Electric Current: Resistance and Flow of Electrons

Key Concepts

• Resistance
• Resistivity

Introduction: 

An ammeter reads a greater value of current when a small conductor is connected in the circuit as compared to that when a long conductor is connected in the circuit. In this section, you are going to learn about resistance, factors affecting resistance, and the mathematical formula for calculating resistance.  

Explanation: 

Resistance: 

Resistance is the property of any material that slows down the flow of electrons. It converts electrical energy into other forms of energy, such as heat energy. Because of this, the temperature increases. 

Example: In the case of a light bulb attached to the electric circuit, when the current passes, the resistance is offered by the tungsten filament of the bulb, which converts electrical energy into heat energy and light energy. 

Factors affecting resistance: 

A few of the factors that affect resistance are length, cross-section, and the resistivity of the material. 

• Resistance (R) depends directly on the length (l) of the material. 

• Resistance depends inversely on the area of cross-section (A) of the material. 
• Resistance depends directly on the property of the material, which is known as resistivity (ρ). Resistivity is the property of a material to conduct electric current. High resistivity means poor conductor. 

Mathematically,  

Resistance

α length 𝛼 length 

Resistance

α𝛼

1Area of cross−section1Area of cross−section

Resistance α lengthArea of cross−sectionResistance 𝛼 lengthArea of cross−section

SI unit of resistivity is Ohm-m, abbreviated as (Ωm) 

Questions and answers 

Question 1: Keeping the material, length and the area of cross-section of resistance the same, if the voltage is increased across the conductor, what will be the effect on the number of electrons passing through the conductor? 

Answer: As the voltage increases, the number of electrons moving through an electric circuit will increase as more electrons will be released from the negative terminal of the battery. 

Question 2: What will be the effect of the increase in resistivity of a material on the current flowing through an electric circuit?  

Answer: As the resistivity increases, the resistance will increase as resistance and resistivity are directly proportional to each other. 

Now since the resistance increases, that means the resistance to the flow of the electrons is increasing. With this increase in the resistance, the number of electrons passing each second through the conductor will decrease. 

Current is the number of electrons passing through a point per second. As the number of electrons decreases, the current flowing through an electric circuit will decrease.

Summary

• Resistance is the property of any material that slows down the flow of electrons. It converts electrical energy into other forms of energy.
• As the resistance of a material increases the amount of electrical energy converted into other form of energy increases.
• Factors which affect the resistance are:

1. Resistivity of the material,ρ
2. Length of the material, ℓ 
3. Area of cross-section of the material, A
4. R= ρℓ / A