The magnetic field of a magnet is a vector quantity that describes the magnetic influence of the magnet at every point in space. It is represented by field lines going from the North pole to the South pole outside the magnet. The smartphone’s magnetometer allows direct measurement.
Discover FizziQ
How to measure it in class
With the FizziQ app, the magnetometer allows you to map the magnetic field of a magnet.
Steps:
- Open FizziQ and select the “Magnetometer” or “Magnetic Field” instrument
- Bring a magnet close to the smartphone and observe the variations
- Measure the field intensity at different distances
- Verify the rapid decrease with distance (approximately 1/r³)
- Identify the North and South poles of the magnet
- Map the field lines with iron filings
Scientific activities on this topic
Possible extensions with FizziQ: measurement of the intensity as a function of distance and comparison of different magnets.
Learn more
Field characteristics:
- Direction: tangent to the field lines
- Sense: from the North pole to the South pole (convention)
- Intensity: in teslas (T) or milliteslas (mT)
Typical values:
| Source | Intensity (mT) |
|---|---|
| Permanent magnet | 10-100 |
| Fridge magnet | 5-10 |
| Earth’s field | 0.03-0.06 |
| Medical MRI | 1500-7000 |
Dipole model:
Far from the magnet, the field decreases as 1/r³ (magnetic dipole). Near the magnet, the actual shape matters and the field is more complex.
Interaction between magnets:
- Like poles: repulsion
- Unlike poles: attraction
Formula
Field of a magnetic dipole at large distance (on the axis):
B = (μ₀/4π) × (2M/r³)
where:
- B: magnetic field intensity (T)
- μ₀ = 4π × 10⁻⁷ T·m/A: permeability of free space
- M: magnetic moment of the magnet (A·m²)
- r: distance from the center of the magnet (m)
Application examples
- Electric motors use the interaction between magnets and coils
- Hard drives store data using magnetic domains
- MRI creates an intense magnetic field to image the body
- Bank cards store information on a magnetic stripe
FAQ
Q: Why does the field decrease so rapidly with distance? A: A magnet is a dipole: the effects of the two poles partially cancel out at a distance. The field decreases as 1/r³, faster than gravity (1/r²).
Q: Does the smartphone magnetometer measure the total field? A: Yes, it measures the three components (x, y, z) and FizziQ can display the magnitude. Caution: the smartphone also measures Earth’s field, which must be subtracted.
Q: How do you find a magnet’s poles with FizziQ? A: Bring the magnet close to the smartphone. The z-component of the field changes sign when passing from one pole to the other.
Q: Do magnets wear out? A: Slowly, especially if they are heated or knocked. Neodymium magnets are very stable at room temperature.
Related concepts
Magnetometer - Earth’s Magnetic Field - Electromagnetic Induction