RESISTANCE

                              RESISTANCE 

WHAT IS RESISTANCE??

IT IS DEFINED AS THE PROPERTY OF A SUBSTANCE DUE TO WHICH IT OPPOSES OR RESTRICTS THE FLOW OF ELECTRICITY i.e. ELECTRONS THROUGH IT.

RESISTANCE IS A MEASURE OF THE OPPOSITION TO CURRENT FLOW IN AN ELECTRICAL CIRCUIT. IT IS USED TO MEASURED IN OHMS.

                                    SYMBOL OF OHM = Ω (OMEGA)

UNIT OF RESISTANCE?

THE PRACTICAL UNIT OF RESISTANCE IS OHM. A CONDUCTOR IS SAID TO HAVE A RESISTANCE OF ONE OHM IF IT PERMITS ONE AMPERE CURRENT TO FLOW THROUGH IT WHEN ONE VOLT IS IMPRESSED ACROSS ITS TERMINALS.

LAWS OF RESISTANCE

THE RESISTANCE R OFFERED BY A CONDUCTOR DEPENDS ON THE FOLLOWING FACTORS:

1. IT VARIES DIRECTLY AS ITS LENGTH

2. IT DEPENDS ON THE NATURE OF THE MATERIAL

3. IT ALSO DEPEND ON THE TEMPERATURE OF THE CONDUCTOR

EFFECTS OF TEMPERATURE ON RESISTANCE

THE EFFECT OF RISING IN TEMPERATURE IS:

1. TO INCREASE THE RESISTANCE OF PURE METALS. THE INCREASE IS LARGE AND FAIRLY REGULAR FOR NORMAL RANGES OF TEMPERATURE.

2. TO INCREASE THE RESISTANCE OF ALLOYS, THOUGH IN THEIR CASE, THE INCREASE IN RELATIVITY SMALL AND IRREGULAR

3. TO DECREASE THE RESISTANCE OF ELECTROLYTES, INSULATORS (LIKE RUBBER, GLASS, PAPER), AND PARTIAL CONDUCTORS SUCH AS CARBON. HENCE INSULATORS ARE SAID TO POSSESS A NEGATIVE TEMPERATURE COEFFICIENT OF RESISTANCE.

OHM'S LAW

THIS LAW APPLIES TO ELECTRIC TO ELECTRIC CONDUCTION THROUGH GOOD CONDUCTORS AND MAY BE STATED:-

THE RATIO OF POTENTIAL DIFFERENCE (v) BETWEEN ANY TWO POINTS ON A CONDUCTOR TO THE CURRENT(I) FLOWING BETWEEN THEM, IS CONSTANT, PROVIDED THE TEMPERATURE OF THE CONDUCTOR DOES NOT CHANGE.

                                                 V/I = R

RESISTANCE IN SERIES 

WHEN CONDUCTORS HAVING RESISTANCES R1, R2, R3, ARE JOINED END-TO-END THEN THEY ARE SAID TO BE CONNECTED IN SERIES.

FOR BEING A SERIES CIRCUIT, IT SHOULD BE KNOWN THAT

1. CURRENT IS THE SAME THROUGH ALL THE THREE CONDUCTORS

2. VOLTAGE DROP ACROSS EACH IS DIFFERENT DUE TO ITS DIFFERENT RESISTANCE AND IS GIVEN BY OHM'S LAW.

3. SUM OF THE THREE VOLTAGE DROPS IS EQUAL TO THE VOLTAGE APPLIED ACROSS THE THREE CONDUCTORS

V= V1+V2+V3 = IR1+IR2 +IR3

V=IR

WHERE R IS THE EQUIVALENT RESISTANCE OF THE SERIES COMBINATION.

IR=IR1+IR2+IR3

OR

R+R1+R2+R3

CHARACTERISTICS OF A SERIES CIRCUIT ARE:

1. THE SAME CURRENT FLOWS THROUGH ALL PARTS OF THE CIRCUIT

2. DIFFERENT RESISTORS HAVE THEIR INDIVIDUAL VOLTAGE DROPS.

3. VOLTAGE DROPS ARE ADDICTIVE.

RESISTANCE IN PARALLEL

THREE RESISTANCE JOINED ARE SAID TO BE CONNECTED IN PARALLEL.

1. POTENTIAL DIFFERENCE ACROSS ALL RESISTANCES IS THE SAME 

2. CURRENT IN EACH RESISTOR IS DIFFERENT AND IS GIVEN BY OHM'S LAW 

3. THE TOTAL CURRENT IS THE SAME AS THE THREE SEPARATE CURRENTS.

I = I1+I2+I3=V/R1 +V/R2 + VR3

I = V/R WHERE V IS APPLIED VOLTAGE.

R = EQUIVALENT RESISTANCE OF THE PARALLEL COMBINATION.

V/R = V/R1 +V/R2 +V/R3 

OR

I/R = I/R1 + I/R2 + I/R3

CHARACTERISTICS OF A PARALLEL CIRCUIT:-

1. SAME VOLTAGE ACTS ACROSS ALL PARTS OF THE CIRCUITS

2. DIFFERENT RESISTORS HAVE THEIR INDIVIDUAL CURRENT

3. BRANCH CURRENTS ARE ADDITIVE