Current Electricity Class 9 ICSE — Physics Chapter 9

Current electricity class 9 icse is the Physics chapter that explains how charge moves through a conductor. In this chapter you will study the sources of direct current, the meaning of electric current, the symbols used in circuit diagrams, potential difference and resistance. Every formula comes with its S.I. unit so you can apply it directly. The scope of the Class 9 Physics syllabus is set by CISCE, and you can confirm it on the official CISCE website.

Download Current Electricity Class 9 ICSE Physics Chapter 9 PDF

Download the Current Electricity Class 9 ICSE Physics chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 14 pages, 10 sections, 53 questions.

What is inside Chapter 9

14 pages · 10 sections · 1 worked examples · 53 exercise questions

• EXERCISE 9(A): 22 questions
• Multiple choice type :: 6 questions
• EXERCISE 9 (B): 9 questions
• Multiple choice type :: 7 questions
• EXERCISE 9(C): 7 questions
• Multiple choice type :: 2 questions

9.1 SOURCES OF DIRECT CURRENT

Direct current (d.c.) is a current of constant magnitude flowing in one direction. A torch cell supplies d.c. to the bulb. The current flows from the positive terminal to the negative terminal through the external circuit. The magnitude stays constant for a long time. When the cell discharges, it stops giving current and becomes useless.

A cell changes chemical energy into electrical energy when it sends current. It has two conducting rods called electrodes. The electrodes sit apart in a solution or jelly called the electrolyte, kept in a vessel. Cells are of two kinds: primary cells and secondary cells.

The reaction type explains why only secondary cells recharge. A primary cell is use-and-throw because its reaction cannot be reversed. A lead accumulator is used as a car battery because it can give high current.

9.2 ELECTRIC CURRENT (AS RATE OF FLOW OF ELECTRIC CHARGE)

Current is the rate of flow of charge across a cross-section normal to the direction of flow. If charge Q flows through the cross-section in time t, then

I = \frac{Q}{t} \tag{9.1}

Current is a scalar quantity. In metals, it flows due to electrons. In electrolytes, both positive and negative ions carry it. If n electrons pass in time t, the total charge is Q = ne, so

I = \frac{Q}{t} = \frac{ne}{t} \tag{9.2}

The S.I. unit of current is the ampere. One ampere is one coulomb per second:

1\ \text{A} = \frac{1\ \text{C}}{1\ \text{s}} \tag{9.3}

One coulomb is carried by 6.25 \times 10^{18} electrons. So 1 A means that many electrons cross per second. Weak currents are measured in milliampere (10^{-3}\,\text{A}) and microampere (10^{-6}\,\text{A}). In an electrolyte where both ions move, the total current is I = n_1 q_1 + n_2 q_2.

This worked pattern is the same one used across this ICSE chapter for numericals. You can explore the other chapters in the Class 9 Physics book for more such problems.

9.3 SYMBOLS USED IN CIRCUIT DIAGRAMS

A circuit diagram uses standard symbols so every component is recognised at a glance. A d.c. cell is two vertical lines of unequal length; the longer line is the positive anode and the shorter is the cathode. A battery joins several cells in series. An a.c. source is a sine curve inside a circle. A key puts current on and off, and is shown open or closed.

A rheostat has three terminals A, B and C. The ends A and B give a fixed resistance equal to the full wire. Using terminal C with the sliding jockey gives a variable resistance. A resistance box provides set values in steps of one ohm.

Three instruments deserve special care because their connections differ:

A galvanometer does not measure magnitude. Its needle deflects both ways from a central zero to show the direction of current. A load is the appliance, such as a bulb or heater, and connecting wires are made of copper or aluminium so their resistance is negligible.

9.4 SIMPLE ELECTRIC CIRCUIT

The simplest circuit uses three components: a cell, a bulb and a switch. When the switch closes, current flows and the bulb glows. Opening the switch stops the flow and the bulb goes off.

The chapter also builds a five-component circuit with a cell, key, rheostat, bulb, ammeter and voltmeter. The ammeter sits in series with the bulb. The voltmeter connects in parallel across the bulb. When the key is open, the ammeter and voltmeter read zero. When it closes, the bulb glows and both instruments show readings.

