Physics 2020 Lab: Magnetism page 1 of 8Lab: MagnetismPART I: INTRODUCTION TO MAGNETSThis week we will begin work with magnets and the forces that they produce. By nowyou are an expert on setting up circuits, and we will look at the interaction betweenmagnetic fields and flowing current. The goals of this lab are to see how magnetismis created by and acts on electrical currents, to learn two ways to use the right-handrule in magnetism, and to see some real-world examples of magnetism.Magnetic fields are caused by moving charges – sometimes by charges moving onthe atomic level (electrons moving around atomic nuclei, for example), andsometimes moving on a macroscopic scale, such as through the wires in an ordinarycircuit. Similarly to how electric fields are both produced by and act on chargedparticles, magnetic fields are both produced by and act on moving charges. The unitof measurement of magnetic field is the Tesla (the earth’s magnetic field is about0.00005 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 barmagnets each have two poles (North and South), but they are not labeled. Once youdetermine which end of the magnet is North and which is South, be sure to keeptrack of it! IMPORTANT: The convention for magnetic field lines is that they pointaway from a magnetic “North” pole, and towards a magnetic “South” pole(analogous to how electric field lines point away from positive charges and towardsnegative charges).Predict what the magnetic field structure would be around this bar magnet. Draw in10 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.University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: Magnetism page 2 of 8Using the compass, measure the direction of the field lines near the tips of your barmagnet. NOTE that the colored compass tip points along the magnetic fielddirection. Does the general shape match your prediction? Can you determine whichend of your magnet is North and which is South?The iron core of the earth acts like a giant bar magnet. Given that the compassneedle (which points towards the geographic north), points along the magnetic fieldlines, draw in the magnetic field lines surrounding the earth. Once you have donethis, label the magnetic poles of the giant magnet in the earth.PART II: MAGNETIC FIELD PRODUCED BY A CURRENTGiven a magnet that is free to rotate in an external magnetic field, which way will itline itself up? Draw your answer below (i.e. draw the field lines, and draw the finalposition of the magnet with the poles labeled).The situation that you drew is exactly how a compass works – namely, a compass isjust a freely-rotating little magnet which aligns itself with any external fields. University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: Magnetism page 3 of 8As mentioned above, magnetic fields are also produced by moving current. Thedirection of the magnetic field around a current-carrying wire can be determined bythe 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 thecurrent.At your table, you should have the pieces to construct a “trapeze” setup similar to thepicture below. NOTE: After you are done with each measurement, disconnectthe battery! The trapeze should be set up to swing freely – be careful that there isno pressure on the joints that will keep it from moving. On the figure, draw thedirection of the current.Using the figure above, predict and draw the direction of the magnetic field lines inthe vicinity of the upward-leg of the trapeze. The four disks in the picture aresupposed to represent little compasses -- draw in the compass needles for the fourcompasses.Connect the circuit and use your compass to check your prediction – were youcorrect? If not, why not?University of Colorado at Boulder, Department of PhysicsPhysics 2020 Lab: Magnetism page 4 of 8PART III: FORCE ON A CURRENT IN A MAGNETIC FIELDAs mentioned in part I, magnetic fields produce a force on any moving charge. Thiscan be observed in the lab by moving charges through a wire (i.e. with an electricalcurrent). The force on a current-carrying wire in a magnetic field is given byBLIFwhere F is the force on the wire, I is the current, L is the length, and B is themagnetic field. Notice that F, L, and B are all vectors, and the “” signindicates a specific kind of vector multiplication called the cross product. To figure out just the magnitude of the force, you can use the formula sinILBF where is the angle between the current / length direction and the magnetic fielddirection. For example, if the current is perpendicular to the magnetic field, =90and sin()=1, so the magnitude of the force just equals ILB. Notice that if the currentis doubled, the force is doubled. Notice also that if the magnetic field strength isdoubled, the force is doubled. Notice also that if the length of wire is doubled, theforce is doubled. In other words, the force has a linear dependence on each of thevariables I, L, and B.If a 5 cm long wire carrying 2 amps is para llel to the magnetic field lines of a 0.1Tesla 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 resultsfrom a cross-product, you need to use the right-hand rule (note that this is the second way we usethe right-hand rule for magnetism). To use theright-hand rule, aim your fingers towards the firstvector in the cross product (L, which is taken to bethe current direction) then curl your fingers in thedirection of the second vector in the cross product (B). Now extend your thumb, which will point in thedirection of the cross-product (F).In the figure below, the magnetic field lines point into the page (as indicated by thelittle circle with the cross in it – that symbol is supposed to represent the view of anUniversity of Colorado at Boulder,
View Full Document
Unlocking...