Magnetism ICSE Class 10 — Selina Chapter 10

Magnetism ICSE Class 10 in Selina Concise Physics Chapter 10, Electro Magnetism, explains how electric current creates magnetic fields and how those fields move conductors and induce currents. You will study Oersted’s experiment, field-line rules, the d.c. motor, electromagnetic induction, the a.c. generator, and the transformer. This page walks through each section of the chapter in book order, with formulas and worked examples. It is part of the Class 10 ICSE Physics book.

Download Electro Magnetism Class 10 ICSE Physics Chapter 10 PDF

Download the Electro Magnetism Class 10 ICSE Physics chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 31 pages, 16 sections, 95 questions.

What is inside Chapter 10

31 pages · 16 sections · 3 worked examples · 95 exercise questions

• EXERCISE-10(A): 31 questions
• MULTIPLE CHOICE TYPE: 2 questions
• EXERCISE-10(B): 13 questions
• MULTIPLE CHOICE TYPE: 1 questions
• EXERCISE-10(C): 42 questions
• MULTIPLE CHOICE TYPE: 6 questions

10.1 OERSTED’S EXPERIMENT ON THE MAGNETIC EFFECT OF ELECTRIC CURRENT

Oersted’s experiment, first performed in 1820, showed that an electric current produces a magnetic field around a wire. A compass needle detects this field. With no current, the needle rests along the earth’s north-south direction. With current, the needle deflects to align with the field at that point.

Four observations from the experiment are listed below. The wire AB lies in the north-south direction, and current from A to B means the current flows from south to north.

The inference is central to this magnetism ICSE Class 10 chapter: a current, or any moving charge, produces a magnetic field around it. Field strength depends on the current’s magnitude. Field direction depends on the current’s direction.

10.2 MAGNETIC FIELD AND FIELD LINES DUE TO CURRENT IN A STRAIGHT WIRE

Iron filings on a cardboard sheet around a vertical wire arrange in concentric circles when current flows. Each circle lies perpendicular to the wire, with its centre on the wire. A compass needle at any point shows the field direction along the tangent to the circle.

Three observations complete the picture. Reversing the current reverses the field direction but not the circular pattern. Increasing the current makes the lines denser and extends them farther from the wire. Far from the wire, the earth’s field dominates and the filings settle into straight lines.

The neutral point is where the wire’s field and the earth’s field are equal and opposite. The resultant field there is zero. A compass needle placed at the neutral point can rest in any direction.

10.3 RULE TO FIND THE DIRECTION OF MAGNETIC FIELD

The right hand thumb rule gives the direction of the magnetic field around a straight conductor. Hold the wire in your right hand with your thumb pointing along the current. Your curled fingers then point along the magnetic field lines.

Worked example: a vertical wire carries current upward. Grip it with your thumb pointing up. Viewed from above, the fingers curl anticlockwise, so the field lines circle the wire anticlockwise.

Use this rule only for field direction. Fleming’s left hand rule, covered in section 10.11, finds the direction of force. Using the wrong hand is a common mistake in this chapter.

10.4 MAGNETIC FIELD DUE TO CURRENT IN A LOOP (OR CIRCULAR COIL)

A current loop produces field lines that are nearly circular near the wire and nearly parallel near the centre. The field near the centre is almost uniform. The loop behaves like a magnetised disc, so it acts as a magnetic dipole.

Polarity at each face follows the clock rule. Looking at a face, if the current around it is anticlockwise, that face is a north pole. If the current is clockwise, the face is a south pole.

Mnemonic: anticlockwise and north both come first — A before C and N before S in the alphabet. So anticlockwise gives north, and clockwise gives the opposite pole, south. Reversing the current reverses both polarities.

Worked example: view a coil from the front. Current flows clockwise around that face, so the front face is a south pole.

10.5 MAGNETIC FIELD DUE TO A CURRENT CARYING CYLINDRICAL COIL (OR SOLENOID)

A solenoid is a cylindrical coil of insulated wire, with diameter less than its length. Its field lines inside are nearly straight and parallel, so the field inside is uniform. The field strengthens with more current, more turns per unit length, or a soft iron core along the axis. Soft iron has high magnetic permeability, which boosts the field.

A current-carrying solenoid resembles a bar magnet in three ways. Its field lines look the same. Suspended freely, it sets in the north-south direction. It attracts iron filings.

It differs in two ways. The solenoid’s strength changes with current, while a bar magnet’s cannot. Reversing current reverses the solenoid’s polarity, while a bar magnet’s poles stay fixed.

10.6 ELECTROMAGNET

An electromagnet is a temporary strong magnet made by passing current through a coil wound around soft iron. Two shapes are common: the I-shaped or bar electromagnet, and the U-shaped or horse-shoe electromagnet. Both need a d.c. source.

The I-shaped form winds a solenoid on a straight soft iron bar. The U-shaped form winds the two arms in opposite senses, so the two ends become opposite poles. This gives a strong field in the gap between the poles.

Two methods increase the field: more turns of winding, or more current through the coil. With a 50 Hz a.c. source, the core still magnetises, but its polarity reverses fifty times each second.

