Basic knowledge of magnetic field strength
- Ethan
- Knowledge base
Magnetic field strength is a physical vector describing a magnetic field. It primarily reflects the magnitude and direction of the magnetic effect produced by a magnetic source in space, and is unaffected by the magnetic permeability of the surrounding medium.It differs from magnetic flux density (B):
- Magnetic flux density (B): Considers the influence of the medium and represents the actual force exerted by the magnetic field on charged particles.
- Magnetic field strength (H): Focuses more on the characteristics of the magnetic field and is often used in magnetic circuit calculations.
How to Measure Magnetic Field Strength
In real-life applications, the most common instruments for measuring magnetic field strength focus on the magnetic induction intensity B, which is a fundamental physical quantity in fields such as physics, materials science, engineering technology, and geophysics.
Search Coil Method
Based on Faraday’s law of electromagnetic induction, this method uses the motion of a coil in a magnetic field to generate an induced electromotive force for measuring B.
Instantaneous Measurement Method: Moving the detection coil within the target magnetic field induces a rapid change in magnetic flux, ΔΦ. This generates an induced electromotive force pulse. By integrating this voltage pulse, the change in magnetic flux ΔΦ = N · A · B (N: number of coil turns, A: effective area) can be calculated, thereby yielding the average value of B.
Continuous Measurement Method: The coil is continuously rotated in the magnetic field at a certain frequency, producing an alternating sinusoidal voltage with a peak value of U_m ∝ B · ω · N · A (where ω is the angular velocity). As long as the amplitude of the voltage is measured, B can be immediately determined.
✅ Advantages: Simple structure, no power supply required, low cost, suitable for strong magnetic fields.
❌ Limitations: Only suitable for measuring changing magnetic fields, cannot measure static magnetic fields.
Hall Effect Method
When a current I flows through a semiconductor material perpendicular to a magnetic field B, a voltage VH is generated between the upper and lower surfaces of the semiconductor. The magnitude of VH is VH = (I · B) / (n · e · t).
✅ Advantages: Suitable for both DC and AC magnetic fields, compact size, and the probe can be miniaturized.
❌ Disadvantages: High temperature sensitivity, requiring temperature compensation.
Fluxgate Method
Applying a high-frequency alternating saturation current to the drive coil will periodically drive the magnetic core into a magnetic saturation state. When an external magnetic field is introduced along the axis, asymmetric saturation will occur, generating even-order harmonics. The strength and direction of the magnetic field can be accurately inferred from the amplitude and phase of these harmonics.
✅ Advantages: High sensitivity, low noise, strong vector measurement capability.
❌ Limitations: Complex structure, high cost.
| Measurement Method | Measurement Range | ✅ Advantages | ❌ Disadvantages | Typical Applications |
|---|---|---|---|---|
| Search Coil | 1 mT - 100 T | Simple structure, withstands strong fields | Cannot measure static fields | Pulsed magnetic fields, motor magnetic field distribution |
| Hall Effect | 10 μT - 30 T | Wide bandwidth, miniaturizable | Requires calibration, large temperature drift | General gaussmeters, position sensing |
| Fluxgate | 0.1 nT - 1 mT | High resolution, high stability | Small dynamic range, easy to saturate | Electronic compass, geological exploration |
| Magnetoresistive Sensor | 10 nT - 1 Gs | Ultra-high sensitivity, extremely small volume | Nonlinear, requires setting | Hard disk read heads, angle sensors |
| Nuclear Magnetic Resonance (NMR) | 0.01 T - tens of T | Extremely high absolute accuracy, can serve as standard | Complex equipment, requires uniform magnetic field | Magnetic field reference devices, scientific research |
| SQUID | 1 fT - 1 mT | Ultimate sensitivity | Requires liquid helium cooling, expensive | Magnetoencephalography, basic scientific research |
Formula for Magnetic Field Strength
In electromagnetism, magnetic field strength and magnetic flux density are two closely related but distinct concepts: H is primarily determined by free current, while B includes the magnetization contribution of the material. The formula for calculating magnetic field strength is: H = B / μ (H: magnetic field strength, B: magnetic flux density, μ: permeability).
This formula originates from Maxwell’s equations and reflects the propagation characteristics of magnetic fields in different media. Physically, H represents the strength of the “magnetic field source,” unaffected by the material. B, on the other hand, describes the actual magnetic effect, influenced by the material’s permeability.
Factors Affecting Magnetic Field Strength
Magnetic Material Properties
The “maximum energy product” (BHmax) of a material is a key indicator of the performance of permanent magnets. It represents the maximum magnetic energy stored per unit volume of the material.
Shape and Size
This phenomenon is technically termed the “Demagnetizing Field.” A long, thin bar magnet has its strongest field near its ends but is weak at its sides, whereas a short, thick magnet struggles to maintain high intensity along its magnetized axis due to its own strong demagnetizing field.
Magnetic Circuit Structure
A well-designed magnetic circuit provides a low-impedance path for magnetic flux, The air gap in the magnetic circuit is a crucial factor, an excessively large air gap reduces the magnetic field strength.
External Environmental Factors
For most permanent magnet materials, increasing temperature directly causes a reduction in magnetic field strength, with a critical temperature point for irreversible loss. For some soft magnetic materials, initial permeability may first increase and then decrease with temperature.
Stress and Mechanical Shock
Applying pressure or strong mechanical shock to a magnetic material can alter its internal domain structure, leading to changes in remanence and magnetic field strength.
Time
For rare earth magnets, even without external disturbances, their magnetic field strength undergoes extremely slow decay over time, known as the “aging effect.”
Some FAQs
What is the difference between Magnetic Field Strength (H) and Magnetic Flux Density (B)?
Magnetic field strength (H) reflects the original intensity of the magnetic source and is unaffected by the medium. Magnetic flux density (B) includes the contribution from the magnetization of the medium and represents the actual magnetic effect.
What is the unit of Magnetic Field Strength?
The SI unit is Amperes per meter (A/m). It originates from Ampère’s Circuital Law and is directly related to current.
How to calculate the Magnetic Field Strength in a vacuum?
H = B / μ₀ (where μ₀ ≈ 4π ×π × 10⁻⁷ H/m).
Which factors affect Magnetic Field Strength?
Main factors include: magnetic source strength, material properties, shape and size, magnetic circuit design, temperature/stress, and aging over time.
What is the role of Magnetic Field Strength in practical applications?
Magnetic field strength (H) is commonly used in applications such as electric motors, transformers, MRI imaging, and magnetic levitation systems.
Conclusion
In modern engineering applications, one of the core tasks for engineers designing electromagnets, transformers, and relays is to construct efficient magnetic circuits. Magnetic field strength serves as the starting point for magnetic circuit calculations. The maximum achievable magnetic flux density is determined based on the B-H curve of the core material. Without magnetic field strength, neither modern power engineering nor electronic technology would be possible.
For more insights, check these related blogs:
- What is The Magnetic Hysteresis Loop?
- What is hysteresis loss? How can it be reduced in motors?
- What is electromagnetic energy? How does it affect us?
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