Key relationship
Br=(µ₀/4π)2m cosθ/r³; Bθ=(µ₀/4π)m sinθ/r³.
Physics · Class 12
Bar magnet and equivalent solenoid
A bar magnet and a small current loop with the same dipole moment have the same far field. Field lines form closed loops; an isolated magnetic pole is absent from this model.
Open the interactive experiment →Bar magnet and equivalent solenoid
சட்டக் காந்தமும் சிறு மின்சுருளும் தூரத்தில் ஒரே இருமுனைப் புலத்தைத் தரும். காந்தப்புலக் கோடுகள் மூடிய வளையங்கள்.
Key notes, relationships and model limits
Remember
A bar magnet and a small current loop with the same dipole moment have the same far field. Field lines form closed loops; an isolated magnetic pole is absent from this model.
Common trap
The distant dipole approximation does not describe the detailed field close to a magnet’s pole faces.
Model limits
Point magnetic dipole approximation, probe far beyond the magnet size; no isolated magnetic monopoles.
What you will learn
Use the controls and plotted relation to explain bar magnet and equivalent solenoid.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
A 1 A m² dipole has axial B=0.2 µT and equatorial magnitude 0.1 µT at 1 m.
Why this matters
The model connects bar magnet and equivalent solenoid with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
Magnetic field lines of a bar magnet form?
- Open lines beginning at an isolated north pole.
- Closed loops, continuing through the magnet.
- Straight lines that end at the magnet surface.
Read the explanation
Closed loops, continuing through the magnet.
Course connection: Class XII Chapter 5. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.