Electric Resistance and Ohm’s Law Explained: Complete Notes for JKSSB Finance Accounts Assistant
Introduction
Electricity is one of the most important topics in General Science for competitive examinations like JKSSB Finance Accounts Assistant (FAA). Many questions are asked from basic concepts of electric current, voltage, resistance, and Ohm’s Law. When electric current flows through a conductor, the movement of electrons is opposed by a property called resistance. The amount of resistance depends on factors such as the length, thickness, material, and temperature of the conductor.
Ohm’s Law explains the relationship between voltage, current, and resistance in an electrical circuit. Understanding this law helps aspirants solve numerical and conceptual questions related to electricity.
Electric Resistance
Electric resistance is the property of a conductor by which it opposes the flow of electric current through it. When electrons move through a conductor, they collide with the atoms of the material, creating opposition to their motion. This opposition is called resistance.
In simple words, resistance controls the flow of electric current in a circuit. A conductor with high resistance allows less current to pass, while a conductor with low resistance allows more current to flow.
Formula of Resistance
The resistance of a conductor is given by:
R = V / I , Where:
R = Resistance (Ohm, Ω)
V = Potential Difference or Voltage (Volt)
I = Electric Current (Ampere)
SI Unit of Resistance
The SI unit of resistance is Ohm (Ω). A conductor has a resistance of 1 Ohm if a potential difference of 1 Volt produces a current of 1 Ampere through it.
Important Points
- Resistance is represented by the symbol R.
- Resistance opposes the flow of electric current.
- Higher resistance → Lower current flow.
- Lower resistance → Higher current flow.
- Resistance is measured using an instrument called Ohmmeter.
Factors Affecting Resistance
The resistance of a conductor does not remain the same in all conditions. It depends on several factors such as the length of the conductor, area of cross-section, nature of material, and temperature.
Understanding these factors is important for JKSSB Finance Accounts Assistant exams as direct conceptual questions are often asked from this topic.
1. Length of Conductor
The resistance of a conductor increases with an increase in its length.
- A longer wire has more resistance because electrons travel a greater distance and face more collisions.
- A shorter wire offers less resistance.
Relation: Resistance ∝ Length
Example: A long wire has more resistance than a short wire made of the same material and thickness.
2. Area of Cross-Section of Conductor
Resistance depends on the thickness of the conductor.
- A thick wire has a larger area of cross-section and offers less resistance.
- A thin wire has a smaller area of cross-section and offers more resistance.
Relation: Resistance ∝ 1 / Area
Example: A thick copper wire carries more current easily compared to a thin wire.
3. Nature of Material
Different materials have different abilities to oppose the flow of current.
- Good conductors like copper and aluminium have low resistance.
- Materials like nichrome have high resistance and are used in heating elements.
This property of a material is called resistivity.
4. Temperature
Temperature also affects resistance.
- In metals, resistance increases when temperature increases.
- Higher temperature causes more vibration of atoms, leading to more collisions with electrons.
Important Points
- Resistance increases with increase in length.
- Resistance decreases with increase in thickness of wire.
- Copper has low resistance and is a good conductor.
- Metals generally show an increase in resistance with temperature.
- Formula:
R = ρL / A ,Where:
R = Resistance
ρ = Resistivity
L = Length of conductor
A = Area of cross-section
Ohm’s Law
Ohm’s Law is one of the most important laws of electricity and is frequently asked in competitive examinations like JKSSB Finance Accounts Assistant (FAA).
Statement of Ohm’s Law
Ohm’s Law states that: “At constant temperature, the current flowing through a conductor is directly proportional to the potential difference (voltage) applied across its ends.”
In simple words, when the voltage increases, the current also increases, provided the temperature and other conditions remain constant.
Mathematical Expression
According to Ohm’s Law: V ∝ I or V = IR
Where:
- V = Potential Difference (Voltage) measured in Volt
- I = Electric Current measured in Ampere
- R = Resistance measured in Ohm (Ω)
Explanation
- If the resistance of a conductor remains constant, increasing voltage increases the current.
- If resistance increases, the flow of current decreases for the same voltage.
Example
If a circuit has a resistance of 5 Ω and a voltage of 10 V is applied: I = V / R
I = 10 / 5
I = 2 A
Therefore, the current flowing through the circuit is 2 Ampere.
Important Points
- Ohm’s Law is valid when temperature remains constant.
- The relationship between voltage and current is directly proportional.
- The graph between voltage and current for an ohmic conductor is a straight line.
- Formula of Ohm’s Law: V = IR
- Resistance is the constant of proportionality between voltage and current.
V-I Graph and Ohmic Conductors
The relationship between Voltage (V) and Current (I) can be represented using a graph. This graph helps us understand Ohm’s Law in a simple way.
