Hello friends, today I am going to tell you **What is Resistance, **Definition of Resistance, Combination of Resistors, Effect of Temperature on Resistance, Laws of Resistance *:*If you also want to know, then keep reading this article completely.

## What is Resistance

**Definition –** Resistance is the property of a material which opposes the flow of electric current, it is called resistance. The unit of resistance is ohm. The resistance in which a constant current of one ampere produces heat at the rate of one joule per second will be one ohm.

Resistance is represented by Ohm and its symbol is in the image below, it is called resistance.

## Combination of Resistors

### Series Combination

Resistors should be connected in such a way that there is only one path of current in them, such a combination is called series combination. The resistors r1, r2, r3 are shown in series in the figure and current (I) is flowing in them at the supply voltage (V).

**R = R1 + R2 + R3**

Voltage across different resistors = V1, V2, V3

The total voltage is equal to the different voltages.

V=V1+V2+V3

**V = IR**

### Parallel Combination

When two or more resistors are connected in such a way that one end of them is connected together and the other end is connected together in parallel, then such combination is called parallel sequence combination. In the figure three resistances r1, r2 and r3 are shown connected to a voltage V used in parallel combination.

**I** = Supply Current

I1, I2, I3 are the current values in the resistors R1, R2, R3 respectively.

*According to Ohm’s law,*

**I=V/R**

## Effect of Temperature on Resistance

The resistance of pure metals like copper, silver, gold etc. increases with increase in temperature, hence their resistance temperature coefficient is positive (+). The resistance temperature coefficient increases very little on increasing the temperature of the alloys. E.g. Constantan, Magnene, German Silver etc. The resistance of semiconductors decreases with increase in temperature, the resistance temperature coefficient of such materials is negative (-).

The change in resistance per degree C is called resistance temperature coefficient. It is of positive (+) and negative (-) type according to the change.

## Laws of Resistance

The following rules can be determined for the resistance of a conductor

**(i) Length of the conductor (l) :-** The resistance of the conductor is proportional to the length of the conductor. Hence

**R∝ l**

The resistance of a conductor increases with increasing length and decreases with decreasing length.

**(ii) The cross-sectional area of the conductor (a):-** The resistance of the conductor is inversely proportional to its cross-sectional area. a Therefore Roc, that is, the resistance of a conductor decreases with increase in cross-sectional area and increases with decrease in cross-sectional area.

**R∝ l / a**

**(iii) The resistance of the conductor depends** on the properties of the material of the conductor, that is, the resistance of wires like Eureka, Manganin, Nikron etc. of the same length and transverse area are many times higher than that of a copper wire of fixed length and cross-sectional area.

**(iv) Resistance of conductor** also depends on temperature, if the temperature is increased then resistance of many materials increases, but there are some materials whose resistance decreases with increase in temperature.

**Unit of Resistance**

The unit of resistance is the ohm (Ω). It is represented by resistance (R).

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