**Resistance**

What causes heat in a light bulb? How do electric heaters provide the required warmth? The answers to these questions lie in understanding the concept of resistance. What is resistance? Whenever an electric current flows through a conductor, it experiences some obstruction. This obstruction is referred to as electrical resistance. While some materials/conductors have a higher resistance and others have a lower. The following article explains resistance, its fundamentals, calculation methods, applications, and more.

**What is Resistance?**

Ohm’s law provides a relation between the current that flows through a conductor and the potential difference across it. The relation can be written as follows: V is the potential difference across the conductor, I is the current flowing through it, and R is a constant. This constant is called electrical resistance.

V ∝ I

V = IR

So, how would you define resistance? Resistance is the opposition posed to current flow in an electrical circuit.

Or

**Resistance definition of electricity: **The ratio between the voltage applied to the current flowing through a conductor gives the conductor resistance.

**Unit of measurement:** Ohm, symbolised by the omega (Ω).

R = V/I

So, Ohm = Volt/Ampere

The unit of resistance —Ohm, is named after German physicist Georg Simon Ohm, who formulated Ohm’s law after thoroughly studying the relationship between current, voltage, and electrical resistance. |

**Factors Affecting Electrical Resistance **

A conductor’s resistance depends on the following factors:

- Length of the conductor
- Its cross-sectional area
- It’s material
- The temperature of the conductor

The relation between the above-mentioned factors can be given as follows:

R = ρL/A

Where ρ is the resistivity of a conductor measured in ohm metre (Ωm)

**What is Resistivity? How is it Different from Resistance?**

Resistivity is an intrinsic property of a given material. It is the material’s resistance of unit length and unit cross-sectional area at a specific temperature. So, resistance can change with the given material geometry, but the resistivity of the material remains the same. For instance, the resistance gets doubled on increasing the length of a wire and halved when you double the area of the cross-section of the material. However, the resistivity remains unaltered as it would still be calculated for the material’s unit length and area.

Since R = ρL/A

ρ= RA/L

The following table shows the key differences between resistance and resistivity.

Characteristics |
Resistance |
Resistivity |

Proportionality | It’s directly proportional to the length and temperature and inversely to the cross-sectional area of the material. | Only proportional to the temperature of the conductor. |

Symbol | R | ρ |

Formula | R = V/I
R = ρ(L/A) |
ρ = (R×A)/L |

SI Unit | Ohm | Ohm metre |

Applications | Used in the creation of heaters, fuses, and more. | Used in quality control test for calcareous soil. |

**Variation in Resistance with Increase in Temperature **

What would happen to the resistance of pure metal when you increase its temperature? An increase in temperature increases the resistance of pure metals. This change in resistance happens because of an increase in the number of electrons in the conduction band and an increase in the atomic vibration within the wire, reducing the mobility.

So, what would happen to the resistance of an insulator if you increase its temperature? The opposite scenario takes place when you raise the temperature of an insulator. Its resistance decreases. The reason behind this change is the increased electron movement from the conduction band to the valence band, as the energy gap is large between these two bands. Therefore, resistance decreases as conductance increases.

**What are resistors?**

An electrical circuit’s electronic components that resist the current flow are called resistors. They help to adjust current and voltage in an electrical circuit, just like faucets help adjust the tap water flow. In addition to adjusting the current flow in a circuit, resistors also allow voltage distribution. When current is reduced in a circuit using a resistor, the surplus current is transformed into heat. The various types of resistors are as follows:

- Fixed resistors
- Variable resistors
- Potentiometers

**How is Resistance Different from Reactance?**

There are several similarities between reactance and resistance. For instance, both oppose current flow and have the same unit that is Ohms (Ω).

However, reactance occurs only when there is a change of current in capacitors and inductors, as it depends on the frequency of the AC (alternating current) through a capacitor or inductor. While resistance is the obstruction to the current flow, i.e., it opposes electron flow and makes them slow.

