Class 12 Physics Chapter 3 Current Electricity

If Current Electricity Class 12 feels confusing, don’t worry - many students feel the same. This chapter has many formulas and numerical problems, which makes it stressful before exams.

That’s why these notes explain everything in simple words, with short summaries, important formulas, and exam-focused points. Nothing extra, nothing complicated.

These notes will help you save time, understand the basics clearly, and feel more confident for board exams and competitive exams. You can also download the PDF and revise anytime without reading long textbook pages. Let’s make Current Electricity easy, step by step.

Current Electricity Class 12 Notes

If you’re looking for Current Electricity Class 12 notes that are easy to understand, you’re in the right place. No textbook-style explanations - just clear concepts, important formulas, and a layout made for quick revision.

From basic ideas and resistor combinations to exam-focused questions and common mistakes, everything you need for this chapter is covered here. You can also download the CBSE Class 12 Current Electricity Notes PDF for easy revision anytime.

What is Electric Current?

Electric current is the flow of electric charges through a conductor. It is measured in amperes (A). Although conventional current flows from positive to negative, the actual flow of electrons flows in the opposite direction. 

The SI unit for electric current is ampere (A). The dimensional formula of electric current is (M⁰ L⁰ T⁰ I¹), where I represent electric current.

Formula for Electric Current: I = Q / t

Where:

• I denote electric current, expressed in amperes (A).
• Q represents the charge, given in coulombs (C).
• t stands for the time, in seconds (s).

Types of Current

1. Direct Current (DC): Direct current refers to the movement of electric charge in one direction. An example of this is the current from batteries.

2. Alternating Current (AC): Alternating current refers to the periodic reversal of direction of the electric charge flow. This type of current is used to power industries and homes.

Electric Potential Against Electric Force

Electric potential at a point is the work done against the electric force in bringing a unit positive test charge from infinity to that point. Electric potential is a scalar quantity and is expressed in volts (V).

‍1. Potential Difference (V): Potential difference between two points is the work used to move a unit positive charge from one point to another

Formula: V = W / Q

Where:

• V is for potential difference in volts,
• W is for work done in joules,
• Q is for charge in coulombs.

The SI unit of potential difference (or voltage) is volt (V). A volt is defined as the potential difference between two points when it takes one joule of work to transfer one coulomb of charge from one point to the other.

What is Ohm’s Law?

Ohm's law is one of the basic principles that links voltage (V), current (I) and resistance (R) with a simple formula. That is, the current passing through a conductor is directly proportional to the potential difference offered to it and inversely proportional to the resistance of that conductor.

Ohm's Law Formula: V = I. R

where V is the potential difference (voltage), I is the current and R is the resistance. This law is useful for understanding the concept of electrical circuits.

Applications of Ohm’s Law

These are some real life applications of Ohm’s Law:

• In Resistors: It can be used to calculate the resistance of a conductor when both the current and the potential difference are known.
• In Electric Circuits: Ohm's Law plays an important role in designing electrical circuits and determining the appropriate resistance values to obtain the desired current flow.

What is Resistance?

Resistance is the opposition that the electric current offers while passing through a conductor. This is dependent on the material, length and cross-sectional area of the conductor. Resistance is measured in ohms (Ω).

Resistance Formula: R = ρ. L / A

Where:

• R is the resistance in ohms, Ω
• ρ is the resistivity of the material in ohm-meters, Ω⋅m
• L is the length of the conductor in meters, m
• A is the cross-sectional area of the conductor in square meters, m^2

Factors Affecting Resistance

Several factors directly affect how easily current can pass through the conductor. 

• Material: Different materials have different resistivities. Conductors such as copper and aluminum have low resistance, while insulators such as rubber and wood have high resistance.

• Length of the conductor: The resistance of a conductor is directly proportional to its length. The longer the conductor is in terms of length, the higher its resistance will be. 

• Cross-sectional area: The cross-sectional area of the conductor is inversely proportional to its resistance. The wider the cross-sectional area, the lesser resistance it will carry.

• Temperature: Usually in materials, the resistance increases with temperature. Conversely, in some materials such as semiconductors, resistance decreases with temperature.

