29.11: Solenoids
A solenoid is a conducting wire coated with an insulating material, wound tightly in the form of a helical coil. The magnetic field for a solenoid is the vector sum of the magnetic field due to its individual turns. For an ideal solenoid, the magnetic field inside is almost uniform and parallel to the solenoid axis, while the magnetic field outside the solenoid is nearly zero.
Each turn in a solenoid can be approximated as a circular current carrying coil that generates a dipole moment. The magnetic dipole moment due to each turn equals the current's product and the loop's area. Thus, the total magnetic dipole moment of all the turns in a solenoid is equal to the dipole moment due to each turn multiplied by the total number of turns.
A solenoid is an electromagnet with north and south poles separated by its length, similar to a bar magnet. When a solenoid is connected to a battery, the end connected to the battery's negative terminal acts as the north pole, while the other end acts as the south pole. Therefore, the magnetic moment of a solenoid and bar magnet is equal. Hence, the magnetic field pattern for a solenoid resembles the pattern due to a bar magnet.
Varying the magnitude of the current and the number of turns in a solenoid changes its magnetic field strength, whereas the magnetic field strength for a bar magnet is fixed. The polarity of a solenoid can be reversed by altering the current direction, while a bar magnet possesses a fixed polarity. Switching off the current in a solenoid demagnetizes it.
Since their magnetic field strength is controllable, solenoids are used in various applications like automobile ignition systems, relays, valves, etc.
