wire wrapped around clay, plastic, or
fiberglass tubing, offers resistance at higher power levels. Those used for
applications that must withstand high temperatures are typically made of
materials such as cermet, a ceramic-metal composite, or tantalum, a rare metal, so
that they can endure the heat.
Ohm’s law
V = I R
R = V / I
UNITS OHM
POWER = I2
R = VI = V2 / R
Polarity marking for resistor
Current enters at positive terminal and leaves
at negative terminal
RULE
: CURRENT ALWAYS MOVES FROM POSITIVE TERMINAL TO NEGATIVE
TERMINAL
In the above
diagram polarities marked opposite to above RULE .so voltage is taken as negative.
NOTE :
while calculating problems first find by marking polarities according to rule
and then check the polarities given are according to rule. If yes then take
answer as it is otherwise take the opposite polarity
NOTE : current always moves from higher potential to lower potential
POTENTIAL RULE
in the above fig current starts from v2.so we can say that v2 is at higher potential(ie., v2 > v1)
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