10.7 PERMANENT MAGNET

A permanent magnet is made from steel. A coil wound around the steel carries current until the steel is magnetised. Steel has high retentivity, so it keeps its magnetism after the current stops. That is why steel is used instead of soft iron.

Permanent magnets are used in electric meters such as galvanometers, ammeters, and voltmeters. They also appear in magnetic compasses.

10.8 COMPARISON OF AN ELECTRO MAGNET WITH A PERMANENT MAGNET

Read the table row by row for board questions. The material, the permanence of the field, and the ease of demagnetisation are the three differences most often asked.

10.9 ADVANTAGES OF AN ELECTRO-MAGNET OVER A PERMANENT MAGNET

An electromagnet has three advantages over a permanent magnet. It can produce a strong magnetic field. Its strength can be changed by adjusting the current or the number of turns. Its polarity can be reversed by reversing the current.

Each advantage appears in a real device. Lifting heavy iron needs the strong field. The electric bell relies on the field switching on and off. Reversing the current reverses polarity, which suits devices that need a changeable field direction.

10.10 USES OF ELECTROMAGNET

Electromagnets are used to lift and transport heavy iron scrap, girders, and plates — up to 20,000 kg in a single lift. They load furnaces with iron, separate iron from ores, and remove iron pieces from wounds. They also appear in the electric bell, telegraph, tram, motor, relay, microphone, and loudspeaker.

Electric bell working: pressing the switch magnetises the core, which attracts the soft iron armature. The hammer strikes the gong. The armature’s motion breaks the contact at the adjusting screw, so current stops and the core demagnetises. The spring pulls the armature back, remaking the contact. This make-and-break cycle continues while the switch is held.

With an a.c. source the bell still rings. The core magnetises, but its polarity changes. Attraction of the armature does not depend on polarity, so the hammer keeps striking the gong.

10.11 FORCE ON A CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD

The force on a current-carrying conductor in a magnetic field is F = IBl. Here I is the current, B the magnetic field strength, and l the length of wire inside the field. The force acts perpendicular to both the current and the field.

The SI unit of B follows from B = F/(Il), giving \text{N A}^{-1}\text{m}^{-1}. This unit is called the tesla (T).

Fleming’s left hand rule gives the force direction. Stretch forefinger, central finger, and thumb mutually perpendicular. Forefinger points along the field, central finger along the current, and the thumb shows the direction of motion, which is the force.

Worked example: I = 2\ \text{A}, B = 0.5\ \text{T}, l = 0.3\ \text{m}. The force is F = IBl = 2 \times 0.5 \times 0.3 = 0.3\ \text{N}.

If the current runs parallel to the field, the force is zero. The force is maximum when current and field are perpendicular.

10.12 SIMPLE D.C. MOTOR

An electric motor converts electrical energy into mechanical energy. A d.c. motor works because a current-carrying conductor in a magnetic field experiences a force. Fleming’s left hand rule gives the direction of that force.

Main parts: an armature coil ABCD on an axle, a split ring commutator with two parts S_1 and S_2, two carbon brushes B_1 and B_2, a horse-shoe electromagnet, and a battery. The coil sits between the magnet’s poles.

Current in arms AB and CD produces equal and opposite forces that form a couple, rotating the coil. When the coil passes the vertical position, the split ring swaps the contacts. Current through the coil reverses, so the couple keeps the coil turning the same way. Without the split ring, the coil would stop after a half turn.

The deflecting couple is BAnI. Here B is the field, A the coil area, n the number of turns, and I the current. Doubling the current doubles the couple and therefore the speed.

Speed increases four ways: more current, more turns, larger area, or a stronger magnetic field. A soft iron core inside the coil makes the field strong and radial. The d.c. motor is a frequent topic in Class 10 ICSE Physics.

10.13 ELECTROMAGNETIC INDUCTION

Electromagnetic induction is the phenomenon in which an e.m.f. is induced in a coil when the magnetic flux linked with the coil changes. Faraday demonstrated it with a magnet and a solenoid connected to a centre-zero galvanometer.

Observations: a stationary magnet gives no deflection. Moving the magnet towards the coil deflects the needle one way. Stopping the motion brings the needle back to zero. Moving the magnet away deflects it the opposite way. Faster motion or a stronger magnet gives a larger deflection.

Only relative motion matters. Moving the coil instead of the magnet gives the same result. During induction, mechanical energy spent in moving the magnet is transformed into electrical energy in the coil.

10.14 FARADAY’S LAWS OF ELECTROMAGNETIC INDUCTION

Faraday’s first law states that an e.m.f. is induced whenever the magnetic flux linked with a coil changes, and it lasts only while the flux keeps changing. The second law states that the magnitude of the induced e.m.f. is proportional to the rate of change of flux.

Induced e.m.f. equals the rate of change of flux per turn multiplied by the number of turns:

e = \frac{N \times \Delta\phi}{t}

Worked example: a coil has 100 turns. The flux through each turn changes by 0.4 Wb in 2 s. Then e = 100 \times 0.4 / 2 = 20\ \text{V}.

