电磁阀

A solenoid is a conducting wire coated with an insulating material, wound tightly in the form of a helical coil.

The turns can be picturized as a stack of closely spaced coaxial circular wires. Thus, a current-carrying solenoid generates a magnetic field equal to the sum of the magnetic field due to each turn.

If the fingers curl along the current direction, then the thumb points to the direction of the magnetic field.

For an ideal solenoid with its length much greater than the radius, the magnetic field inside the solenoid is almost uniform and parallel to the solenoid axis.

This pattern is analogous to the field due to a bar magnet. Each turn in a solenoid is equivalent to a small circular current carrying loop generating a magnetic dipole moment. Thus, a solenoid is an electromagnet with the poles separated by its length.

Unlike the bar magnet, the polarity of a solenoid can be reversed by altering the direction of the current and can be demagnetized by switching off the current.