Units
| Quantity | SI unit | CGS unit | Conversion |
|---|---|---|---|
| Magnetic flux density B | Tesla (T) | Gauss (G) | 1 T = 10 000 G |
| Magnetic field strength H | A/m | Oersted (Oe) | 1 Oe = 79.577 A/m |
| Magnetic flux Φ | Weber (Wb) | Maxwell (Mx) | 1 Wb = 10⁸ Mx |
| Permeability μ | H/m | — | μ₀ = 4π × 10⁻⁷ H/m |
Typical field strengths
| Source | Field |
|---|---|
| Earth's magnetic field | 25–65 µT (0.25–0.65 G) |
| Refrigerator magnet | ~5 mT (50 G) |
| Strong neodymium magnet (surface) | 0.5–1.5 T (5–15 kG) |
| MRI (clinical) | 1.5 – 3 T |
| Strongest continuous lab magnet | 45 T |
| Pulsed lab magnet | hundreds of T |
| Neutron star surface | 10⁸ – 10¹¹ T |
Key relations
| B in free space | B = μ₀ H |
|---|---|
| Magnetic flux | Φ = B · A (perpendicular area) |
| EMF (Faraday) | ε = −dΦ/dt |
| Force on moving charge | F = qv × B |
| Force on wire | F = IL × B |
Worked examples
Converting units. Since 1 T = 10,000 G, a 5 mT refrigerator magnet is 5 × 10 = 50 G, and Earth’s field of 50 µT is 0.5 G. A neodymium magnet surface reading of 12,000 G is 1.2 T.
Flux through a coil. A uniform 0.2 T field passes through a 5 cm × 4 cm area (20 cm² = 0.002 m²) at right angles. Flux Φ = B × A = 0.2 × 0.002 = 4 × 10−4 Wb, which is 0.4 mWb, or 4 × 104 maxwells. If a 100-turn coil around that area sees the flux fall to zero in 20 ms, Faraday’s law gives an average EMF of 100 × 0.0004 / 0.020 = 2 V.
B versus H in air. For a flux density of 0.2 T in air, H = B / μ₀ = 0.2 / (4π × 10−7) = 159,155 A/m. Dividing by 79.577 gives 2,000 Oe, which is the same number as 2,000 G, a convenience of the CGS system in free space only. Inside iron the two are very different, because the material’s permeability multiplies B.
Common mistakes
- Treating B and H as the same. B (tesla, gauss) is flux density in a material; H (A/m, oersted) is the field applied. They are linked by permeability.
- Comparing magnet ratings. A gauss reading depends on where and how it is measured. A surface reading of a small magnet and a pull-force rating are different specifications.
- Mixing weber and tesla. A weber is flux (total); a tesla is a weber per square meter (density).
- Ignoring distance. A dipole field falls off rapidly with distance, so a magnet that reads 1 T at its face is far weaker a few centimeters away.