Two aluminum strips with a mass of 2.6 g/cm^2, length 23 cm and width 2 cm are used to suspend a thin aluminum bar, which is placed above a magnet. A voltage is then applied to the circuits consisting of the strip and bar, and the resulting displacement of the bar from its original equilibrium position is observed.

• From this information and some assumptions we can estimate the magnetic field of the magnet.
• We begin by estimating the resistance of the aluminum strip.
• The resistance of the strip will be proportional to the length of the strip and inversely proportional to its area (more length implies less potential gradient and less area implies fewer electrons available per unit of length).
• The constant proportionality is called the resistivity and is a designated by the Greek letter `rho. For aluminum we have `rho = 2.65 * 10^-8 ohm meters.
• We therefore obtain a resistance estimate of .011 ohms.

The resistance of the aluminum bar, with its much greater cross-sectional area, will be considered negligible.

• Under the circumstances, as shown below, we would expect a current of approximately 180 amps.

A current of 180 C / s through a 4-volt potential difference would imply a 720 watt power requirement.

• Since the generator was cranked by hand, this power requirement is obviously way too high to be accurate.

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The .7 cm deflection of the aluminum bar, which has a mass of 2.2 grams, is easily found to result from a force of .0006 N.

• This force is found from the proportionality displacement / length = required force / weight for a pendulum with a small displacement.

The force exerted when a current I runs perpendicular to a magnetic field B whose effect is felt for distance L is equal to the product of the current, the distance and the magnetic field.

• The precise relationship is F = I L B.
• Assuming that the magnetic field acts over a distance of approximately 1 cm, we solve this relationship for the magnetic field B, using the 180 A current, and find the magnetic field is approximately 3 * 10^-4 N / (amp meter), or .0003 Tesla.
• The current, however, is greatly over estimated and the magnetic field is somewhat higher.
• A more realistic estimate of the current might be about 1/10 of this estimate, making the magnetic field 10 times greater, or about .003 Tesla.