Electricity and magnetism

Circuits, Ohm's law, and electromagnetic induction

See the relation I = U/R visually (drifting electron density), and observe the Faraday–Lenz law: the induced current direction always opposes the flux change that created it. Measure circuit current and induced emf, checking $I = U/R$ and $ε = −dΦ_B/dt$ .

Middle school

Equipment

3D circuit: single resistor, series circuit, parallel circuit
Virtual bar magnet with reversible polarity
Coil with adjustable number of turns and field strength

Procedure

Measure current in series and parallel circuits
Switch between the three circuit modes. For the series circuit, compare U₁, U₂ with I·R₁, I·R₂. For the parallel circuit, add I₁ + I₂ and compare with the displayed total current I. Compare the observed quantities with $I = I₁ + I₂$ .
Reverse the magnet's poles and observe Lenz's law
Let the magnet oscillate, and read the ε gauge on the left. Click 'Reverse poles' and watch the induced current direction (blue dots) flip. Stop the magnet at the center of the coil and watch ε drop near zero even though flux Φ is at its maximum. Compare the observed quantities with $ε = −dΦ_B/dt$ .
Check the induced electromotive force
Move the magnet quickly and then slowly through the coil; read ε and watch the induced-current direction. Compare with the magnet held still where flux is largest, and use ε = −dΦ_B/dt to explain why ε is near zero at rest. Compare the displayed values with $ε = −dΦ_B/dt$ .

Simulation

Experiment history

Electrical science advanced through quantitative studies of currents and circuits during the eighteenth and nineteenth centuries. Georg Ohm published the relation among voltage, current, and resistance in 1827; Michael Faraday's work on electromagnetic induction later extended the picture from conducting circuits to interactions between current and magnetic fields. Heinrich Lenz supplied the rule for the induced-current direction: its effect opposes the change in magnetic flux that produced it, consistent with energy conservation. For an ideal resistor, Ohm's law is I = U/R. In a coil, induced electromotive force follows ε = −dΦ_B/dt; the minus sign expresses Lenz's rule, and current appears when magnetic flux through the circuit changes. Adjust voltage and resistance to compare electron drift, then move the magnet to observe induced voltage. Electrons drift slowly even though an electrical signal propagates rapidly; the animation shows qualitative behavior, not literal particle sizes or speeds.

Related physicists

Georg Simon Ohm Michael Faraday Heinrich Lenz Joseph Henry

Equipment

Procedure

Simulation

Experiment history

Related physicists

Related library topics