For a conductor that follows Ohm’s Law, the current increases directly with an increase in voltage. Therefore, the V-I graph is a straight line passing through the origin.
Features of V-I Graph
- Voltage is taken on the Y-axis.
- Current is taken on the X-axis.
- A straight-line graph shows that voltage and current are directly proportional.
- The slope of the V-I graph represents the resistance of the conductor.
Ohmic Conductors
A conductor that follows Ohm’s Law is called an Ohmic conductor.
Examples:
- Copper wire
- Aluminium wire
In these conductors:
- Resistance remains constant.
- Current increases proportionally with voltage.
Non-Ohmic Conductors
Conductors that do not follow Ohm’s Law are called Non-Ohmic conductors.
Examples:
- Diode
- Filament bulb
In these materials, the relationship between voltage and current is not linear.
Important Points
- Straight line V-I graph indicates an Ohmic conductor.
- Slope of V-I graph gives resistance.
- Ohm’s Law is not applicable when temperature changes.
- Filament bulbs do not obey Ohm’s Law.
Difference Between Resistance and Resistivity
Resistance and resistivity are related terms in electricity, but they are not the same. Understanding their difference is important for JKSSB Finance Accounts Assistant (FAA) examinations.
| Resistance | Resistivity |
| Resistance is the opposition offered by a conductor to the flow of electric current. | Resistivity is the property of a material that determines how strongly it opposes current flow. |
| It depends on length, area, and material of the conductor. | It depends only on the nature of the material. |
| It changes when the size of the conductor changes. | It remains the same for a given material at a particular temperature. |
| SI unit is Ohm (Ω). | SI unit is Ohm metre (Ω m). |
| Formula: R = V/I | Formula: ρ = RA/L |
Important Points
- Resistance is a property of a particular conductor.
- Resistivity is an inherent property of a material.
- Copper has low resistivity, making it a good conductor.
- Materials with high resistivity are used as heating elements.
Important Facts and Quick Revision for JKSSB FAA
Electric Resistance and Ohm’s Law are important topics from General Science where direct factual and conceptual questions are asked in competitive examinations.
Quick Revision Points
- Electric resistance is the opposition offered by a conductor to the flow of electric current.
- Resistance is represented by the symbol R.
- SI unit of resistance is Ohm (Ω).
- Resistance is measured using an instrument called Ohmmeter.
- Formula of resistance: R = V / I
- According to Ohm’s Law: V = IR
- Ohm’s Law is valid only when temperature remains constant.
- Increase in length of a conductor increases its resistance.
- Increase in area of cross-section decreases resistance.
- Copper and aluminium are good conductors due to low resistance.
- Nichrome is used in heating elements because of its high resistance.
- Metals generally show an increase in resistance with increase in temperature.
- The slope of a V-I graph represents resistance.
- Conductors that follow Ohm’s Law are called Ohmic conductors.
Frequently Asked Questions (FAQs) on Electric Resistance and Ohm’s Law
Q1. What is electric resistance?
Electric resistance is the property of a conductor that opposes the flow of electric current through it. Its SI unit is Ohm (Ω).
Q2. What is the formula of resistance?
The formula of resistance is: R = V / I
Where R is resistance, V is voltage, and I is current.
Q3. What is Ohm’s Law?
Ohm’s Law states that at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.
Formula: V = IR
Q4. Which factors affect the resistance of a conductor?
The resistance of a conductor depends on:
- Length of conductor
- Area of cross-section
- Nature of material
- Temperature
Q5. Why does a longer wire have more resistance?
A longer wire has more resistance because electrons have to travel a greater distance and face more collisions with atoms.
Q6. Why is copper used for electrical wiring?
Copper is used in electrical wiring because it has very low resistance and allows electric current to flow easily.
Q7. Which instrument is used to measure resistance?
Resistance is measured using an instrument called an Ohmmeter.
Q8. Does Ohm’s Law apply to all conductors?
No. Ohm’s Law applies only to Ohmic conductors under constant temperature conditions. Devices like filament bulbs and diodes do not obey Ohm’s Law.
Q9. What happens to resistance when the thickness of a wire increases?
When the thickness (area of cross-section) of a wire increases, its resistance decreases.
Q10. What is the SI unit of resistivity?
The SI unit of resistivity is Ohm metre (Ω m).
Conclusion
Electric Resistance and Ohm’s Law are fundamental concepts of electricity and are important topics for JKSSB Finance Accounts Assistant (FAA) General Science preparation.
Resistance explains how a conductor opposes the flow of electric current, while Ohm’s Law describes the relationship between voltage, current, and resistance. A clear understanding of formulas, units, and factors affecting resistance helps aspirants solve both conceptual and numerical questions.
About The Author