The rheostat changes the circuit. Sliding the jockey left decreases resistance, so the bulb glows brighter and both readings rise. Sliding right increases resistance, so the glow dims and the readings fall.

This simple circuit is one topic in the wider set of Class 9 Physics chapters.

Conductors allow current easily because they have many free electrons. Metals, the human body, acidulated water and mercury are conductors. Insulators have almost no free electrons and offer very high resistance. Cotton, rubber, plastic, wood, glass and pure water are insulators.

9.6 CLOSED AND OPEN CIRCUITS

Current flows only when the circuit is complete, or closed. Every component must pass current through it. If any part is broken, or an insulator sits in the path, the circuit is open and current stops.

The chapter shows three cases with a dry cell and a bulb. With metal wires joining both terminals, the circuit is complete and the bulb glows. With one wire broken, the circuit is incomplete and the bulb stays off. With a thread, an insulator, replacing one wire, the bulb again stays off.

What carries the current differs by material. In a metallic conductor, free electrons carry the current. In an electrolyte, both positive and negative ions carry it. This is why a circuit needs conducting material throughout — one insulator anywhere breaks the path.

9.7 FLOW OF ELECTRONS BETWEEN THE CONDUCTORS

Join two conductors with a metal wire and electrons flow from the one with a higher concentration of electrons to the one with a lower concentration. The movement stops when both have equal concentration.

Three cases show the rule. A positively charged conductor joined to an uncharged one pulls electrons from the uncharged body. A negatively charged conductor pushes electrons into an uncharged body. Two charged conductors, one negative and one positive, send electrons from the negative to the positive until balance is reached.

The inference matters for circuits. Flow lasts only while one conductor keeps an excess and the other a deficit. An electric cell maintains exactly this imbalance by chemical reaction, so electrons keep flowing in the external circuit from cathode to anode.

9.8 DIRECTION OF THE ELECTRIC CURRENT — CONVENTIONAL AND ELECTRONIC FLOW

The direction of flow follows a pattern seen across physics. A falling body moves from a higher level to a lower one. Water flows from a higher level to a lower level between joined vessels. Heat flows from a hotter body to a cooler one. Electric current follows the same habit: it flows from a higher potential to a lower potential.

A conductor with an excess of electrons is at a negative, or lower, potential. One with a deficit is at a positive, or higher, potential. Electrons therefore flow from low potential to high potential. Conventional current flows from high potential to low potential — opposite to the electrons.

To remember this: conventional current takes the high road, electrons take the low road.

9.9 ELEMENTARY IDEA ABOUT WORK DONE IN TRANSFERRING CHARGE THROUGH A CONDUCTING WIRE; POTENTIAL DIFFERENCE (V = W/Q)

Moving charges against the forces between them needs work. Potential at a point is the work done in bringing a unit charge from infinity to that point. If work W' brings charge q, then

V = \frac{W'}{q} \tag{9.4}

Potential difference between two points is the work done in transferring a unit positive charge from one to the other. If work W transfers charge q, then

V_1 - V_2 = \frac{W}{q} \tag{9.5}

Both potential and potential difference are scalar quantities. The S.I. unit is joule per coulomb, named the volt. One volt is one joule per coulomb: 1\ \text{V} = \frac{1\ \text{J}}{1\ \text{C}}. A potential difference of 1 volt means 1 joule of work moves 1 coulomb of charge.

9.10 ELECTRICAL RESISTANCE

Resistance is the obstruction a wire offers to the flow of current. Its cause is collision. Free electrons in a wire drift toward the positive terminal, but they keep colliding with the fixed positive ions and other free electrons. Each collision slows them and changes their direction, so they never speed up without limit. The wire therefore resists the flow.

Ohm’s law connects the three quantities. If a potential difference V sends current I through a wire, the resistance is

R = \frac{V}{I} \quad \text{or} \quad V = IR \tag{9.6}

The S.I. unit is volt per ampere, named the ohm: 1\ \Omega = \frac{1\ \text{V}}{1\ \text{A}}. Resistance is a scalar quantity.

