Physics 2020 Lab: Magnetism page 1 of 8 University of Colorado at Boulder, Department of Physics Lab: Magnetism PART I: INTRODUCTION TO MAGNETS This week we will begin work with magnets and the forces that they produce. By now you are an expert on setting up circuits, and we will look at the interaction between magnetic fields and flowing current. The goals of this lab are to see how magnetism is created by and acts on electrical currents, to learn two ways to use the right-hand rule in magnetism, and to see some real-world examples of magnetism. Magnetic fields are caused by moving charges – sometimes by charges moving on the atomic level (electrons moving around atomic nuclei, for example), and sometimes moving on a macroscopic scale, such as through the wires in an ordinary circuit. Similarly to how electric fields are both produced by and act on charged particles, magnetic fields are both produced by and act on moving charges. The unit of measurement of magnetic field is the Tesla (the earth’s magnetic field is about 0.00005 Tesla, and a refrigerator magnet creates a field of about 0.01 Tesla). 1 Tesla = 1 T = 1 N / (A*m) In this lab you will be using bar magnets as the source of the magnetic field. The bar magnets each have two poles (North and South), but they are not labeled. Once you determine which end of the magnet is North and which is South, be sure to keep track of it! IMPORTANT: The convention for magnetic field lines is that they point away from a magnetic “North” pole, and towards a magnetic “South” pole (analogous to how electric field lines point away from positive charges and towards negative charges). Predict what the magnetic field structure would be around this bar magnet. Draw in 10 or so magnetic field lines. Predict which way a compass would point in the field of this magnet. In each of the circles, draw an arrow in the direction that a compass should point.Physics 2020 Lab: Magnetism page 2 of 8 University of Colorado at Boulder, Department of Physics Using the compass, measure the direction of the field lines near the tips of your bar magnet. NOTE that the colored compass tip points along the magnetic field direction. Does the general shape match your prediction? Can you determine which end of your magnet is North and which is South? The iron core of the earth acts like a giant bar magnet. Given that the compass needle (which points towards the geographic north), points along the magnetic field lines, draw in the magnetic field lines surrounding the earth. Once you have done this, label the magnetic poles of the giant magnet in the earth. PART II: MAGNETIC FIELD PRODUCED BY A CURRENT Given a magnet that is free to rotate in an external magnetic field, which way will it line itself up? Draw your answer below (i.e. draw the field lines, and draw the final position of the magnet with the poles labeled). The situation that you drew is exactly how a compass works – namely, a compass is just a freely-rotating little magnet which aligns itself with any external fields.Physics 2020 Lab: Magnetism page 3 of 8 University of Colorado at Boulder, Department of Physics As mentioned above, magnetic fields are also produced by moving current. The direction of the magnetic field around a current-carrying wire can be determined by the right-hand rule. Namely, if you point your thumb in the direction of the current, your fingers will curl in the direction of the magnetic field lines which surround the current. At your table, you should have the pieces to construct a “trapeze” setup similar to the picture below. NOTE: After you are done with each measurement, disconnect the battery! The trapeze should be set up to swing freely – be careful that there is no pressure on the joints that will keep it from moving. On the figure, draw the direction of the current. Using the figure above, predict and draw the direction of the magnetic field lines in the vicinity of the upward-leg of the trapeze. The four disks in the picture are supposed to represent little compasses -- draw in the compass needles for the four compasses. Connect the circuit and use your compass to check your prediction – were you correct? If not, why not?Physics 2020 Lab: Magnetism page 4 of 8 University of Colorado at Boulder, Department of Physics PART III: FORCE ON A CURRENT IN A MAGNETIC FIELD As mentioned in part I, magnetic fields produce a force on any moving charge. This can be observed in the lab by moving charges through a wire (i.e. with an electrical current). The force on a current-carrying wire in a magnetic field is given by BLIFrrr×= where Fr is the force on the wire, I is the current, Lr is the length, and Br is the magnetic field. Notice that Fr, Lr, and Br are all vectors, and the “×” sign indicates a specific kind of vector multiplication called the cross product. To figure out just the magnitude of the force, you can use the formula ()θsinILBF =r where θ is the angle between the current / length direction and the magnetic field direction. For example, if the current is perpendicular to the magnetic field, θ=90° and sin(θ)=1, so the magnitude of the force just equals ILB. Notice that if the current is doubled, the force is doubled. Notice also that if the magnetic field strength is doubled, the force is doubled. Notice also that if the length of wire is doubled, the force is doubled. In other words, the force has a linear dependence on each of the variables I, L, and B. If a 5 cm long wire carrying 2 amps is parallel to the magnetic field lines of a 0.1 Tesla field, what is the force on the wire? Make a sketch to go with your answer. To figure out the direction of the vector that results from a cross-product, you need to use the right-hand rule (note that this is the second way we use the right-hand rule for magnetism). To use the right-hand rule, aim your fingers towards the first vector in the cross product (Lr, which is taken to be the current direction) then curl your fingers in the direction of the second vector in the cross product (Br). Now extend your thumb, which will point in the direction of the cross-product ( Fr).Physics 2020 Lab: Magnetism page 5 of 8 University of Colorado at Boulder, Department of Physics In the figure below, the magnetic field lines point into the page (as indicated by the little circle with the cross in it – that symbol
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