Fleming’s right hand rule gives the direction of induced current. Stretch thumb, forefinger, and central finger of the right hand mutually perpendicular. Forefinger points along the field, thumb along the motion, and the central finger along the induced current.

Lenz’s law states that the induced current opposes the change that produces it. When a north pole approaches a solenoid, the near end becomes a north pole to repel it. This follows from conservation of energy: mechanical work against the opposing force becomes the electrical energy of the induced current.

An a.c. generator converts mechanical energy into electrical energy using this same principle. A coil rotates in a magnetic field, so the flux through it changes continuously. The induced e.m.f. is alternating, reversing direction after each half rotation.

Check which hand to use: the left hand gives force on a conductor, the right hand gives induced current. If the question asks for force, use the left. If it asks for induced current, use the right. Mixing them up is a common error.

10.17 DISTINCTION BETWEEN AN A.C. GENERATOR AND D.C. MOTOR

Two similarities tie them together. Both have a coil rotating in a magnetic field between the poles of a powerful electromagnet. Both involve a transformation of energy from one form to another. This comparison is a frequent board question in the Class 10 ICSE syllabus.

10.18 TRANSFORMER

A transformer changes the magnitude of an alternating e.m.f. without changing its frequency. It has a primary coil, where input is applied, and a secondary coil, where output is taken. Both coils are wound on a laminated soft iron core. Transformers supply appliances that need voltages different from the 220 V mains, which the household circuits chapter explains.

The turns ratio is n = N_s / N_p. For an ideal transformer, E_s / E_p = N_s / N_p = n, and output power equals input power, so E_s I_s = E_p I_p.

A step-up transformer has N_s \gt N_p, so n \gt 1, E_s \gt E_p, and I_s \lt I_p. A step-down transformer has N_s \lt N_p, so n \lt 1, E_s \lt E_p, and I_s \gt I_p. The coil carrying the larger current uses thicker wire to reduce copper loss.

A transformer cannot work on d.c. A constant primary current produces a constant flux, so no e.m.f. is induced in the secondary. The laminated core reduces eddy-current loss.

Worked example: a transformer steps 220 V down to 11 V. The primary has 1000 turns. Then n = 11/220 = 1/20, so N_s = 1000/20 = 50 turns. A 22 W device on the secondary draws I_s = 22/11 = 2\ \text{A}. The primary draws I_p = 22/220 = 0.1\ \text{A}. The secondary wire must be thicker because it carries the larger current.

What to practise from Magnetism ICSE Class 10

The exercise blocks in this ICSE Class 10 chapter test the sections in order. Work through them after you have understood each section’s rule and worked example. For the current syllabus and exam updates, see the official CISCE website.

• Exercise-10(A) — 31 questions on the magnetic effect of current (sections 10.1 to 10.10)
• Multiple Choice Type — 2 questions
• Exercise-10(B) — 13 questions on force and the d.c. motor (sections 10.11 to 10.12)
• Multiple Choice Type — 1 question
• Exercise-10(C) — 42 questions on induction, the generator, and the transformer (sections 10.13 to 10.18)
• Multiple Choice Type — 6 questions

Magnetism ICSE Class 10 frequently asked questions

What is the right hand thumb rule in magnetism ICSE Class 10?

Hold the conductor in your right hand with the thumb pointing along the current. Your curled fingers then show the direction of the magnetic field lines around the wire. For current flowing upward in a vertical wire, the field circles the wire anticlockwise when viewed from above.

Why does a d.c. motor need a split ring commutator?

After each half rotation, the arms of the coil swap positions, which would reverse the direction of the couple. The split ring reverses the current through the coil at that moment, so the couple keeps acting in the same direction. Without it, the coil would stop turning after half a rotation.

How do I calculate the turns ratio in a transformer?

The turns ratio is n = N_s / N_p, the number of turns in the secondary divided by the primary. For an ideal transformer this equals E_s / E_p. A ratio greater than 1 gives a step-up transformer; a ratio less than 1 gives a step-down transformer.

What is the difference between Fleming’s left hand rule and right hand rule?

The left hand rule finds the force on a current-carrying conductor in a magnetic field, as in a motor. The right hand rule finds the direction of induced current in a moving conductor, as in a generator. If the question asks for force, use the left; if it asks for induced current, use the right.

Why can’t a transformer work on a d.c. source?

A constant d.c. current in the primary produces a constant magnetic flux. Induction needs a changing flux, so no e.m.f. appears in the secondary. A transformer works only with alternating current.

Reference: ICSE Class 10 Physics textbook material, chapter 10: Electro Magnetism — CISCE-aligned.

More from this book

• Chapter 1: Force
• Chapter 2: Work Energy and Power
• Chapter 3: Machines
• Chapter 4: Refraction of Light At Plane Surfaces
• Chapter 5: Refraction Through a Lens
• Chapter 6: Spectrum
• Chapter 7: Sound
• Chapter 8: Current Electricity
• Chapter 9: Household Circuits
• Chapter 11: Calorimetry
• Chapter 12: Radioactivity