Currents and Magnetic Forces Laboratory: Measuring Magnetic Fields and Forces, Lab Reports of Physics

Download Currents and Magnetic Forces Laboratory: Measuring Magnetic Fields and Forces and more Lab Reports Physics in PDF only on Docsity! Phys 132L Spring 2008 Laboratory 10: Currents and Magnetic Forces 1 Forces between Two Currents The force exerted by one current carrying wire on another parallel current carrying wire of the same length is F = µ0I1I2 2πd L (1) where I1 and I2 are the currents in the two wires, d is the distance between the wires and L the length of the wires. This relationship can be investigated using a current balance, whose essential working parts are two rigid parallel wires through which the same current is passed. The current balance is designed so that one of the upper wire is free to pivot and the lower wire is fixed. When currents pass through the wires in opposite directions the lower wire exerts force on the upper wire which causes it to rotate upwards. This can be counteracted by adding masses to the small balance pan on the upper wire, eventually returning the upper wire to its equilibrium position. When the balance is in equilibrium, the gravitational force due to the additional mass equals the force exerted by the lower wire on the upper wire. a) The distance between the two currents can be measured by noting how far the reflected laser beam moves when the upper wire is completely depressed against the lower wire. Note the equilibrium position of the beam, place a coin on the mass pan and measure the distance, ∆y through which the beam moves on the wall. Geometrical reasoning shows that d = ∆y 2D b where D is the distance from the pivot point to the wall, and b is the distance from the pivot point to the wire. Determine d. b) Predict the current required to balance the current balance arm when there is 10mg masspiece in the balance pan. Repeat this for the case where there is 20mg masspiece in the balance pan. c) Set up the current balance with no additional mass in the pan. Record the equilib- rium point by marking the position of the reflected laser beam on the wall. Measure the distance between the two wires and the length of the upper wire. d) Place a 10mg masspiece in the balance pan. Adjust the current until the arm balances and measure the current. Reverse the direction of the current through the apparatus (this should partially cancel the effects of the earth’s magnetic field) and again balance the arm and measure the current. Determine the average current and calculate the difference between this and the predicted current. e) Repeat the previous part using a 20mg masspiece in the balance pan. 2 Force Exerted by Current Carrying Coil on a Magnet: Determining the Earth’s Magnetic Field This experiment uses a circular current-carrying coil with N complete loops of wire. The magnetic field at the center of the loop points along the axis of the loop in a direction given by the right hand rule. The magnitude of the magnetic field at the center of the loop is given by B = N µ0I 2R (2) where R is the radius of the loop and I the current through the loop. This can be used in the following way to determine the earth’s magnetic field. a) Place the compass at the center of the loop and orient the loop so that the plane of the loop lies along the north-south direction. b) Suppose that current passes through the coil. Indicate the two possible directions of the magnetic field produced by the coil. In each case, sketch qualitatively the net magnetic field vector at the center of the loop, indicating the contributions from the earth’s magnetic field vector and the magnetic field vector produced by the coils. c) Adjust the current through the coil so that the compass needle deflects by about 20◦. Measure the current through the coil, determine the magnitude of magnetic field produced by the coil and use this to determine the magnitude of the earth’s magnetic field. d) Repeat the previous part for compass needle deflections of 30◦, 40◦, 50◦, and 60◦. e) Determine an average value for the earth’s magnetic field based on your measure- ments. 3 Magnetic Field Produced by a Helmholtz Coil Standard electromagnetic theory predicts that the field at the center of a pair of Helmholtz coils is B = µ08NI 5 √ 5R (3) where N is the number of coils and R is the radius of the coil. The aim of this experiment is to check this relationship. a) Align the Helmholtz coils so that they are parallel to the north-south direction. Adjust the current in the coils so that the compass deflection is about 45◦. Measure deflection of the compass needle and the current in the coils. b) Based on the measurement of the earth’s magnetic field and the compass deflection, calculate the magnetic field produced by the Helmholtz coils. 2