Download Electric Field

Magnetic Fields Created By Currents
I. Introduction: The source of magnetic fields are moving charges. The charges can
be moving freely through the air, moving within the material of a permanent magnet or
moving along a conductor.
II. Long Straight Wire:
A. The magnetic field generated by a long
straight wire is given by the equation
B
μo I
2 r
where o is a constant called the
permeability of free space. (Note: This is
the magnetic constant that is similar to the
electric constant o.) It has a value of o =
T m
N
4 x 10-7
= 4 x 10-7 2 .
A
A
B. The magnetic field lines form circles centered on the wire.
C. The direction of the magnetic field can be determined by imagining you grab
the wire with your right hand with your thumb pointing in the direction of the
current. The direction your fingers curl around the wire gives the direction of
the magnetic field.
D. Draw the magnetic field lines around the two separate wires.

I
1
X
I
E. Example 1: A proton moves at 2.0 x 105 m/s parallel to a long straight wire at
a distance of 4.0 cm from the wire. If the current is 20 A in the wire, what is
the magnetic force on the proton?
vx
+e
I
III. Two Long Straight Wires: Two long straight parallel wires are separated by a
small distance r and have currents I1 and I2 running through them.
A. At a point on wire 2, draw the magnetic field
vector B1, the length vector L2 and the force
vector F2.
B. Using F  IL  B , find the magnetic force per
unit length.
C. In what direction does the force on wire 1 point? _______________
D. How does the force experienced by wire 1 compare to the force experienced
by wire 2? ____________________
E. If I1 is reversed, now moving to the left, and I2 remains the same, moving to
the right, how does this affect the forces on each of the wires?
________________________________________
IV. Magnetic Field of a Solenoid: If you wind a wire in a helix around a hollow tube,
you have formed a solenoid. If N is the total number of turns and L is the length of
the solenoid, then the number of turns per unit length is n = N/L.
A. The strength of the magnetic
field inside the solenoid is given
by
B = onI
B. The magnetic field is constant
everywhere inside the solenoid.
C. The magnetic field can be
increased by _____________
the number of turns per unit
length or by _____________
the current.
D. Label the north and south poles ends of the solenoids.
E. Example: A solenoid whose length is 30 cm and diameter is 2.0 cm has 10,000
turns of wire. If the wire carries a current of 5.0 A, what is the magnitude
of the magnetic field inside the solenoid?
I1
I2