5H15.40 Solenoid and Iron Filings
Abstract
Moving charges create magnetic fields. When current flows through a solenoid, a strong uniform magnetic
field is created inside the solenoid, and a weak curved field exists outside the solenoid. Iron filings align themselves
in straight lines while inside the solenoid, and begin to diverge near the openings of the solenoid, indicating the
shape of the magnetic field lines.
Picture
Setup
Setup is 5 minutes.
Safety Concerns
Ensure that iron filings are not sprinkled near power supply, as they will damage the equipment.
Equipment
• EICO1064S Power Supply
• Iron Filings around a Solenoid Apparatus
• Iron Filings
• Connecting Leads
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Procedure
Connect terminals on apparatus to the power supply. Change the range to the 12V/10A setting and turn on
the power supply. Adjust the voltage knob until a current of 10 A is reached. Slowly sprinkle iron filings onto the
plexiglass. Tap the top of the plexiglass to increase the visibility of the field lines.
Place the iron filings back into the container. A piece of paper folded in half can be used as a funnel for the
iron filings. Use a paper towel to clean dust off of the plexiglass before storing.
Theory
Moving charges create magnetic fields. The direction of the magnetic field created by a moving charge at a
given point in time and space must be perpendicular to both the velocity vector of the moving charge and the
position vector extending from the charge to the point of interest. In a circular current loop there are straight
magnetic field lines passing through the center of the loop and perpendicular to the plane of the loop, while
elsewhere in the loop the field lines are curved.
A solenoid is made by ”stacking” many loops of wire together. The superposition of the magnetic fields
passing through the centers of the loops creates a strong, nearly uniform field in the center of the solenoid. The
strength of the magnetic field inside a solenoid is
B = µ0 nI,
(1)
where B is the magnetic field strength, I is the current running through the solenoid, n is the number of turns
per meter, and µ0 (µ0 = 4π × 10−7 T m/A) is the permeability of free space.
The solenoid in this demonstration has 13 turns and spans approximately 15 cm, therefore n ≈ 86.667 turns/m.
If a current of 10 A is supplied to the solenoid, the theoretical magnetic field strength inside the solenoid is 1.1
mT.
The right hand rule is used to determine the orientation of the field lines of a solenoid. If the fingers are curled
in the direction of current flow, the thumb points in the direction of the field lines inside the solenoid. To find
the orientation of the field lines outside the solenoid, the thumb is pointed in the direction of current flow, and
the fingers curl in the direction of the field lines. These lines are illustrated in Figure 1.
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2
S
1
N
Current
Field Lines
Figure 1: Diagram showing the magnetic field lines surrounding a current carrying solenoid.
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With a current of 10 A supplied to the solenoid, the magnetic field strengths at the various points shown in
Figure 1 are measured with a Hall probe and the data is recorded in Table 1.
Position
Field Strength (mT)
Inside Solenoid [1]
0.904
Opening of Solenoid [2]
0.282
3 cm Away from Solenoid [3]
0.209
Table 1: Table listing the magnetic field strengths at 3 labelled regions around the solenoid.
When a bar magnet is placed in a magnetic field, the magnetic field exerts a torque on the poles of the magnet
until the bar magnet is aligned parallel to the magnetic field lines. Ferromagnetic materials, such as iron filings,
contain many randomly oriented magnetic domains that will also align parallel to an external magnetic field due
to torque. These materials then behave as bar magnets. In the case of a solenoid, iron filings align to form lines
inside the solenoid that diverge near the openings.
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References
[1] Freier, G. D. and Anderson, F. J. A Demonstration Handbook for Physics, ”Ei-10. Field of a Solenoid”,
American Association of Physics Teachers One Physics Ellipse, College Park MD, 1996. pg E-32.
[2] Meiners, Harry F. et al. Physics Demonstration Experiments Vol. II, ”31–1.21”, American Association of
Physics Teachers, The Ronald Press Company, New York, 1970. pg 922-923.
[3] Knight, Randall D. Physics for Scientists and Engineers, ”Ampere’s Law and Solenoids”, 2nd ed, California:
Pearson Addison-Wesley, 2008. pg 1015-1017.
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