Resistance |
Reactance |

It reacts to both AC and DC. | It concerns variations of currents or AC only. |

Resistance measures the opposition to a flow of current. | Reactance measures the opposition to a change in current. |

It has a simple value. | It has a complex value in mathematical analysis. While one part is the real part, the other is imaginary. |

Resistance is only brought about by a resistor. | Reactance requires all three: resistor, inductor, and capacitor. |

**How to Calculate Total Resistance in Circuits?**

Electrical components are connected in either of the two ways:

- Series circuits: The components are connected one after the other.
- Parallel circuits: The components are connected along parallel branches.

**Calculating Total Resistance in a Series Circuit**

In a series circuit, you can calculate the total resistance by adding the resistance of all the components.

For instance, a series circuit has the following resistors: a 6 Ω, a 5 Ω, and an 8 Ω resistor.
The total resistance in a series circuit is given by R1 + R2 + R3 R = 6 + 5 + 8 R = 19 Ω |

If you are not given the individual resistance values, you can use Ohm’s law and calculate the resistance using the formula: V = IR. Keep in mind that:

- The current in a series circuit is the same at all points.
- The total voltage is the same as the voltage of the supply.
- Using the current and the voltage, you can find the total resistance in the circuit and then calculate individual resistances.

**Calculating Total Resistance in a Parallel Circuit**

A circuit that branches into multiple paths that join back later is called a parallel circuit. When current flows through each branch of the parallel circuit, you can calculate the total resistance using the following formula:

Total resistance = 1R1+1R2+1R3

For instance, if a parallel circuit has four branches with resistances of 10 Ω, 2 Ω, 5Ω, and 1 Ω, the total resistance can be calculated as follows:
R = 110+12+15+11 R = 1+5+2+1010 R= 1810 R= 1.8 Ω |

If you do not know the individual resistances, you can use Ohm’s law to find resistance. However, keep the following points in mind:

- The voltage across a branch in a parallel circuit is the same as the total voltage across the circuit.
- The current can be different in each branch, so you must know the total current.
- Using total current and voltage across the circuit, you can calculate the total resistance in the circuit and then find individual resistances.

**Examples of Resistance from Everyday Life**

Resistance and Ohm’s law can be observed in day-to-day life. Following are some examples of electrical resistance:

**Conventional Domestic Fans**

There are regulators to control the speed of the fans. It is possible to check the current flowing through the fan by altering the resistance via a regulator. The circular knob rotation helps attain variable resistance on the output terminals.

**Electric Heaters**

The electrical heaters have a metal coil that has high resistance. It permits only a certain amount of current through it and gets heated up in the process. One can easily calculate the power needed to be supplied to the heater using Ohm’s law.

**Electric Irons and Kettles**

The electric kettle and irons have multiple resistors that limit the amount of current flowing through them to supply the needed heat. The resistors’ size can be determined by using Ohm’s law.

**Fuse **

Electrical fuses are protective components that play a vital role in limiting the amount of current flowing through the household circuit. The wires inside the fuse have high resistance and low melting point. The high resistance prevents a current higher than the prescribed value from passing through it, while the low melting point allows it to break the circuit when a high current passes through it.

**Frequently Asked Questions **

**Q1. How is resistivity different from conductivity?**

The reciprocal of resistivity gives conductivity. While conductivity is the measure of how readily current flows in a conductor, resistivity provides a measure of how much the material would resist the current flow. Resistivity is denoted by ρ and measured in Ωm; conductivity is given by σ and measured in Siemens ( 1/Ωm). So, when resistivity is low, conductivity is high, and vice versa.

**Q2. What is the relationship between resistance and conductance?**

Conductance is the reciprocal of resistance. The electrical conductance (G) is given by the following relation: G= 1/R. As the unit of resistance is Ω, the unit of conductance is called mho (Ω -1).

**Q3. Can a material have zero resistance?**

Yes, and they are called superconductors. These materials carry electrical current with zero electrical resistance. So, no energy is lost in the form of heat when electrons move through it. Examples of superconductors are aluminium and niobium at temperatures below a certain value, called the critical temperature.

**Conclusion**

Resistance may appear to be a negative word, but it has a huge role in electrical circuits. It is analogous to friction. Higher resistance puts greater hindrance in the current flow path, while lower resistance allows easier flow. This concept is the backbone of many electrical appliances and makes various technologies possible.