Magnetism & Electromagnetism
Learn how magnets and currents create magnetic fields, how changing magnetic flux induces emf, and how these ideas power motors, generators, transformers, trains, and more. Use the interactive simulator below to explore the magnetic field around a bar magnet with compass needles, vector fields, and iron-filings view. Use Launch to start; then Play / Pause and Reset to explore.
- Magnetic field — region where a magnet or current exerts force
- Field lines — N → S, closer where field is stronger
- Fleming’s rules — directions for force and induced current
- Electromagnetic induction — emf from changing magnetic flux
- Motors & generators — electrical ↔ mechanical energy
Magnetic Field (Bar Magnet)
Direction N → S · Stronger near poles · Weaker with distance.
Field is strongest near poles.
Parameters
Key formulas
Real-world applications
MRI machines
Magnetic Resonance Imaging uses strong, highly uniform magnetic fields and changing radio-frequency fields to image the body.
Key insight: Precise magnetic fields and induction in nuclei are central to MRI.
Electric trains
Powerful electric motors drive trains; during braking, the same motors can act as generators (regenerative braking).
Key insight: Motor and generator are inverse processes based on the same principles.
Transformers
Transformers use changing magnetic flux in iron cores to step up or step down AC voltages efficiently.
Key insight: Electromagnetic induction in coupled coils; EMF ∝ N·dΦ/dt.
Loudspeakers
A current in a voice coil in a magnetic field produces a force on the coil, moving the speaker cone and creating sound.
Key insight: Force on current-carrying conductor (F = BIL) produces mechanical vibration.
Induction cooktops
Rapidly changing magnetic fields induce eddy currents in metal cookware, heating it directly.
Key insight: Changing magnetic flux induces currents that dissipate energy as heat.
Wind turbines
Turbines spin generator rotors in magnetic fields to produce electricity from wind energy.
Key insight: Mechanical rotation → changing flux → AC generation.
Common misconceptions & tips
Magnetic field lines are not physical wires or strings; they are imaginary curves we draw to represent the direction and strength of the field. The field exists everywhere in space, even where we do not draw lines, and lines are closer where the field is stronger.
📘 Thinking of field lines as objects can lead to confusion. They are a visual tool; only the underlying field and its effects on magnets and currents are physical.
🔢 Field strength ∝ density of field lines
🧪 In the field-line simulators, change magnet strength and see how line density changes.
In circuits, electrons drift from negative to positive, but we define conventional current direction from positive to negative. All right-hand/left-hand rules and formulas are based on conventional current; it is just a consistent sign convention.
📘 Mixing electron flow with conventional current can flip directions incorrectly. Use conventional current for rules and diagrams.
🔢 Use I (conventional current) for F = B I L and right/left-hand rules.
🧪 In the conductor and motor simulators, arrows show conventional current direction.
For a bar magnet, the field is strongest near the poles and weakens rapidly with distance. Field lines spread out as you move away. Only between special pole shapes can we approximate a small region as uniform.
📘 Assuming uniform field everywhere gives wrong forces and torques. Real fields vary in magnitude and direction.
🔢 Approx. near a straight conductor: B ∝ I / r
🧪 Move the probe in the bar magnet and current simulators to see how |B| changes with r.
An electric generator converts mechanical energy (from moving water, wind, steam turbines, etc.) into electrical energy. The induced emf and current always come from changing magnetic flux due to mechanical work.
📘 Energy is conserved: generators transform energy from one form to another, they do not create energy.
🔢 Electrical power out ≈ mechanical power in (minus losses)
🧪 In the generator simulator, increasing rotation speed (more mechanical power) increases induced emf and electrical output.
Chapter Guide
How to Study This Chapter
- Start with magnetic fields and field lines around bar magnets
- Build: fields due to currents and Fleming’s rules
- Explore electromagnetic induction and EMF vs time
- Apply principles to motors, generators, and real-world devices
What You'll Learn
- Describe and visualise magnetic fields and field lines
- Use Fleming’s left-hand and right-hand rules correctly
- Explain electromagnetic induction and EMF ∝ dΦ/dt
- Understand how motors and generators interconvert energy
Subtopics – Magnetism & Electromagnetism
Each subtopic has a dedicated page with clear explanations and an interactive simulator.
Magnetic Field
A magnetic field is the region around a magnet or current-carrying conductor where magnetic effects can be detected, represented by field lines and vectors.
Read more →Magnetic Field Lines
Magnetic field lines are imaginary lines used to represent the direction and strength of the magnetic field; they emerge from the north pole and enter the south pole outside a magnet.
Read more →Magnetic Field due to Current-Carrying Conductor
A current-carrying conductor produces a magnetic field with circular field lines around it; the direction is given by the right-hand thumb rule.
Read more →Fleming’s Left-Hand Rule
Fleming’s left-hand rule gives the direction of force on a current-carrying conductor placed in a magnetic field: thumb = force, forefinger = field, middle finger = current.
Read more →Fleming’s Right-Hand Rule
Fleming’s right-hand rule gives the direction of induced current when a conductor moves in a magnetic field: thumb = motion, forefinger = field, middle finger = induced current.
Read more →Electromagnetic Induction
Electromagnetic induction is the production of emf in a conductor when the magnetic flux linked with it changes, described qualitatively by Faraday’s and Lenz’s laws.
Read more →Electric Motor
An electric motor converts electrical energy into mechanical energy using the force on a current-carrying coil in a magnetic field, plus a commutator to keep it rotating in one direction.
Read more →Electric Generator
An electric generator converts mechanical energy into electrical energy by rotating a coil in a magnetic field to induce emf and current.
Read more →