Magnetic Field of a Current-Carrying Wire

1 · Predict

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?

• 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.
• Yes — it's a simple 1/r² falloff, same as a point charge.
• The field doesn't depend on distance from the wire.

2 · Set Up

1. Open the wire-field preset and press Reset. The wire carries a fixed current of 8 A over a 4.8 m segment.
2. Enable the field-strength readout.
3. Set the perpendicular distance from the wire's midpoint for each trial and record the field strength.

3 · Collect Data

Plot |B_z| (y-axis) against 1/d (x-axis) for your three trials. Does it look more linear than plotting against 1/d²?

4 · Analyze

1. 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.
2. 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.

5 · Extend

1. 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?
2. 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.

The Physics Behind This Experiment

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