Electromagnetism
Current-carrying wire
B arrows circulate around the wire; raise current I and read B in mT.
Current-carrying wire — interactive Electromagnetism simulation. B arrows circulate around the wire; raise current I and read B in mT. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Magnetic field of wire
B arrows circulate around the wire; raise current I and read B in mT.
A long straight wire carrying current I produces circular B-field lines around the wire (Ampère's law: B ∝ I/r). The right-hand rule gives direction: thumb along I, fingers curl with B. ⊗/⊙ symbols mark field into/out of the page. Raise I and read B in mT at a fixed distance — doubling I doubles B.
- B ∝ I/r
- Right-hand rule
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 straight wire carries a steady current. As you move a test point farther from the wire, does the magnetic field strength fall off the same way an electric charge's field does?
Predictions to weigh
- Yes — it's a simple 1/r² falloff, same as a point charge.
- The field doesn't depend on distance from the wire.
- No — a current-carrying wire's field falls off differently than a point charge's, since current is an extended (line) source, not a point.
Variable roles
What you set:
- Perpendicular distance d (m)
What you measure:
- Field strength |B_z| (µT)
How the investigation runs
- Open the wire-field preset and press Reset. The wire carries a fixed current of 8 A over a 4.8 m segment.
- Enable the field-strength readout.
- Set the perpendicular distance from the wire's midpoint for each trial and record the field strength.
Governing equation
Field of a Finite Current-Carrying Wire — B = μ₀I/(2πr)
The Biot–Savart law, integrated along a finite straight wire, gives a magnetic field that circles the wire and depends on both the perpendicular distance and the wire's length through the same interpolating shape as a finite line of charge.
What the printable worksheet asks students to work out
- For one trial, compute |B| = μ₀|I|L_half/(2πd√(L_half² + d²)) using μ₀ = 4π×10⁻⁷ H/m, I = 8 A, L_half = 2.4 m. Compare to the table.
- Compare this formula's shape to the line-charge experiment's electric-field formula. Explain why both a finite charged rod and a finite current-carrying wire produce fields with the same kind of distance dependence — close to 1/d near the wire, closer to 1/d² far away.
Where this shows up beyond the lab
- For an idealized infinite wire, the field simplifies to the classic B = μ₀I/(2πd) — a clean 1/d falloff. At what distance compared to this wire's 4.8 m length would you expect this experiment's finite-wire formula to closely match that infinite-wire approximation?
- A compass needle placed near a current-carrying wire deflects to align with the wire's magnetic field. Explain, using the right-hand rule, why the field circles around the wire rather than pointing radially outward like an electric field would.
- 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 Wire Field
- Select the wire
- Press Play
- B field around a wire
- 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.