Solenoid Topics

• magnetic field lines
• movable core
• output ports
• controller circuit
• pneumatic valve
• physical explanation
• mechanical force
• solenoid valve
• loop c
• requisite number
• armature
• side 1
• psi
• distances
• vent
• density
• direction
• electric field lines

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Solenoid, Solenoid

In the diagram to the right, we immediately know that the field points in the positive z direction inside the solenoid, and in the negative z direction outside the solenoid.

We have shown above that the field is pointing upwards inside the solenoid, so the horizontal portions of loop "c" don't contribute anything to the integral. Thus the integral up side 1 is equal to the integral down side 2. Since we can arbitrarily change the dimensions of the loop and get the same result, the only physical explanation is that the integrands are actually equal, that is, the magnetic field inside the solenoid is constant. A similar argument can be applied to loop "a" to conclude that the field outside the solenoid is constant.

Magnetic field lines only exist as loops, they cannot diverge from or converge to a point like electric field lines can. The magnetic field lines go up the inside of the solenoid, so they must go down the outside so that they can form a loop. However, the volume outside the solenoid is much greater than the volume inside, so the density of magnetic field lines outside is greatly reduced. Recall also that the field outside is constant. In order for the total number of field lines to be conserved, the field outside must go to zero as the solenoid gets longer.

The armature is used to provide a mechanical force to some mechanism (such as controlling a pneumatic valve). Although typically weak over anything but very short distances, solenoids may be controlled directly by a controller circuit, and thus have very low reaction times.

A solenoid consists of a balanced or easily movable core, which channels the gas to the appropriate port, coupled to a small linear solenoid. The valve allows a small current applied to the solenoid to switch a large amount of high pressure gas, typically up to 100 psi (7 bar, 0.7 MPa, 0.7 MN/m). Some solenoids are capable of operating at far greater pressures. Pneumatic solenoids may have one, two, or three output ports, and the requisite number of vents. The valves are commonly used to control a piston or other linear actuator.

Hydraulic machinery uses solenoids to control the flow of oil to rams or actuators to (for instance) bend sheets of titanium in aerospace manufacturing.

Also called a starter relay , the starter solenoid receives a large electric current from the car battery and a small electric current from the ignition switch. When the ignition switch is turned on (when the key is turned to start the car), the small electric current forces the starter solenoid to close a pair of heavy contacts, thus relaying the large electric current to the starter motor.

This can be caused by a low or dead battery, by corroded or loose connections in the cable, or by a broken or damaged positive (red) cable from the battery. Any of these will result in some power to the solenoid, but not enough to hold the heavy contacts closed, so the starter motor itself never spins, and the engine is not rotated (does not start).

Source: Wikipedia > Solenoid