The word "Resistance" literary means to oppose or resist something. This word can be used in terms of a resistance generated by police or arm forces to stop the crowd or mob to stop the protest or violence, or it can be used as a technical term in the field of electronics and electrical engineering to show opposition of flow of electricity.

In this article, we will discuss about the word "Resistance" in domain of Electrical and Electronics Engineering.

**What is Resistance:**

*Resistance is the property of a material or tendency to oppose or resist the flow of current or electricity.*

The unit of measurement of resistance is "ohms" Ω. It is denoted by capital "R" and it is a real number mathematically. The terms "resistance" in electrical and electronics is used for DC "Direct Current" circuits. The instrument used to measure resistance is called "Ohmmeter"

**What is a DC Circuit:**

The DC circuit is the one that has a DC voltage or current source and the flow of direction of current is unidirectional. Any circuit that is operated by battery or DC power supply or any DC voltage source like solar panels they are said to be DC circuits.

**What is Impedance:**

The PCB Impedance is resistance used in AC circuits. The circuit where the power source is AC like alternator, generator or commercial 220V/110 V outlet. The AC circuit is the one in which line frequency (50Hz or 60Hz) plays important role. The AC circuit contains Inductor and Capacitors which have frequency dependant resistance called "Reactance". This reactance plus the DC resistance forms AC Impedance of AC circuits.

In this article however we shall only focus on the DC circuits "Resistance" and AC circuits "Impedance or Reactance" shall be discussed in upcoming articles.

**What exactly 1****Ω**** Resistance is ..?**

*A resistance is said to be 1**Ω,** if a circuit is operated by a 12V battery and results in the flow of 12 Ampere current in the circuit. So the resistance of the circuit is said to be 1**Ω**.*

**What is a Circuit:**

A circuit is a closed loop path for the flow of current to leave the power source and return back to the power source. A circuit is said to be complete if it follows this rule. If not then the circuit is said to be "open" or "incomplete".

**What is the Circuit Made Of..?**

*A circuit is made of electronic components connected to each other by means of conductor to complete the circuit in order to attain a desired function.*

The electronic components are of two types passive and active. These components are interconnected from each other from terminal to terminal by means of conductors.

**Conductors and Insulators:**

* A conductor is a material that allows the flow of current with least resistance. *A conductor actually supports the flow of current and hence used to electrically connect components each other. Very good example is copper, gold, silver and aluminum.

An insulator is a material that opposes or stops the flow of current. Example Paper, Teflon, mica, rubber, ceramic, PVC, glass, wood, porcelain.

*" A resistor is basically a controlled insulator that limits the flow of current to a designed extent. "*

**Ohm's Law:**

The fundamental of electronics and electrical engineering is Ohm's Law. It states that *"The amount of current flowing through a circuit is directly proportional to the voltage applied and inversely proportional to the resistance of the circuit. "*

**Calculation of Resistance and Resistivity:**

Resistance is created by every material either it is a conductor or insulator. The insulator creates resistance in range of Mega ohms and Giga Ohms and conductors in the range of milli ohms or micro ohms.

*The smaller the resistance the greater the flow of current and hence the material is conductor*

*The greater the resistance the smaller the flow of current and hence the material is insulator*

** Resistivity: **The resistivity of a material is the ability or power to resist the flow of current. Its unit Ohm-meter (Ω-m). By definition resistivity is directly proportional to the cross section area of wire and inversely proportional to the length.

where R = Resistance, A = Area of cross section , L = length

**Example:**

Let's talk about the conductor copper wire.

A bare annealed copper wire AWG 14 gauge and 100 meters long is purchased by a company to be used in transformer core windings. Find out the total resistance of the wire.

**Solution:**

AWG (American Wire Gauge) is the standard system to determine the gauge or diameter of the wire thickness. We can see that AWG 14 is 0.0633 inch diameter.

1 inch = 2.54 cm

1 inch = 0.0254m

Therefore

Now put this value in

We know that the resistivity of copper is

Therefore

Therefore

**Example of Light Bulb and Resistance:**

An electric lamp is made up of a filament that provides an electrical DC resistance when it is supplied by a voltage source. This resistance is generated as soon as the wires are connected in "close loop" circuit. The close loop circuit is the one that is not broken from any point in the circuit. Continuity is not broken. If a circuit breaks from any one single point the circuit will become "Open". In open circuits, current will not flow and hence bulb will not glow. Continuity is broken.

Now assume that the bulb/lamp is rated at 12V and the 12V lead acid battery is used to power the lamp. As soon as the conducting wires are shorted to the bulb the bulb will glow very bright. This is because the circuit is the "series circuit" and there is no *current limiting resistor* connected in series. This will result in maximum glow of bulb. As a result a filament will dissipate lot of heat energy and that heat energy in turn generates light.

*The series circuit is on in which components are connected in series or from one terminal to other terminal. The circuit is series if the positive terminal of one component is connected negative terminal of another component. The flow of current remains same but the voltage is divided as per the resistance of each component.*

Now here we have connected am adjustable resistor, to control the flow of current through lamp and a switch is also connected in series to ON and OFF the circuit. When the switch is "open" the circuit will be OFF and when switch is closed the circuit will be ON. Now suppose this adjustable resistor is 10K ohm value. When it is turned to maximum extent it will become short circuit means "zero ohm" and hence allow maximum current flow and full glow bulb. And when it is turned maximum to other way it becomes maximum resistance (10K ohm) and creates heavy resistance. Thus dimming the bulb or completely turn off the bulb due to very high resistance equivalent to open circuit. In order to glow the circuit, switch should be remain "closed". An open switch will not turn ON the bulb no matter what value of adjustable resistor is.

**A Basic Revision of What We Learnt:**

1- The opposition to the flow of current is "Resistance"

2- A DC circuit is the one having DC source and unidirectional flow of current

3- AC circuit resistance is called Impedance or reactance

4- Copper is a good conductor of electricity widely used in wiring and circuits

5- Insulator blocks the flow of current. Controlled insulator can be called "Resistor"

6- Ohm's Law say, the current flow is directly proportional to voltage and inversely proportional to resistance

7- Ability or property of a conductor to resist flow of current is called resistivity.