Magnetic Fields of Currents

What you will investigate

This magnetic field virtual lab uses compasses and probes to connect electrical current with the field around a wire and coil. Change current, reverse its direction, vary the distance from a wire, and compare the magnetic contributions from more than one source. The compass is a measurement device in the investigation, not simply a decoration placed on an already labeled field-line diagram.

A compass responds to the combined magnetic field, including the course's Earth-field reference. Near a wire, it therefore need not point exactly along the circle representing the wire's field alone. Keep the measured direction separate from a drawing of an individual contribution. This distinction is especially useful when a local current-produced field is comparable to the background field.

A controlled straight-wire comparison

For the ideal long straight wire, B = μ₀I/(2πr). Change current at fixed distance to test proportionality, then change distance at fixed current to test the inverse-distance relationship. The direction follows the right-hand rule and reverses when the current reverses.

Ideal compass south of the wire	Compass deflection
Current 2.0 A, distance 2.0 cm	45.00000°
Current 1.0 A, distance 2.0 cm	26.56505°
Current 2.0 A, distance 4.0 cm	26.56505°
Current −2.0 A, distance 2.0 cm	−45.00000°

These are the prompt's ideal validation angles with its stated horizontal Earth field. Halving current and doubling distance produce the same wire-field magnitude in this comparison. The angle itself is not directly proportional to that magnitude; the course uses the tangent of the deflection to connect perpendicular field contributions. Do not turn an angle comparison into an unsupported claim that twice the current always gives twice the compass angle.

Coils, cores, and force measurements

At a circular coil's center, the ideal expression is B = μ₀NI/(2R). Compare turns and current while keeping the geometric setup fixed. For a long solenoid, the familiar approximate relation uses turns per unit length. A finite coil and its position-dependent field require the course's stated geometry rather than treating every point as the center of an infinite solenoid.

The core comparison connects a stronger field with the magnetization of the material. Iron's domain response is different from the much smaller effects of aluminum and copper in the selected probe setup. The course does not treat permeability as a universal fixed multiplier independent of material shape and field strength.

The force bench supplies another measurement: for a perpendicular current-carrying wire segment, F = IℓB. The balance supports the magnet, so its reading reflects the reaction force on the magnet. Keep that force's direction separate from the force on the wire. This prevents a correct scale reading from being paired with the wrong force arrow.

Fields are not streams of electric charge

The bar-magnet comparison emphasizes closed magnetic field lines and paired poles. Cutting a magnet does not isolate one pole. For the moving-charge extension, the magnetic force is perpendicular to the relevant velocity and field directions; a stationary charge does not receive the same magnetic-force effect as a moving charge. A field picture should therefore not be described as a literal stream carrying compass needles around the wire.

Standards and classroom discussion

The course supports the current-produces-field part of NGSS HS-PS2-5, along with AP Physics magnetic-field and force topics. The electromagnetic-induction part of that standard is a separate investigation. Ask students to combine a current–distance comparison, a compass direction, and an explanation using vector addition. Continue with Ohm's law and circuits or Newton's second law.

Build an explanation from your records

Make a prediction before changing the conditions, then record a comparison that actually tests it. Keep the conditions attached to each row: a number without its setup can be misleading even when the arithmetic is correct. Explain what changed, what stayed fixed, and which observation supports your conclusion. If the result differs from your prediction, revise the explanation using the evidence rather than choosing a new answer without a reason.

The reference examples on this page describe the course model or its stated source data. They are not measurements collected from a physical laboratory by the reader. Compare like conditions, keep units visible, and distinguish rounded instrument displays from underlying model quantities. Use the investigation's stated limits when deciding whether a result can be generalized beyond the simulated setup. This guide supports preparation and interpretation; practical laboratory technique and safety training remain separate learning activities.