# B Position of a coil relative to the magnetic flux

1. Jul 3, 2016

Does a magnet have to penetrate the plane of a coil in order to generate an emf in the coil?
Would an emf be obtained if the coil only passes through the magnetic lines of force, without the magnet crossing the plane of the coil?
Thank you.

2. Jul 3, 2016

### Simon Bridge

It is the magnetic field that counts.
You can test this easily enough with a wire, a magnet, and a sensitive galvinometer.

3. Jul 3, 2016

### David Lewis

The coil just needs to enclose magnetic flux. When the flux changes, voltage is induced. The coil does not need to pass through the magnetic lines of force, and moving lines of magnetic force do not need to cut the coil.

4. Jul 4, 2016

### Staff: Mentor

That's correct. Relative motion is not required, all that's needed is that there be a change in the flux passing through the coil, regardless of how that change is brought about.

5. Jul 5, 2016

Thanks to each one for the reply!
1) Clarification:
Enclose the magnetic field means surround the magnetic field. Am I understanding this right?
If so, does it mean that running, back-and-forth, a diamagnetic screen between the magnet and the solenoid would ensure the variation of the flux?
2) New question:
Looking at some drawings of linear generators, it seemed to me that the translator (plunger) needed only to translate in front, or above, or beneath the stator windings. No enclosing.
Have I misinterpreted the drawings?

6. Jul 6, 2016

### Staff: Mentor

A coil can act as a sensor of magnetic field changes, for changes occuring inside the volume enclosed by the coil, typically a cylindrical region. Varying the effectiveness of shielding around a magnet can be one way of affecting the sensor.

Changes that occur entirely outside the region enclosed by the coil cannot affect it. It reacts only to those field lines that actually cut through the region the coil encloses. A magnet waved around near a coil can affect it because magnetic field lines of a magnet penetrate through the region around it in huge arcs (ideally these extend out to infinity, though becoming vanishingly weaker at increasing distance).

To maximize the induced voltage, the magnet must be moved rapidly and in close proximity to the coil (or, equivalently, the coil moved in close proximity to the magnet), and their relative orientation must be optimized to produce greatest flux change within the coil.

7. Jul 6, 2016

Excellent!
The arcs are the clincher in the linear case, then.

Thank you very much!

8. Jul 8, 2016

### David Lewis

Right. It's the flux that passes through an imaginary surface whose boundary is the wire loop. Flux outside the coil has no effect.

9. Jul 9, 2016