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These are two related but different quantities that are easy to confuse. Magnetic flux density (B), measured in Tesla (T) in SI units or Gauss (G) in the older CGS system, describes how concentrated a magnetic field is at a point — like the strength of a magnet's field right next to its surface. Total magnetic flux (Φ), measured in Weber (Wb), is that flux density multiplied by the area it passes through: Φ=B×A. Flux density tells you "how strong," while total flux tells you "how much" is passing through a specific surface.
| Quantity | Unit | Relationship |
|---|---|---|
| Flux density (B) | Tesla (T) or Gauss (G) | 1 T = 10,000 G |
| Total flux (Φ) | Weber (Wb) | Φ = B×Area (Wb = T×m²) |
| Earth's magnetic field | ∼0.25–0.65 G (25–65 µT) | varies by location |
| Common fridge magnet | ∼50 G (5 mT) | |
| MRI scanner | 1.5–3 T (15,000–30,000 G) |
The conversion itself is simple — 1 T = 10,000 gauss — but the useful part is knowing what values actually occur:
| Source | Flux density |
|---|---|
| Earth’s magnetic field | ~50 µT (0.5 gauss) |
| Refrigerator magnet | ~5 mT (50 gauss) |
| Ferrite core, saturation | 0.3–0.5 T |
| Neodymium magnet, surface | 1.2–1.4 T |
| Silicon steel, saturation | 1.5–2.0 T |
| MRI scanner | 1.5–3 T |
The gauss survives mainly where the numbers would otherwise be awkwardly small, which is why magnetometers and field-strength probes still use it while transformer and motor design has moved to tesla.
Faraday’s law gives the transformer EMF equation:
Vrms = 4.44 × f × N × Ac × Bmax
where 4.44 is 2π/√2. Rearranged, the core cross-section needed for a given voltage is inversely proportional to both frequency and peak flux density. This is the whole reason switch-mode supplies are small: a 100 kHz transformer needs roughly one two-thousandth the core area of a 50 Hz one for the same voltage and turns.
It also explains the material choice. Silicon steel saturates around three to four times higher than ferrite, so it gives a much smaller core at mains frequency — but its eddy-current losses make it unusable at high frequency, where ferrite’s high resistivity wins despite its lower saturation.
Above Bsat the core’s permeability collapses toward that of air, so inductance falls sharply and magnetising current rises almost without limit. In a switching converter this appears as a current waveform that starts as a clean ramp and then turns sharply upward near the end of the on-time — a characteristic shape worth recognising on an oscilloscope, because it usually precedes a destroyed switching device by a very short time.
Multiply by 10,000: e.g. 1.5 T = 15,000 G. This is an exact conversion between the SI (Tesla) and CGS (Gauss) systems.
Tesla measures flux density (field strength per unit area, like intensity at a point), while Weber measures total flux (flux density multiplied by the area it passes through). One Weber equals one Tesla acting over one square metre.
Roughly 0.25-0.65 Gauss (25-65 microtesla) at the surface, varying by location — strongest near the magnetic poles and weakest near the magnetic equator.
Around 50 Gauss (5 millitesla) at its surface for a common flexible fridge magnet, though small rare-earth (neodymium) magnets can be dramatically stronger, sometimes exceeding 5000 Gauss (0.5 Tesla).
Clinical MRI scanners commonly operate at 1.5 Tesla or 3 Tesla (15,000-30,000 Gauss), tens of thousands of times stronger than Earth's natural field, which is why MRI rooms have strict metal-object safety protocols.
Gauss (CGS system) was historically dominant, especially in the US and in older/legacy documentation, while Tesla (SI system) is now the international scientific standard — many modern datasheets, especially outside the US, use Tesla or millitesla exclusively.
Multiply the flux density (B, in Tesla) by the area it passes through (in square metres): Φ=B×A, giving flux in Weber. This assumes the field is uniform and perpendicular to the surface; angled fields require multiplying by the cosine of the angle as well.
It is a sensor that measures magnetic flux density directly, commonly used for position/proximity sensing and current measurement (via a conductor's magnetic field). Sensitivity and range specifications are typically given in Gauss or millitesla/microtesla.
Transformer and inductor cores have a maximum flux density (Bmax, in Tesla) before the core material saturates and loses its useful magnetic properties — core datasheets specify this saturation flux density, which directly limits how much voltage-time product the core can safely support, as used in the Transformer Turns (Faraday) calculator.
Yes, the same 10,000× ratio applies at any prefix scale — for example, 1 milligauss = 0.0001 microtesla×... more directly: 1 microtesla = 10 milligauss, since the base Tesla-to-Gauss ratio of 10,000 is preserved regardless of which metric prefix is used on either side.
Exactly 10,000. So 1 mT is 10 gauss, and the Earth's field of roughly 50 µT is about 0.5 gauss.
Ferrite saturates at roughly 0.3–0.5 T and silicon steel at about 1.5–2.0 T. The difference is why steel suits mains frequency and ferrite suits switch-mode converters.
Vrms = 4.44 × f × N × Ac × Bmax, where 4.44 is 2π/√2. Core area is inversely proportional to both frequency and peak flux density.
Because required core area falls in inverse proportion to frequency. Operating at 100 kHz instead of 50 Hz reduces the core area needed for the same voltage and turns by a factor of around two thousand.
Permeability collapses toward that of air, so inductance falls and magnetising current rises very steeply. On an oscilloscope the current ramp turns sharply upward near the end of the on-time, which usually precedes device failure.
Because ferrite has very high resistivity, so eddy-current losses stay manageable at high frequency. Silicon steel would overheat badly at switch-mode frequencies despite its better saturation figure.
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