Meaning and mechanism
The magnetic field created by the current is added at each turn of the coil. In the interior of a long solenoid, the magnetic field can be approximated as relatively uniform.
Choi’s self-portrait enhances the coil’s strength; however, there is a limit to how well the magnetic domains align. As the current approaches saturation, increasing the current further results in a smaller increase in effect.
Relationships and conditions
For a long air-core coil, B≈μ₀NI/ℓ. This approximation is proportional to turn count N and current I and inversely proportional to length ℓ. An iron core requires a separate magnetization relationship.
The actual core contains histrionic and residual magnetization, and the coil ends exhibit uneven gaps. It does not accurately predict the number of objects that can be attached solely based on the size of the gaps.
What to observe in the simulation
Air-core field is proportional to NI; an iron core saturates and its increase slows.
This is a long-solenoid approximation. Core hysteresis and the actual mechanics of clip lifting are omitted. Experiment on screen; do not reproduce this with real power supplies, outlets or high voltage.
Key relationships and symbols
Bair = μ₀NI/ℓ · Bcore = Bsat tanh(100Bair/Bsat)
| Symbol | Meaning | Unit |
|---|---|---|
| B | Magnetic field magnitude | T |
| N/ℓ | Number of turns per unit length | m⁻¹ |
| I | Coil current | A |
Worked example
What happens to the poles if only the direction of the same current is reversed?
N ↔ S
Common misconceptions
- The magnetic field of an electromagnet with an iron core does not increase indefinitely in proportion to current.