Electromagnetism
Solenoid field
Two test charges: inside the coil F_B ≠ 0; outside F_B = 0. Select each to compare.
Solenoid field — interactive Electromagnetism simulation. Two test charges: inside the coil F_B ≠ 0; outside F_B = 0. Select each to compare. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Solenoid field
Inside the solenoid B ≈ μ₀(N/L)I — uniform arrows along the axis.
A solenoid with N turns over length L and current I produces an approximately uniform B ≈ μ₀(N/L)I along its axis inside the coil. Field lines run through the interior and loop back outside — like a bar magnet. Increase I or turn density and watch the interior B arrows grow in the readout.
- B ≈ μ₀NI/L
- Uniform inside
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A solenoid (a tightly wound coil) carries current through its many turns. If you increase the current, does the interior field strength scale in direct proportion?
Predictions to weigh
- The field grows faster than the current, since each turn's contribution compounds.
- Yes — the interior field is directly proportional to the current.
- The interior field doesn't depend on current.
Variable roles
What you set:
- Coil current I (A)
What you measure:
- Interior field B (mT)
How the investigation runs
- Open the solenoid-field preset and press Reset. The solenoid has 400 turns over a 0.45 m length.
- Enable the interior field-strength readout.
- Set the coil current for each trial and record the interior field strength.
Governing equation
Solenoid Interior Field — B = μ₀(N/L)I
A long, tightly wound solenoid produces a nearly uniform interior magnetic field set by its turns density and current: B = μ₀NI/L, where N is the total number of turns and L is the solenoid's length.
What the printable worksheet asks students to work out
- For one trial, compute B = μ₀NI/L using μ₀ = 4π×10⁻⁷ H/m, N = 400, L = 0.45 m. Compare to the table.
- Explain why the interior field depends on the number of turns per unit length (N/L), not on the turns and length separately — a longer solenoid with proportionally more turns gives the same field.
Where this shows up beyond the lab
- An electromagnet (like in a junkyard crane) is essentially a solenoid wrapped around an iron core, which can multiply the field by a factor of hundreds or thousands (the core's high permeability). Explain why a plain air-core solenoid like this one produces a much weaker field for the same current and turns.
- Outside an ideal solenoid, the magnetic field is close to zero, unlike a single wire's field which extends outward from it. Explain, using the many individual turns' fields, why the fields from opposite sides of the coil largely cancel outside but reinforce inside.
- AP Physics 2 — Unit 12: Magnetism and Electromagnetism
- AP Physics C: Electricity and Magnetism — Unit 12: Magnetic Fields and Electromagnetism
- IB Physics — D.2 Electric and magnetic fields
- General High School Physics — Magnetism & electromagnetism
- NGSS High School Physics — Electric current and magnetic fields
- Middle School Physical Science — Electric and magnetic force strength
- Middle School Physical Science — Fields without contact
- Welcome to Solenoid Field
- Select the solenoid
- Press Play
- Field inside a solenoid
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.