Download Solenoids

Solenoids
In the 4th Grade
• 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.
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
1. Curve your fingers.
2. Place them along wire loop
so that your fingers point in
direction of current.
3. Your thumb gives the
direction of the magnetic
field in the center of the
loop, where it is straight.
4. 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
B
A
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