CBSE Class 12 Physics Revision Notes Chapter 3

Class 12 Physics Chapter 3 Notes – Current Electricity

Physics is a required subject for all competitive examinations like NEET, JEE or any other technical exams.  Physics should be studied by understanding the concepts well and practising end-text exercises along with additional exercises to master the topic.  . Hence, it becomes necessary to learn from authentic and reliable notes because most of the CBSE questions are picked from NCERT books. To help students with their preparation, Extramarks has exclusively prepared Chapter 3 Physics Class 12 Notes. These notes cover all the concepts explained in simple language. It will help students resolve their queries and prepare well for examinations. 

Physics Class 12 Chapter 3 is about Current Electricity. The current is a flow of electric charge through any cross-section of wire. And these charges are carried by electrons in a wire. This chapter includes all concepts revolving around current electricity. Class 12 Physics Chapter 3 Notes created by the Extramarks includes a detailed explanation of the entire chapter. 

Key Topics Covered In Class 12 Physics Chapter 3 Notes 

Below we have given a brief summary of the key topics covered in our Extramarks Notes of Class 12 Physics Chapter 3.

Electric Current:

Electric Current is defined as the rate of flow of charges in any system. For a steady approach, it is ensured that the flow of charges is in a closed loop. The flow of charges is influenced by an external force and is always considered concerning time.

Some important properties:

• Electric Current is denoted by ‘I’. 
• It flows from higher potential energy to lower potential energy.
• Electric Current ‘I’ can also be written as I = dQ/dt, where dQ is a change in charge and dt is a change in time.
• The SI unit of electric  current is Amperes, denoted by A.

Current Density:

Current Density is the electric charge flowing per unit area of cross-section. It’s a vector quantity.  

Some important properties of current density as covered in Class 12 Physics Chapter 3 Notes:

• Current Density is denoted by J.
• J = I/A where I = electric current, A = cross-sectional area.
• SI unit of current Density is ampere m-2.
• The dimensional formula is [ A T-2 ].

Drift Velocity:

The drift velocity of the electron is the velocity with which free electrons move in a conductor in the direction opposite to the electric field when a potential difference is applied to its ends. 

Some important properties:

• Drift velocity is denoted by vd = eE?/m = eV?/ml.
• Where ? = relaxation time, e = charge on the electron, E = electric field intensity, l = length of the conductor, V = potential difference applied at the ends, m= mass of electron.

Ohm’s Law:

According to Ohm’s Law, the electric current flowing through any conductor is proportional to the voltage applied across its ends. 

If I is the current flowing through the conductor, and V is the potential difference between two points, then according to Ohm’s law,

V ∝ I 

Ohm’s Law is a fundamental concept that will be applied to many advanced levels later. Therefore, we encourage students to have a sound knowledge of each topic and avoid rote learning. . We recommend students to study from our Class 12 Physics Chapter 3 Notes where our Physics faculty experts have explained Ohm’s Law in greater detail.

Kirchoff’s Law:

There are two laws under this:

1. Kirchhoff’s Current Law.
2. Kirchhoff’s Voltage Law.

Kirchoff’s Current Law as described in our Class 12 Physics Chapter 3 Notes:

• This law states the conservation of electric charges.
• According to this law, the sum of currents entering the junction at any node in an electrical circuit equals the sum of currents leaving the junction.

For example:

In the following figure, there are five currents. 

According to Kirchoff’s current law, 

I1 + I3 = I2 + I4 + I5

Kirchoff’s voltage law:

• According to Kirchoff’s voltage law, the directed sum of voltages (electric potential difference) around any closed network is always zero. 

Electrical Resistance: 

Resistance is the obstruction offered by any conductor to the flow of current. It is denoted by R. 

As per Ohm’s law,

V ∝ I, 

Therefore, V = IR 

R is the resistance of a substance. 

Some important properties:

• It is denoted by ‘R’.
• SI unit of resistance is Ohm / Ω.

Electrical resistance is also noted as,

R = ⍴l/A where, ⍴ = resistivity of the material, l = length of the conductor, and A = cross-sectional area of the conductor. 

Resistivity:

Resistivity is the electrical resistance of the conductor considering unit cross-sectional area and unit length. 

Some important properties about resistivity that are further elaborated in our Class 12 Physics Chapter 3 Notes:

• It is dependent on the material of the conductor.
• Increases with an increase in temperature.
• High resistivity designates poor conductors.
• Resistivity is low for metals, high for semiconductors, and very high for alloys like nichrome. 

Electrical conductivity:

Electrical conductivity is the reciprocal of resistivity. 

Some important properties: 

• It is denoted by σ = 1/⍴ = l/RA.
• The SI unit of electrical conductivity is ohm-1 m-1 or mho m-1.
• J = σ E, where J = Current density, σ= electrical conductivity, and E = electrical field intensity. 

Students can register on Extramarks website and get access to our Class 12 Physics Chapter 3 Notes where the electrical conductivity topic is explained with real-life examples.

Superconductivity:

Superconductivity as explained in Class 12 Physics Chapter 3 Notes, is when substances get their electrical resistance to zero, below a specific critical temperature. Examples include aluminum, niobium, etc.

Colour coding for carbon resistors:

The value of resistance in carbon resistors is denoted by four coloured bands marked on their surface. Resistance of any carbon resistor can be determined by the following colour scheme. 

 The color-coding system for numbers is listed below: .

The following phrase has been given in our Class 12 Physics Chapter 3 Notes. This can help students to  remember the above color-coding scheme: “B B ROY Great Britain Very Good Wife.”

