Introduction
HSA Notes brings complete, exam-focused solved notes to help Class 11 (1st Year, HSSC-I) students score high marks in their FBISE Physics board exam.
This page covers Unit 12, Electromagnetism, prepared strictly according to the FBISE New Book (NBF) syllabus, Cantab publisher, for the current academic session, covering magnetic fields, force on conductors, electromagnetic induction, and Faraday’s and Lenz’s laws, along with fully solved examples, derivations, short and long questions, numericals, and assignments from HSA Notes.
Electromagnetism builds directly on the current and resistance concepts from Unit 11, introducing the magnetic effects of moving charges. This is a formula-dense unit requiring careful use of multiple right-hand and left-hand rules alongside numerical formulas.
Magnetic Fields and Force on a Conductor
A magnetic field is the region around a magnet or current-carrying conductor within which magnetic forces act; it is a vector quantity.
Field lines run from North to South outside a magnet, form closed loops, never intersect, and their density shows field strength.
Magnetic poles always exist in pairs; no monopole has ever been observed. Oersted’s experiment showed that current deflects a compass needle, proving current produces a magnetic field. The Right-Hand Rule gives field direction: thumb along current, fingers curl in the field’s direction.
A current-carrying conductor in an external field experiences a force:
F = BIL sin θ
This is maximum (F = BIL) when θ = 90°, and zero when θ = 0° or 180°. Fleming’s Left-Hand Rule (forefinger = field, middle finger = current, thumb = force) gives its direction.
Magnetic Induction, Moving Charges, and Circular Motion
Magnetic flux density (B) is the force per unit length on unit current placed perpendicular to the field:
B = F / (IL), measured in Tesla (T), where 1 T = 1 N/(A·m); 1 Gauss = 10⁻⁴ T.
A moving charge experiences a force F = qvB sin θ, directed by the right-hand rule for positive charges. If velocity is not perpendicular to B, the path becomes helical. When perpendicular, the charge moves in a circle of radius:
r = mv / (qB)
A velocity selector uses perpendicular E and B fields so only particles with v = E/B pass undeflected.
Magnetic Field Patterns and Forces Between Conductors
Standard field formulas: a straight conductor, B = μ₀I/(2πr); a circular coil of N turns, B = μ₀NI/(2R); and a solenoid, B = μ₀nI, where n is turns per unit length. Polarity in each case follows the right-hand rule. A ferrous core inside a solenoid enhances the field to B = μ₀μᵣnI, where μᵣ is relative permeability.
Two parallel current-carrying conductors attract when currents flow in the same direction and repel when opposite — the basis of the formal definition of the Ampere.
Magnetic Flux, Faraday’s Law, and Lenz’s Law
Magnetic flux: Φ = BA cos θ, measured in Weber (Wb); maximum when perpendicular, zero when parallel to the surface. For a coil of N turns, the flux linkage is Λ = NΦ.
Faraday’s Law states the induced emf equals the negative rate of change of flux linkage:
ε = −dΦ/dt
Its magnitude depends on the rate of flux change, number of turns, and coil area.
Lenz’s Law states the induced current always opposes the change producing it, consistent with energy conservation. Fleming’s Right-Hand Rule gives the direction of induced current.
A seismometer applies this principle, detecting ground motion through the emf induced by relative motion between a magnet and coil.
Exercises Solved
Short Question: Reversing the magnetic poles reverses the direction of a current-carrying rod’s motion, since Fleming’s Left-Hand Rule gives an opposite force direction.
Long Question: The magnetic force does no work on a moving charge, since it acts perpendicular to velocity, giving W = F·d = 0.
Assignment 12.1: Draw current direction and explain compass deflection using the right-hand rule.
Assignment 12.3: Electron orbit radius, r ≈ 0.0948 m.
Numerical 6: Force on a current-carrying wire, F ≈ 12.99 N.
Numerical 7: Force on a moving charged particle, F ≈ 0.866 N.
Numerical 8: Magnetic flux calculation using Φ = BA cos θ.
Full solved MCQs, short and long questions, numericals, and assignments are available in the complete PDF.
HSA Notes Features
- Solved short and long questions with clear, detailed answers
- Solved numericals with full working and correct units
- Solved assignments matching the current session’s syllabus
- Solved derivations for every major formula
- Full Unit 12 Electromagnetism notes for Class 11, based on the FBISE NBF course (Cantab publisher), at hsanotes.com
Why This Unit Matters
Electromagnetism ties together current and resistance from Unit 11 with the vector and force concepts introduced earlier in the course.
Its various right-hand and left-hand rules are frequently confused, making repeated practice essential. Faraday’s and Lenz’s laws explain the working principle behind generators, transformers, and devices like the seismometer, concepts that carry directly into real-world engineering applications.
Available Across All FBISE Institutions
These notes are built for students across all FBISE-affiliated institutions, including APS, IMCB Islamabad, F.G. Colleges, Islamabad Model Colleges, Punjab Colleges, Dr. A.Q. Khan Colleges, Fazaia Inter Colleges, IMCG, and Sir Syed Colleges.
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Frequently Asked Questions
What is the difference between magnetic flux density and magnetic flux?
Flux density (B) is force per unit length per unit current at a point, measured in Tesla; flux (Φ) is the total field through an area, Φ = BA cos θ, measured in Weber.
What does Faraday’s Law state?
Induced emf equals the negative rate of change of flux linkage, ε = −dΦ/dt.
What does Lenz’s Law state?
The induced current always opposes the change in flux that produced it.
Why does the magnetic force do no work on a moving charge?
Because it always acts perpendicular to velocity, so W = F·d = 0.
Conclusion
With Unit 12, Electromagnetism, fully covered, Class 11 FBISE students now have a clear understanding of magnetic fields, forces on conductors and charges, flux, and electromagnetic induction. Practice these solved exercises alongside past papers, and consider joining our online classes for guided, exam-focused preparation.
Best of luck for your Class 11 Physics exam!