Resistivity and Conductivity

Resistivity, represented by the symbol ρ, is a characteristic of a material that resists the flow of electric current. It is influenced by the type of material, its temperature and its physical state. The standard unit of resistivity is ohm-meter (Ω·m). 

On the other hand, conductivity, denoted by σ, is the reciprocal of resistivity and measures how easily a material allows the flow of electric current. The relationship between resistivity and conductivity can be expressed as: ρ = 1 / σ

Both resistivity and conductivity are affected by factors such as temperature and the presence of impurities in the material.

‍Series and Parallel Combination of Resistors

Resistors can be connected in two arrangements: series and parallel.

Series Combination: If resistors are connected in a straight line along a single path, then they are said to be connected in series. In this connection, the sum of the individual resistances is equal to the total resistance R_t

Formula: R_t = R₁ + R₂ + R₃ + …

Where R₁, R₂, R_3, … denote the individual resistances of the resistors.

Parallel Combination: Resistors connected in parallel allow the current to have more than one path. The total resistance is less than that of any single resistor.

Formula: 1 / Rt = 1 / R1 + 1 / R2 + 1 / R3 + …….

Where R1, R2, R3,…… represent the individual resistances.

Electrical Power and Energy

Power is defined as the rate at which an electrical device changes electrical energy into other forms, such as heat, light or mechanical energy. Power is measured in watts (W).

P = V. I

Here,

• P is the power expressed in watts, W
• V is the potential difference that is measured in volts, V
• I is the current that is measured in amperes, A

An alternative method to calculate the expression of power is through resistance. This is by using the Ohm's law: P = I². R = V² / R

Important Practice Questions

Some important questions based on the chapter are given below that can help students prepare for exams.

Q1: What is electric current? Discuss the SI unit of current and its dimensional formula.

Q2: Explain Ohm's Law. Derive it and explain its significance.

Q3: What is the difference between series and parallel combinations of resistors? Derive the equivalent resistance for both cases.

Q4: What is electric power? Derive the relation between power, voltage, current and resistance.

Q5: Derive the expression for the resistance of a conductor in terms of its resistivity, length and cross-sectional area.

Q6: Define resistivity and conductivity. How are these properties related to the material of a conductor?

Q7: Explain the concept of electrical energy consumption and the cost of electrical energy used in homes.

Q8: Explain the phenomenon of heating effect of current with the help of a practical example.

Q9: What is the principle behind the working of an ammeter and a voltmeter?

Q10: What is the effect of temperature on the resistance of a conductor?

Common Mistakes Students Make While Studying Current Electricity

While preparing this chapter, many students end up losing marks because of small but avoidable mistakes. Some of the most common ones are:

• Confusing the direction of current by mixing up conventional current with the actual flow of electrons.
• Applying Ohm’s Law incorrectly, especially without checking whether the conductor follows Ohm’s law.
• Ignoring the effect of temperature on resistance, which can change values in numerical problems.
• Forgetting to use proper SI units while solving numericals, leading to calculation errors.
• Using the wrong power formula (P = VI, I²R, or V²/R) without understanding when to apply each one.
• Not clearly understanding the heating effect of current, especially Joule’s law of heating.
• Making mistakes while calculating equivalent resistance in series and parallel combinations.

FAQs

Q1. What is electric current in simple words?

Ans. Electric current is the flow of electric charge through a conductor. It tells us how fast charges move in a circuit and is measured in amperes (A).

Q2. What is the main formula used in Current Electricity?

Ans. The most important formula is Ohm’s Law: V = I × R, where V is voltage, I is current and R is resistance.

Q3. What is the difference between series and parallel circuits?

Ans. In a series circuit, all components are connected one after another, so the current is the same everywhere. In a parallel circuit, components have separate paths and the total resistance becomes lower.

Q4. What factors affect the resistance of a conductor?

Ans. Resistance depends on the material, length, thickness (area) and temperature of the conductor.

Q5. How can I score well in the Current Electricity chapter?

Ans. Understand the basic concepts, memorize key formulas, practice numerical problems and revise using short notes. Solving previous year questions also helps boost confidence and accuracy.