Tolerance percentage:

In the figure below, a carbon conductor with multiple rings is illustrated. There are four bands, given they are used to calculate the resistance. The first two bands provide the significant digits. The third band gives the multiplier value, and the fourth/last band gives the tolerance percentage. The resistance value is determined by the first three bands a,b,c, and the resistance tolerance is determined by the fourth band d.

Combination of resistors:

Resistors are objects used to restrict the flow of charges in any conductor. However, these individual resistors can be arranged in different patterns to obtain different results. 

Resistors can be connected in 2 main ways:

• In series
• In parallel

Resistors in series:

The resistor in series means all the resistors are connected sequentially, one after the other.

The above figure shows that resistors R1, R2, and R3 are connected in series. Students can register on Extramarks’ website and access our Class 12 Physics Chapter 3 Notes that will help students understand the topic of resistors in series much better.

Some important properties:

• Effective resistance is the sum of individual resistance

R = R1 + R2 + R3.

• The current flowing through each resistor is the same.
• The total potential difference applied is equal to the sum of the potential difference at each resistor.

Resistors in parallel:

Resistors in parallel means when resistors are connected one below the other. All ends are connected to a common junction. 

The above image shows that four resistors, R1, R2, R3, and R4, are connected in parallel. A potential difference V is applied at its ends. 

Some important properties:

• Equivalent resistance R is 

1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4  

• The potential difference across each resistor is the same.
• The electric current ‘I’ drawn from the source equals the sum of currents flowing through individual currents. 

I = I1 + I2 + I3 + I4

Cell:

A cell or electric cell is a device that helps to convert chemical energy into electrical energy. There are two types of cells:

• Primary Cell
• Secondary Cell

Primary Cell:

These cells cannot be charged again. And only can be used once.  e.g., Voltic, Daniel Cells. 

Secondary Cell:

These cells can be recharged numerous times . e.g., Acid & alkali accumulators. 

Potentiometer:

It is a device used to compare the potential differences. It can also be used to calculate the internal resistance of a cell. 

Construction of potentiometer:

The potentiometer consists of a long resistance wire AB, with a uniform cross-sectional area, where a steady current flows through a battery setup.

The diagram above represents a potentiometer with a wire AB and a connected cell and key. It also contains a rheostat. 

If R is the total resistance of potentiometer wire, having length l, then the potential gradient will be,

K = V/L = IR/L

K = Eo R / (RO + R)L

Where Eo  = emf of battery, RO = resistance of rheostat Rh. 

Class 12 Physics Chapter 3: Exercises &  Solutions

Class 12 Physics Chapter 3 Notes explain all concepts and terminologies based on the  current  electricity. It explains the significance of current, its use cases, and its applications. The solution set is prepared exclusively by subject matter experts at Extramarks.  They strictly follow the NCERT textbook  and revise the content adhering to the CBSE  guidelines.  These solutions are curated for students to help them prepare for their Examinations.  Extramarks provides a repository of study materials with  answers to all the questions from CBSE NCERT books, Important Questions, CBSE Sample Papers, CBSE previous year question papers, and CBSE Extra Questions to feed their insatiable curiosity  to learn more. It encourages students to write their own answers and then check with the solutions provided. To learn and practice more in less time.

Following are the solutions for all questions mentioned in NCERT Class 12 books. Click on the links below to refer to Solutions. 

Students can refer to the respective exercise to access the NCERT solutions Class 12 Physics Chapter 3. Students can also explore a repository  of educational content available on the Extramarks’ website for all classes from 1 to 12. Click on the respective links below to know more. 

Key Features of NCERT Solutions Class 12 Physics Chapter 3

Chapter 3 Physics Class 12 Notes will help students get an in-depth understanding of subject topics in the following ways: 

• In addition to NCERT books, the  Class 12 Physics Chapter 3 Notes offer a detailed explanation of all the topics mentioned in the chapter. .
• Physics is an important subject required for examinations like JEE, NEET or any other technical examination. Hence while studying Physics, authentic and reliable  notes and solutions provided by Extramarks will definitely prove useful. .
• CBSE Class 12 Physics Chapter 3 Notes by Extramarks encourages  students to study comprehensively for examinations because Extramarks tries to bridge the knowledge gap and help them to excel in academics by clearing their doubts if any.
• These notes include a detailed explanation of each and every topic present in the chapter.
• Illustrative diagrams and figures are used in notes, which will help students understand the logic behind theoretical  knowledge and help them understand the important points easily and make them revise quickly. 
• These chapter notes are helpful for all types of school examinations or competitive examinations. It saves a lot of time in exam preparation.

Some Effective Ways To Score Good Marks Using Class 12 Physics Chapter 3 Notes 

Physics is a complex subject. And hence, it becomes vital to understand the topic well and not  learn the definitions by heart. Extramarks doesn’t believe in rote learning, in fact it follows the latest CBSE guidelines to provide experiential learning to students.

• While preparing for examinations,  get to know the CBSE Syllabus. . It will help you to plan your studies intelligently and effectively.  
• After knowing the syllabus, read concepts and topics included in NCERT textbooks and understand them.
• Also, memorise essential formulas. This would help you to   get more marks in  numerical questions. 
• You can always refer to Class 12 Physics Chapter 3 Notes for a detailed explanation of the topics. 
• While writing answers, you must adhere  to specific guidelines. For this, read  through solutions, and learn how to answer questions without making careless slips.. 
• After learning the concepts, one must always practise solving CBSE Previous Year’s Question Papers, CBSE sample questions, and other Important Questions from the chapter to nail any exam and come out with flying colours