Four factors change the resistance of a wire:

• Material: more free electrons means less resistance.
• Length: resistance is directly proportional to length.
• Area of cross-section: resistance is inversely proportional to area.
• Temperature: hotter ions vibrate more, so resistance rises.

9.11 EFFICIENT USE OF ENERGY

Efficient use of energy means reducing the cost and amount of energy needed to provide the same products and services. It comes from adopting more efficient, eco-friendly technologies. The result is lower energy cost and lower greenhouse gas emissions.

The chapter lists practical ways to save energy:

• Insulate homes to cut heating and cooling cost.
• Use fluorescent lights or natural skylight instead of incandescent bulbs.
• Use CFLs, which save 67% energy and last 6 to 10 times longer.
• Use LED bulbs, which consume far less and avoid mercury.
• Choose star-rated appliances that use less electricity.
• Place windows and white roofs to reduce artificial lighting and cooling.
• Use advanced boilers and lighter vehicles with efficient engines.

The International Energy Agency projects that improved efficiency could cut the world’s energy need by one-third by 2050. Social initiatives also matter. Awareness campaigns, eco-club activities in schools, and involvement of non-government organisations help reduce the misuse of electricity.

What to practise from Current Electricity Class 9 ICSE

Work through the exercise blocks in order. Each one tests the sections that come just before it.

• EXERCISE 9(A): 22 questions plus 6 multiple choice questions. It tests sections 9.1 to 9.6.
• EXERCISE 9(B): 9 questions plus 7 multiple choice questions. It tests sections 9.7 to 9.10.
• EXERCISE 9(C): 7 questions plus 2 multiple choice questions. It tests section 9.11.

Watch for these common errors when you practise:

• Connecting an ammeter in parallel instead of series.
• Connecting a voltmeter in series instead of parallel.
• Marking conventional current in the direction electrons flow.
• Mixing milliampere and ampere values in a calculation.

The full list of Class 9 books, including this Physics volume, is available in the Class 9 books collection.

Current Electricity Class 9 ICSE frequently asked questions

What is the difference between conventional current and electronic current in current electricity class 9 icse?

Conventional current flows from a higher potential to a lower potential. Electronic current is the flow of electrons, which moves from a lower potential to a higher potential. The two directions are opposite. A conductor with an excess of electrons is at lower potential, while one with a deficit is at higher potential.

Why is an ammeter connected in series while a voltmeter is connected in parallel in Class 9 ICSE circuits?

An ammeter measures the current flowing through a circuit, so the whole current must pass through it. That is why it is joined in series and has very low resistance. A voltmeter measures the potential difference between two points, so it is connected in parallel and has very high resistance to avoid drawing appreciable current.

How many electrons flow through a conductor when 1 ampere of current flows for 1 second in Class 9 ICSE Physics?

One ampere means one coulomb of charge flows per second. One coulomb is carried by 6.25 \times 10^{18} electrons. So that many electrons pass across the cross-section in one second.

Why does the resistance of a wire increase when its temperature rises in Class 9 ICSE Physics?

When temperature rises, the fixed positive ions in the wire vibrate more violently. The moving electrons collide with them more often. More collisions mean more obstruction, so the resistance of the wire increases.

What is the difference between a primary cell and a secondary cell in Class 9 ICSE Physics?

A primary cell works by an irreversible chemical reaction and cannot be recharged. A secondary cell works by a reversible reaction and can be recharged again and again. Secondary cells have low internal resistance and can give high current, which is why a lead accumulator serves as a car battery.

Reference: ICSE Class 9 Physics textbook material, chapter 9: Current Electricity — CISCE-aligned.

More from this book

• Chapter 1: Measurements and Experimentation
• Chapter 2: Motion in One Dimension
• Chapter 3: Laws of Motion
• Chapter 4: Pressure in Fluids and Atmospheric Pressure
• Chapter 5: Upthrust in Fluids Archimedes Principle and Floatation
• Chapter 6: Heat and Energy
• Chapter 7: Reflection of Light
• Chapter 8: Propagation of Sound Waves
• Chapter 10: Magnetism