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Current Balance revised October 2009

# Current Balance revised October 2009 - HB Current Balance...

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Current Balance Lab 7 1 i Current Balance Lab 7 Equipment Current balance apparatus, laser on tripod, paper and tape, Fluke multimeter, leads, 9 V DC from wall strip, 5 Ω 9.2 A rheostat, two 6 inch ﬂat 1 mm thick rulers, weights (50 mg, 100 mg, 200 mg) Comment: Disconnect the circuit by removing the red banana plug from the red receptacle on the wall strip. Reading Your textbook. Review the vector product (also known as the cross product). Reread Electrical Safety at the beginning of this manual. Important Information 1. This experiment uses a laser beam as an optical lever arm. DO NOT LET THE LASER BEAM OR ITS REFLECTION ENTER YOUR EYE. SERIOUS DAMAGE TO YOUR EYE MAY RESULT. 2. A wire pivots on two knife edges. The knife edges rest on ﬂat surfaces. Both the knife edges and ﬂat surfaces are easily damaged. Please handle them with care, using the centering rod, described below, to center the knife edges and to gently lower the knife edges onto the ﬂat surfaces. 3. A current passes through a rheostat (variable resistor). At the higher currents one end of the rheostat gets quite hot. DO NOT TOUCH THIS END OF THE RHEOSTAT. 1 Introduction The French Physicist Andre Marie Ampere noticed that when one wire carrying an electrical current is brought near a second wire also carrying a current the two wires experience forces. The force is proportional to the product of the two currents and depends on the relative orientations of the wires. A moving charge in one wire must exert a force on a moving charge in the other wire. The force between the two wires is the sum of the individual forces between the moving charges. This force has distinctly diﬀerent characteristics from the well- known Coulomb force between charged particles. The force between charged particles due to their motion is called the “magnetic” force. In this experiment you will verify that the force between two wires is proportional to the product of the currents, and estimate the proportionality constant involved. 2 Description of the Experiment The experiment is similar in principle to the Coulomb balance and indeed uses the same platform. See Fig. 1. A single horizontal rod or wire is pivoted so that it can move freely up and down. Below this wire is a long 10 turn rectangular coil whose long sides are horizontal and parallel to the single wire. One long side of the coil is substantially closer to the single wire than the other long side of the coil. The single wire and coil are connected in series and a current passed through them so that this current is in opposite directions for the single wire and nearby long side of the coil, and in the same direction for the single wire and faraway long side of the coil. The net force between wire and coil is a repulsion. With zero current, the vertical position of the single wire is adjusted to a convenient value. A weight is then added to a pan on the single wire and a DC (constant) current established so that the position of the single wire returns to its initial value. This procedure is repeated for a

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Current Balance revised October 2009 - HB Current Balance...

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