5H15.10 Iron Filings around a Wire
Abstract
Moving charges create magnetic fields. When current flows through a wire, it produces circular magnetic field
lines around the wire in planes perpendicular to the flow of current. Iron filings near the wire align themselves
with the magnetic field lines indicating the location and shape of the 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.
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Equipment
• EICO1064S Power Supply
• Iron Filings around a Wire Apparatus
• Connecting Leads
• Iron Filings
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 the case of conventional current flowing
through a wire, the right hand rule is used to determine the orientation of the field lines.
If the thumb is pointed along the direction of the current, the direction in which the fingers curl coincides
with the direction of the field lines. The magnetic field lines around a current-carrying wire, as seen in Figure 1,
are circles perpendicular to the wire. The direction of the field at a point from the wire will be tangential to the
circular field line at that point.
Wire with current
going into the page.
Figure 1: Diagram showing the concentric magnetic field lines around a current carrying wire with the current
traveling into the page.
The magnetic field strength near the wire is
B=
µ0 I
,
2πd
(1)
where B is the magnetic field strength, I is the current, d is the perpendicular distance from the wire to the point
at which the field is measured, and µ0 (µ0 = 4π × 10−7 T m/A) is the permeability of free space.
A Hall probe is used to measure the field strength at various distances from a wire with a current of 10 A.
The results are shown in the experimental curve in Figure 2. A theoretical curve calculated using Equation 1 is
also presented for comparison.
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Figure 2: Graph of the experimental and theoretical magnetic field strengths for various distances from a current
carrying wire.
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 also align parallel to an external magnetic field. These
materials then behave as bar magnets. In the case of a current-carrying wire, the iron filings align themselves in
circular patterns around the wire.
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References
[1] Freier, G. D. and Anderson, F. J. A Demonstration Handbook for Physics, ”Ei-9. Magnetic Field Around a
Wire”, 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, ”31–1.7”, American Association of Physics
Teachers, The Ronald Press Company, New York, 1970. pg 920-921.
[3] Knight, Randall D. Physics for Scientists and Engineers, ”The Magnetic Field of a Current”, 2nd ed, California:
Pearson Addison-Wesley, 2008. pg 1005-1009.
[4] Sutton, Richard Manliffe. Demonstration Experiments in Physics, ”E-123. Magnetic Field about Various
Conductors”, McGraw-Hill Book Company Inc., New York and London, 1938. pg 301-302.
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