**Resistance:** It is defined as the property of the conductor due to which it opposes the flow of current, and material which has this property is called a Resistor. The resistance of Resistor is denoted by "**R**." Unit of resistance is "**ohm**."

**R = ρl/A **where, R = Resistance in Ω, ρ in ohm-m, A = Area in m2

Resistance depends upon

- Specific Resistivity of the conductor (
**ρ**) - Length of the conductor(
**l**) - Area of the conductor(
**A**) - The temperature of the conductor(in °
**C**)

Specific Resistivity of the conductor depends upon

- The material of the conductor
- The temperature of the conductor

**Relation of Resistance with specific Resistivity (R ∝ ρ):** With an increase in specific Resistivity, the resistance of the conductor is increasing.

**Relation of Resistance with specific Resistivity (R ∝ l):** With an increase in the length of the conductor, the resistance of the conductor is increasing.

**Relation of Resistance with specific Resistivity (R ∝ 1/A):** With an increase in cross-sectional area, the resistance of the conductor is increasing.

**Relation of Resistance with specific Resistivity (R ∝ temperature):** With an increase in temperature, the resistance of the conductor is increasing.

### Effect of Temperature on Resistance

## A resistor divided into two types

1. Fixed Resistor

2. Variable Resistor

## Fixed Resistor

That resistor which has a fixed value of resistance. It has two terminals. It does not have any sign convention. It means you can connect either side of the terminal to a positive terminal of the source(battery).

## Variable Resistor

That resistor which has a variable value of resistance. It has three terminals positive, negative, and ground terminal either the combination of two terminals that is positive and ground or negative and ground can provide you variable resistance. The terminal which is in the center is a ground terminal, and either side may be positive and negative terminal. As shown in the below figure.

## Why resistor?

Why is a resistor such an essential element in the circuit? As there are different elements(Capacitor, Inductor, LED(Light Emitting Diode), Motor, etc.) connected in the circuit, each element needs a different value of voltages. So, by using these resistors, it is possible to provide a different level of voltages in the circuit.

Suppose you have voltage source of 5 volts and you have to connect a LED in the circuit but when you connect the 5-volt battery directly to a LED. LED is burned. In this case, we have to use 100 or 220-ohm resistor to protect a LED.

## How to connect a resistor in series?

When a resistor is connected in series then total resistance of the circuit increases, when we connect different resistor in series than the same amount of current is flowing through all resistor which is connected in series but the voltage drop across each resistor is different and total voltage drop is equal to sum of all voltage drop across the resistor.

**Total Resistance = R1 + R2 + R3**

Example : If three 100 Ω resistor is connected in series then,

Total Resistance = R1 + R2 + R3 = (100 + 100 + 100 )Ω = 300 Ω

**Note: Total resistance connected in series always increase.**

## How to connect resistors in parallel?

When a resistor is connected in parallel, then total resistance of the circuit decreases. When we connect a different resistor in parallel than the same amount of voltage is applied to each resistor which is connected in parallel, but the amount of current flowing through each resistor is different and total current is equal to the sum of all current in each resistor.

**Total Resistance = R1 * R2 / (R1 + R2)**

Example: Two 100 Ω resistor are connected in parallel then,

Total Resistance = (100 * 100 / (100 + 100))Ω = 50 Ω

**Note: When resistor is connected in parallel then total resistance will decrease.**

Ajay Kumar Bharti

Ajay is an electrical engineer and android developer which share knowledge on different concepts of electrical topics. He believe in Learning by Doing.