A solenoid is a long coil of wire (with many turns or loops, as opposed to a flat loop). Because of its shape, the field inside a solenoid can be very uniform, and also very strong. The field just outside the coils is nearly zero. Figure 3 shows how the field looks and how its direction is given by RHR-2.

The magnetic field inside of a current-carrying solenoid is very uniform in direction and magnitude. Only near the ends does it begin to weaken and change direction. The field outside has similar complexities to flat loops and bar magnets, but the magnetic field strength inside a solenoid is simply

B
=
μ
0
nI
(
inside a solenoid
)
,
B
=
μ
0
nI
(
inside a solenoid
)
,
size 12{B=μ rSub { size 8{0} } ital "nI"` \( "inside a solenoid" \) ,} {}

(4)where nn size 12{n} {} is the number of loops per unit length of the solenoid (n=N/l(n=N/l size 12{ \( n=N/l} {}, with NN size 12{N} {} being the number of loops and ll size 12{l} {} the length). Note that BB size 12{B} {} is the field strength anywhere in the uniform region of the interior and not just at the center. Large uniform fields spread over a large volume are possible with solenoids, as Example 2 implies.

What is the field inside a 2.00-m-long solenoid that has 2000 loops and carries a 1600-A current?

*Strategy*

To find the field strength inside a solenoid, we use B=μ0nIB=μ0nI size 12{B=μ rSub { size 8{0} } ital "nI"} {}. First, we note the number of loops per unit length is

n
−
1
=
N
l
=
2000
2.00 m
=
1000
m
−
1
=
10
cm
−
1
.
n
−
1
=
N
l
=
2000
2.00 m
=
1000
m
−
1
=
10
cm
−
1
.
size 12{n rSup { size 8{ - 1} } = { {N} over {l} } = { {"2000"} over {2 "." "00" m} } ="1000"" m" rSup { size 8{ - 1} } ="10"" cm" rSup { size 8{ - 1} } "." } {}

(5)
*Solution*

Substituting known values gives

B
=
μ0nI=4π×10−7T⋅m/A1000m−11600 A
=
2.01 T.B
=
μ0nI=4π×10−7T⋅m/A1000m−11600 A
=
2.01 T.

(6)
*Discussion*

This is a large field strength that could be established over a large-diameter solenoid, such as in medical uses of magnetic resonance imaging (MRI). The very large current is an indication that the fields of this strength are not easily achieved, however. Such a large current through 1000 loops squeezed into a meter’s length would produce significant heating. Higher currents can be achieved by using superconducting wires, although this is expensive. There is an upper limit to the current, since the superconducting state is disrupted by very large magnetic fields.

There are interesting variations of the flat coil and solenoid. For example, the toroidal coil used to confine the reactive particles in tokamaks is much like a solenoid bent into a circle. The field inside a toroid is very strong but circular. Charged particles travel in circles, following the field lines, and collide with one another, perhaps inducing fusion. But the charged particles do not cross field lines and escape the toroid. A whole range of coil shapes are used to produce all sorts of magnetic field shapes. Adding ferromagnetic materials produces greater field strengths and can have a significant effect on the shape of the field. Ferromagnetic materials tend to trap magnetic fields (the field lines bend into the ferromagnetic material, leaving weaker fields outside it) and are used as shields for devices that are adversely affected by magnetic fields, including the Earth’s magnetic field.

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