ソレノイド

A solenoid is a conducting wire coated with insulating material, wound tightly in a helical coil pattern. The turns function as closely spaced coaxial circular wires stacked together. When current flows through the solenoid, each turn generates a magnetic dipole moment, and the total magnetic field equals the vector sum of fields from all individual turns.

Inside an ideal solenoid, the magnetic field is nearly uniform and parallel to the solenoid axis. Outside the solenoid, the magnetic field is nearly zero. This pattern resembles the magnetic field of a bar magnet, with field lines concentrated along the axis and minimal external field.

Use the right-hand rule: curl your fingers along the current direction, and your thumb points in the direction of the magnetic field. This method works because each turn in the solenoid generates a magnetic field, and the thumb indicates the resulting field direction along the solenoid axis.

The magnetic field strength depends on the magnitude of the current flowing through the solenoid and the total number of turns. Increasing either the current or the number of turns increases the overall magnetic field strength. Unlike bar magnets with fixed field strength, solenoids allow controllable field variation.

Both solenoids and bar magnets have north and south poles separated by their length and produce similar magnetic field patterns. However, solenoids offer advantages: their polarity reverses by changing current direction, their field strength varies with current and turn count, and they demagnetize when current switches off. Bar magnets have fixed polarity and field strength.

Each turn in a solenoid acts as a circular current loop generating a magnetic dipole moment equal to the current multiplied by the loop's area. The total magnetic dipole moment of the solenoid equals the dipole moment per turn multiplied by the total number of turns, making it proportional to turn count.

Solenoids are used in automobile ignition systems, relays, and valves because their magnetic field strength is controllable and reversible. Unlike permanent magnets, solenoids can be switched on and off by controlling current, and their field intensity can be adjusted by varying current magnitude or turn count, enabling precise electromagnetic control.