The field inside a solenoid
"The field inside a solenoid" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "The solenoid" in Moving Charges and Magnetism. A solenoid cut-away with uniform B = μ₀nI inside and almost none outside as turns, length, layers and radius change. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 56 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson The solenoid in Moving Charges and Magnetism. Change the values and watch what happens.
What it shows
NCERT §4.7
A solenoid cut-away with uniform B = μ₀nI inside and almost none outside as turns, length, layers and radius change.
Controls
5 to try
- Slidertotal turns 100–2000
- Sliderlength 10–100 cm
- Sliderlayers 1–4
- Slidercurrent 0–10 A
- Sliderradius 1–5 cm
Try this
Reach 6.28 mT inside, then double the radius and show B does not change.
Common mistake
Taking n as the total number of turns instead of turns per metre.
In NEET
10 years of papers
Asked 2 times in 10 years of NEET.
See the past questions: Ampere's Circuital Law, Moving Charges and Magnetism (chapter)
In JEE Main
42 shifts analysed
56 questions from Moving Charges and Magnetism across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §4.7
- A solenoid is a long insulated wire wound as a closely spaced helix; a long solenoid is one whose length is much more than its radius. Each turn acts as a circular loop.
- Between neighbouring turns the fields cancel. Inside, the field is strong, uniform and along the axis; just outside it is weak, and for a very long solenoid it is taken as zero.
- A rectangular Amperian loop with one side h inside along the axis and one outside gives Bh = μ₀(nh)I, so B = μ₀nI, with n counting the turns in each metre of winding.
- The inside field does not depend on the radius or on the position inside, only on n and I. Its direction follows the right-hand rule.
- A solenoid is the usual way to make a uniform field; a soft iron core inside makes it much larger.
- Example 4.8: 500 turns on 0.5 m (n = 1000 per metre) with 5 A: B = 4π × 10⁻⁷ × 10³ × 5 = 6.28 × 10⁻³ T. The ratio of length to radius is 50, so the long-solenoid formula is fair.
Questions
5 answers
- What does the "The field inside a solenoid" simulation show?
- A solenoid cut-away with uniform B = μ₀nI inside and almost none outside as turns, length, layers and radius change.
- What mistake does the "The field inside a solenoid" simulation fix?
- A common mistake it clears up: taking n as the total number of turns instead of turns per metre.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 2 times in 10 years of papers.
- Does JEE Main ask questions from Moving Charges and Magnetism?
- Yes. JEE Main asked 56 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "The solenoid" in Moving Charges and Magnetism (Class 12 Physics).
Embed this simulation
Free, with a link back
Teaching this idea? Put the simulation on your own page or LMS with this code.