Ohm's Law and Resistance Class 10 Notes | Electricity Chapter ( PART 2)
Introduction
Have you ever wondered why some electrical devices consume more electricity while others consume less? Why does an electric heater become hot while a copper wire simply carries current? The answer lies in a property called resistance.
In the study of electricity, one of the most important relationships is between current, voltage, and resistance. This relationship is explained by Ohm's Law, a fundamental principle discovered by the German physicist Georg Simon Ohm.
Understanding Ohm's Law helps us explain how electrical circuits work and forms the foundation for solving many numerical problems in electricity.
What is Ohm's Law?
Ohm's Law states that: "At constant temperature, the potential difference across the ends of a conductor is directly proportional to the current flowing through it."
In simple words, if the voltage across a conductor increases, the current flowing through it also increases, provided the temperature remains constant.
This relationship can be expressed mathematically as:
Where:
V = Potential Difference (Voltage)
I = Electric Current
R = Resistance
This formula is one of the most important equations in electricity and is widely used to solve numerical problems.
Important Forms of Ohm's Law
Depending on what value we need to calculate, the formula can be rearranged as:
To find Resistance
To find Current
I= current
R= resistance
What is Resistance?
Resistance is the property of a conductor that opposes or resists the flow of electric current through it.
Every material offers some resistance to the movement of electrons. The greater the resistance, the more difficult it is for current to flow.
SI Unit of Resistance
The SI unit of resistance is Ohm (Ω).
What is One Ohm?
A conductor is said to have a resistance of one ohm if a potential difference of one volt causes a current of one ampere to flow through it.
1 ohm= 1 volt/1 ampere
Formal for ohms law
V= I*R
R= V/I
I= V/ R
Relationship Between Current and Resistance
Current and resistance are inversely related.
This means:
If resistance increases, current decreases.
If resistance decreases, current increases.
For example, if the resistance of a conductor is doubled, the current flowing through it becomes half, provided the voltage remains the same.
Variable Resistance and Rheostat
In many electrical circuits, it is necessary to control or adjust the current flowing through the circuit.
A variable resistance is a component that allows us to change the resistance without changing the voltage source.
A device called a rheostat is commonly used for this purpose. By changing the resistance, a rheostat helps regulate the amount of current flowing in a circuit.
Examples include:
Fan regulators
Light dimmers
Laboratory electrical circuits
Classification of Conductors Based on Resistance
Different materials offer different amounts of resistance to the flow of current.
Good Conductors
Good conductors offer very low resistance and allow current to flow easily.
Examples:
Copper
Aluminium
Silver
These materials are commonly used in electrical wiring.
Resistors
A resistor is a conductor designed to provide a specific amount of resistance in a circuit.
Resistors are used to control current and voltage in electrical and electronic devices.
Poor Conductors
Poor conductors offer higher resistance to the flow of current.
Examples:
Iron
Nichrome
Current flows through them less easily than through good conductors.
Insulators
Insulators offer extremely high resistance and practically prevent the flow of current.
Examples:
Rubber
Glass
Plastic
Wood
Insulators are used to protect us from electric shocks.
Factors Affecting Resistance
The resistance of a conductor depends on several factors:
1. Nature of Material
Different materials have different resistances.
For example:
Copper has low resistance.
Rubber has very high resistance.
2. Length of the Conductor
Resistance increases with the length of the conductor.
A longer wire provides more opposition to the flow of electrons.
Therefore:
Longer wire → More resistance
Shorter wire → Less resistance
3. Area of Cross-Section
The thickness of a wire also affects resistance.
A thick wire allows electrons to flow more easily than a thin wire.
Therefore:
Larger area → Less resistance
Smaller area → More resistance
4. Temperature
The resistance of most conductors changes with temperature.
As temperature increases, resistance generally increases.
Resistivity
The resistance of a material is also determined by a property known as resistivity.
The relationship is given by:
Where:
R = Resistance
ρ (rho) = Resistivity
l = Length of conductor
A = Area of cross-section
SI Unit of Resistivity
The SI unit of resistivity is Ohm metre (Ω m).
Solved Numerical Problems & Some important questions frequently asked in exam
Why Are Alloys Used in Heating Devices?
Alloys such as nichrome are commonly used in heating appliances like:
Electric irons
Heaters
Toasters
Electric stoves
This is because:
They have higher resistance.
They can withstand very high temperatures.
They do not oxidize or burn easily.
Why is Tungsten Used in Electric Bulbs?
Tungsten is almost exclusively used as the filament in electric bulbs because:
It has a very high melting point.
It can withstand extremely high temperatures without melting.
It glows brightly when heated.
Why are Copper and Aluminium Used for Transmission Wires?
Copper and aluminium are widely used for electrical transmission because:
They are excellent conductors of electricity.
They have very low resistance.
They allow efficient transmission of electric current over long distances.
Draw the symbol for variable resistance, bulb,cell,wire joint, wire crossing without joining
If a wire is having 5 cm in diameter, and another wire is having 2.5 CM in diameter in which wire do we find less resistance and why
5 cm diameter wire has less resistance because resistance is inversely proportional to area of cross section
Problem 1 : How much current will an electric bulb draw from a 220 V source if the resistance of the bulb filament is 1200 Ω?
Solution:
I = V/R
= 220/1200
= 0.18 A
Answer: 0.18 Ampere
Problem 2 : How much current will an electric heater draw from a 220 V source if its resistance is 100 Ω?
Solution:
I = V/R
= 220/100
= 2.2 A
Answer: 2.2 Ampere
Problem 3 : The potential difference across an electric heater is 60 V and it draws a current of 4 A. What current will it draw if the voltage is increased to 120 V?
Solution:
First calculate resistance:
R = V/I
= 60/4
= 15 Ω
Now,
I = V/R
= 120/15
= 8 A
Answer: 8 Ampere
🎥 Video Explanation
Quick Revision
Ohm's Law states that voltage is directly proportional to current.
Resistance opposes the flow of electric current.
SI unit of resistance is Ohm.
Resistance depends on length, area, material, and temperature.
Copper and aluminium are good conductors.
Rubber and glass are insulators.
Alloys are used in heating devices because of their high resistance and heat tolerance.
Conclusion
Ohm's Law is one of the most important concepts in electricity. It explains the relationship between current, voltage, and resistance and helps us understand the working of electrical circuits. A clear understanding of resistance and the factors affecting it will make it easier to study advanced topics such as electrical power, household circuits, and electrical energy in later chapters.
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