Solenoids

In the 4th Grade N S You learned that coils with current in them make magnetic fields. The iron nail was not necessary to cause the field; it merely intensified it. B I

Solenoid A solenoid is a coil of wire. When current runs through the wire, it causes the coil to become an “electromagnet”. Air-core solenoids have nothing inside of them. Iron-core solenoids are filled with iron to intensify the magnetic field.

Magnetic Field Inside a Solenoid B = on I o: 4  10-7 T m / A n: number of coils per unit length (# / m) I: current (A) You are not required to memorize this formula, but only to use it.

Magnetic Field around Curved Current B

Right Hand Rule for magnetic fields around curved wires Curve your fingers. Place them along wire loop so that your fingers point in direction of current. Your thumb gives the direction of the magnetic field in the center of the loop, where it is straight. Field lines curve around and make complete loops. B I

Sample Problem An air-core 10 cm long is wrapped with copper wire that is 0.1 mm in diameter. What must the current be through the wire if a magnetic field of 20 mT is to be produced inside the solenoid?

Sample Problem What is the direction of the magnetic field produced by the current I at A? At B? I A B

Magnetic Field around Curved Current B

Sample Problem What is the magnetic field inside the air-core solenoid shown if the resistance of the copper wire is assumed to be negligible? There are 100 windings per cm. Identify the north pole. 120 V I 100-W