Chapter 4 of 25

16%

Tamil Nadu (TNBSE)Class 9 Science★ 5-7 marks in assessments

Electric Charge and Electric Current

Samacheer Kalvi (TN State Board) Class 9 Science — Physics Chapter 4, Electric Charge and Electric Current. Electricity powers our modern world. Understanding electric charges, current, voltage, resistance, and Ohm's law forms the core foundation of electrical engineering and home appliance safety. Notes, worked examples, common mistakes, and quick revision.

⏱ 21 min readMedium difficulty✓ Free · NCERT-aligned

Worksheet

Podcast

Listen as a conversation

A teacher & a student talk through the chapter — in your language.

Mind Map

The chapter at a glance

Every concept and exam-favourite branch — revisable in 5 minutes.

…

Study Guide

Night-before notes

Key points, glossary and the questions most likely to show up.

…

VocabularyBuild your vocabularyLexiGenome mines the key words and schedules them with spaced repetition.Mine words →

By the end of this chapter you'll be able to…

1Explain electric charge, current, and potential difference
2State and mathematically apply Ohm's law
3Analyze series and parallel combinations of resistors
4Describe the heating, magnetic, and chemical effects of current

💡

Why this chapter matters

Electricity powers our modern world. Understanding electric charges, current, voltage, resistance, and Ohm's law forms the core foundation of electrical engineering and home appliance safety.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

🔗

Class 8 Science — Electricity

Static charge and basic circuits.

Electric Charge and Electric Current — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Physics — Chapter 4. Electricity powers our modern world. Understanding electric charges, current, voltage, resistance, and Ohm's law forms the core foundation of electrical engineering and home appliance safety.

1. About this chapter

This chapter covers electric charge, current, potential difference, circuits, Ohm's law, and effects of current.

2. Electric Current and Potential Difference

Electric Charge: Measured in coulombs (C). Electron charge is $- 1.6 \times 1 0^{- 19}$ C.
Electric Current: Flow of charges per unit time ( $I = Q / t$ ). Measured in amperes (A).
Potential Difference (V): Work done per unit charge ( $V = W / Q$ ). Measured in volts (V).

3. Ohm's Law and Resistance

Ohm's Law: At constant temperature, current is directly proportional to potential difference ( $V = I R$ ).
Resistance (R): Opposition to current flow. Unit: ohm ( $Ω$ ).

4. Resistor Networks

Series: $R_{s} = R_{1} + R_{2} + R_{3}$ . Total resistance increases.
Parallel: $\frac{1}{R _{p}} = \frac{1}{R _{1}} + \frac{1}{R _{2}} + \frac{1}{R _{3}}$ . Total resistance decreases.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Electric Current

I = Q / t

Q = charge (coulombs), t = time (seconds). Unit: ampere (A).

Ohm's Law

V = I R

V = potential difference (volts), I = current, R = resistance (ohms).

Resistors in Series

Rs = R1 + R2 + R3

Current remains the same, voltage splits.

Resistors in Parallel

1/Rp = 1/R1 + 1/R2 + 1/R3

Voltage remains the same, current splits.

⚠️

Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Confusing electric current flow direction.

✓ Conventional current flows from positive to negative terminal, which is opposite to the actual flow of electrons (negative to positive).

WATCH OUT

✗ Incorrect calculation of equivalent parallel resistance.

✓ Remember to take the reciprocal of the final sum. $R_p = 1 / (1/R_1 + 1/R_2 + 1/R_3)$.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Multiple Choice Questions

Questions on circuit components, Ohm's law, and resistance units.

Questions

Numerical Problems

Solving resistor networks in series and parallel.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Numerical

Calculate the current if a charge of 300 C flows through a conductor in 2 minutes.

▶ Show solution

Q = 300 C, t = 2 min = 120 s. I = Q / t = 300 / 120 = 2.5 A.

Q2MEDIUM· Numerical

Two resistors of 6 ohms and 3 ohms are connected in parallel. Find the equivalent resistance.

▶ Show solution

1/Rp = 1/6 + 1/3 = (1 + 2)/6 = 3/6 = 1/2. Rp = 2 ohms.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Current is charge flow rate ($I = Q / t$).
•Ohm's law: $V = IR$.
•Series resistors: $R_s = R_1 + R_2$. Parallel: $1/R_p = 1/R_1 + 1/R_2$.
•Heating effect: $H = I^2Rt$ (Joule's law of heating).

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 5-7 marks in assessments

Question type	Marks each	Typical count	What it tests
MCQ	1	1-2	Base concepts and definitions
Short Answer	2-3	1-2	Descriptive and application points

Prep strategy

Understand core definitions and solve standard textbook problems.
Review common mistakes to avoid losing easy marks.

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Household Wiring

Appliances are connected in parallel so they operate at the same voltage and can be switched independently.

Electric Fuse

Uses the heating effect of current to break circuits and protect appliances during overloading.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Write definitions precisely as defined in the textbook.
Draw neat, labeled diagrams for biology and physics chapters.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Read advanced reference materials to explore concepts beyond the school syllabus.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

Class 9 Annual ExamsHigh

NTSE Stage 1Medium

AI helpers for this chapter

Generate more practice on the fly, or have the chapter explained at a different level.

⚡

Generate 5 more questions

Fresh MCQs + short-answer items based on this chapter, plus answer keys.

🧠

Explain at a different level

Re-explain the chapter at the level you actually want.

📝 My notes

Saved on this device — sign in to sync · how this works

0/600

Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

The work done in moving a unit positive charge from one point to another in an electric field ($V = W / Q$). Unit: volt (V).

Copper is a good conductor with low electrical resistance, reducing energy loss as heat.

Practice more, ask doubts, find a tutor

Tamil Nadu (TNBSE) Class 9 mock tests

Tamil Nadu (TNBSE) Class 9 PYQs

Ask in Q&A

Science flashcards

Related chapters

Students who read Electric Charge and Electric Current usually read these next.

Measurement

Samacheer Kalvi (TN State Board) Class 9 Science — Physics Chapter 1, Measurement. Measurement is the foundation of all physical sciences. It teaches how to quantify observations using standard units and precise instruments like vernier calipers and screw gauges. Notes, worked examples, common mistakes, and quick revision.

Motion

Samacheer Kalvi (TN State Board) Class 9 Science — Physics Chapter 2, Motion. Motion describes how objects change position over time. Understanding speed, velocity, acceleration, and equations of motion is essential for engineering, space exploration, and everyday transport. Notes, worked examples, common mistakes, and quick revision.

Fluids

Samacheer Kalvi (TN State Board) Class 9 Science — Physics Chapter 3, Fluids. Fluids (liquids and gases) exert pressure and show unique properties like buoyancy. Understanding pressure, density, and buoyancy explains how ships float, hydraulic brakes work, and hot air balloons fly. Notes, worked examples, common mistakes, and quick revision.

Magnetism and Electromagnetism

Samacheer Kalvi (TN State Board) Class 9 Science — Physics Chapter 5, Magnetism and Electromagnetism. Magnetism and electricity are deeply linked. Electromagnetism explains how motors convert electricity into mechanical power, how generators produce electrical energy, and how transformers work. Notes, worked examples, common mistakes, and quick revision.