P. J. Ouseph
University of Louisville
Physics Department
Louisville, KY
40292
502-852-0918
pjouse01@louisville.edu
Electromagnetic Spring Balance
Abstract
The
Electromagnetic Spring Balance consists of a coil (100 turns) suspended from a
spring. A pan for holding weights is attached to the coil. The bottom part of
the coil is in a magnetic field produced by two permanent magnets. A variable
dc current source is connected to the coil. The magnet, the coil, and the
spring are enclosed in a Plexiglas tube. This instrument can be used to
demonstrate magnetic force acting on a current carrying wire and to study
dependence of the force on current.
Construction of Apparatus:
The apparatus is shown in Fig.1. It consists of a rectangular coil, a spring, and a magnet inside a Plexiglas cylindrical enclosure. The rectangular coil is wound around a rectangular aluminum plate. The aluminum core of the coil is glued perpendicular to a pan that can hold weights. Using aluminum for core has the advantage of completely damping coil oscillations. The coil has 100 turns of #62 insulated wire. The width of the coil is 1.5 cm, and its length is 5.7 cm. The pan on the top of the coil is fixed to a spring attached to a copper rod. The copper rod can be moved vertically up or down to position the coil at the proper height in the magnetic field. The rod can also be rotated to orient the coil perpendicular to the magnetic field. The spring, obtained from Daedalon (EA-34), is 5 cm long and it is made of 5-mm stainless steel wire. The magnetic field is between two NdFeB permanent magnets glued to the inside of a U-shaped soft iron. The 2.5 x 2.5 cm square magnets have a thickness of 0.3 cm. These magnets are commercially available from several sources including Edmund industrial optics. The bottom of the cylinder is fixed to a base. A bubble level fixed to the base enables us to keep the spring along the axis of the cylindrical enclosure. A circular Teflon disk is fixed over the U-shaped soft iron. The coil can move up and down with little friction in the rectangular opening at the center of the disk. This eliminates any possibility of coil moving horizontally from its equilibrium position.
Use of Apparatus:
1. Demonstration of electromagnetic force
Connect the battery to the coil through a rheostat and an ammeter and observe the motion of the coil. The proportionality between the current and force can be demonstrated by rotating the potentiometer knob and thus changing current. Current changes produce changes in the position of the coil. Also we can demonstrate the direction of force on the coil reversing as the direction of the current is reversed.
Figure 1. Coil in a magnetic field. Parts of the electromagnetic spring balance are
identified by labels in figure.
2. General Physics Laboratory Experiment
The Electromagnetic Spring balance is used in our General Physic Laboratory for an experiment entitled ÒForce acting on a current carrying wire in a magnetic fieldÓ. In this experiment students determine the force acting on the coil in the magnetic field as a function of current. Procedures followed in the experiment are shown below.
Procedure:
1.
Level the electromagnetic spring balance by adjusting the
screws.
2.
Connect the battery to the coil through a rheostat and
ammeter. Connect the positive side of the battery to the red banana plug and
the negative side to the black plug fixed to the cover of the cylinder. This
assures the coil movement in the up direction for any current input.
3.
Place a 0.5-g mass in the pan. The spring will stretch and
the pan will be below the equilibrium level.
4.
Adjust the current by rotating the rheostat knob so that
the pan and the spring come back to the original position. The increased
gravitational force on the added mass in the pan and the electromagnetic force
are equal at this time. Read the current.
5.
Repeat steps 3 through 5 for masses 1, 1.5, 2, 2.5, 3, 3.5,
4, 4.5, and 5 g. Enter your data in table below.
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Mass,
m |
Gravitational
force, N |
Current,
i. |
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6.
Open the Excel Program and obtain a plot of current (A) vs.
gravitational force (N) in the Table. Obtain a fit to the plot and the slope of
the fitted line. Slope of the line = ______________.
Calculations
Gravitational force on the added weight = Fg =
mg.
Magnetic force acting on the coil in the magnetic field = Fm
= iLB, where L is the total length of the wire in magnetic field.
Total length of the wire in magnetic field = L = nw,
where n is the number of turns and w is the width of the
coil
Therefore, the magnetic force = inwB.
When the forces are balanced; that is, when the current is
sufficient to balance gravitational force,
Fm
= Fg
inwB = mg.
Obtain an equation for the slope of the i vs.m line.
Slope of the line (with proper units) = ______________
Calculate B, using the experimentally obtained slope, the
given values of n and w, and the known value of g.
B = _________________
Fig.2. A plot of current vs. mass obtained by a student