Labo Élec
electrical lab simulator
Reminder sheet
A current creates a magnetic field, and a field acts on a current: that is Laplace's force. The three-finger rule gives you the direction without hesitation.
A magnet has two poles, north and south: like poles repel. Field lines leave from the north and enter at the south. The field is described by the flux density B (teslas); the flux Φ = B·S (webers) measures what crosses a surface.
A current creates a field: a coil of N turns behaves like a magnet (right-hand rule: fingers follow the current, the thumb gives north). A conductor carrying I in a field B experiences the Laplace force F = B·I·l (three-finger rule).
| Quantity | Symbol | Unit |
|---|---|---|
| Flux | Φ | Wb |
| Magnetic flux density | B | T |
| Area | S | m² |
| Quantity | Symbol | Unit |
|---|---|---|
| Vacuum permeability (4π × 10⁻⁷) | µ₀ | T·m/A |
| Number of turns | N | - |
| Length | l | m |
| Quantity | Symbol | Unit |
|---|---|---|
| Force | F | N |
| Current | I | A |
| Conductor length | l | m |
A long coil of 500 turns and 20 cm carries 2 A: what flux density B inside it (µ₀ = 4π × 10⁻⁷)? A 10 cm conductor carrying 5 A is placed perpendicular to a 0.5 T field: what force?
B = µ₀ · N · I / l = 4π × 10⁻⁷ × 500 × 2 / 0.20 = 6.3 × 10⁻³ T (6.3 mT), the length being in metres.
F = B · I · l = 0.5 × 5 × 0.10 = 0.25 N, perpendicular to the conductor and to the field (three-finger rule).
Field lines of a bar magnet: they leave the north and enter at the south.