Physics 2020, Sping 2005 Lab 5 page 1 of 8 Lab 5. Magnetism PART I: INTRODUCTION TO MAGNETS This week we will begin wok with magnets and the foces that they poduce. By now you ae an expet on setting up cicuits, and we will look at the inteaction between magnetic fields and flowing cuent. The goals of this lab ae to see how magnetism is ceated by and acts on electical cuents, to lean two ways to use the ight-hand ule in magnetism, and to see some eal-wold examples of magnetism. Magnetic fields ae caused by moving chages sometimes by chages moving on the atomic level (electons moving aound atomic nuclei, fo example), and sometimes moving on a macoscopic scale, such as though the wies in an odinay cicuit. Similaly to how electic fields ae both poduced by and act on chaged paticles, magnetic fields ae both poduced by and act on moving chages. The unit of measuement of magnetic field is the Tesla (the eath s magnetic field is about 0.00005 Tesla, and a efigeato magnet ceates a field of about 0.01 Tesla). 1 Tesla = 1 T = 1 N / (A*m) In this lab you will be using ba magnets as the souce of the magnetic field. The ba magnets each have two poles (Noth and South), but they ae not labeled. Once you detemine which end of the magnet is Noth and which is South, be sue to keep tack of it! IMPORTANT: The convention fo magnetic field lines is that they point away fom a magnetic Noth pole, and towads a magnetic South pole (analogous to how electic field lines point away fom positive chages and towads negative chages). Pedict what the magnetic field stuctue would be aound this ba magnet. Daw in 10 o so magnetic field lines. Pedict which way a compass would point in the field of this magnet. In each of the cicles, daw an aow in the diection that a compass should point. Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 2 of 8 Using the compass, measue the diection of the field lines nea the tips of you ba magnet. NOTE that the coloed compass tip points along the magnetic field diection. Does the geneal shape match you pediction? Can you detemine which end of you magnet is Noth and which is South? The ion coe of the eath acts like a giant ba magnet. Given that the compass needle (which points towads the geogaphic noth), points along the magnetic field lines, daw in the magnetic field lines suounding the eath. Once you have done this, label the magnetic poles of the giant magnet in the eath. PART II: MAGNETIC FIELD PRODUCED BY A CURRENT Given a magnet that is fee to otate in an extenal magnetic field, which way will it line itself up? Daw you answe below (i.e. daw the field lines, and daw the final position of the magnet with the poles labeled). The situation that you dew is exactly how a compass woks namely, a compass is just a feely-otating little magnet which aligns itself with any extenal fields. Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 3 of 8 As mentioned above, magnetic fields ae also poduced by moving cuent. The diection of the magnetic field aound a cuent-caying wie can be detemined by the ight-hand ule. Namely, if you point you thumb in the diection of the cuent, you finges will cul in the diection of the magnetic field lines which suound the cuent. At you table, you should have the pieces to constuct a tapeze setup simila to the pictue below. NOTE: Afte you ae done with each measuement, disconnect the battey! The tapeze should be set up to swing feely be caeful that thee is no pessue on the joints that will keep it fom moving. On the figue, daw the diection of the cuent. Using the figue above, pedict and daw the diection of the magnetic field lines in the vicinity of the upwad-leg of the tapeze. The fou disks in the pictue ae supposed to epesent little compasses -- daw in the compass needles fo the fou compasses. Connect the cicuit and use you compass to check you pediction wee you coect? If not, why not? Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 4 of 8 PART III: FORCE ON A CURRENT IN A MAGNETIC FIELD As mentioned in pat I, magnetic fields poduce a foce on any moving chage. This can be obseved in the lab by moving chages though a wie (i.e. with an electical cuent). The foce on a cuent-caying wie in a magnetic field is given by F = IL! B whee F is the foce on the wie, I is the cuent, L is the length, and B is the magnetic field. Notice that F, L, and B ae all vectos, and the! sign indicates a specific kind of vecto multiplication called the coss poduct. To figue out just the magnitude of the foce, you can use the fomula F = ILB sin (! ) whee! is the angle between the cuent / length diection and the magnetic field diection. Fo example, if the cuent is pependicula to the magnetic field, θ=90 and sin(θ)=1, so the magnitude of the foce just equals ILB. Notice that if the cuent is doubled, the foce is doubled. Notice also that if the magnetic field stength is doubled, the foce is doubled. Notice also that if the length of wie is doubled, the foce is doubled. In othe wods, the foce has a linea dependence on each of the vaiables I, L, and B. If a 5 cm long wie caying 2 amps is paallel to the magnetic field lines of a 0.1 Tesla field, what is the foce on the wie? Make a sketch to go with you answe. To figue out the diection of the vecto that esults fom a coss-poduct, you need to use the ighthand ule (note that this is the second way we use the ight-hand ule fo magnetism). To use the ight-hand ule, aim you finges towads the fist vecto in the coss poduct ( L, which is taken to be the cuent diection) then cul you finges in the diection of the second vecto in the coss poduct ( B ). Now extend you thumb, which will point in the diection of the coss-poduct ( F ). Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 5 of 8 In the figue below, the magnetic field lines point into the page (as indicated by the little cicle with the coss in it that symbol is supposed to epesent the view of an aow shooting away fom you). Cuent is unning though the cicuit as indicated by the aows on the ectangula wie. On each side of the cicuit, daw an aow epesenting the diection of the foce (use the ight-hand-ule to figue out the diection). If this wie wee flexible, what would happen to it? Repeat the pevious poblem with the figue below, which is simila except that the cuent is now going in the opposite diection. If this wie wee flexible, what would happen to it? Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 6 of 8 On the figue below, daw the diection of the cuent. The figue below is a zoom-in of the bottom of the tapeze as the end of the ba magnet is moved close to the tapeze. Re-daw the cuent diection, and now daw in the magnetic field lines fom the ba magnet. Which way will the tapeze swing? (Will it swing at all?) Hook up the cicuit and ty the expeiment. Is you pediction coect? If not, why not? Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 7 of 8 Pedict what would happen if the othe end of the magnet wee used, then check you pediction wee you coect? If not, why not? Pedict what would happen if the battey was connected in the othe diection, then check you pediction wee you coect? If not, why not? PART IV: PUTTING IT ALL TOGETHER Imagine you have a flexible wie with cuent unning though it, bent into a kink as shown below. The cuent in each side of the kink will poduce a magnetic field which acts on the cuent flowing though the othe side. Will these foces kink the wie up moe o un-kink the wie? Univesity of Coloado at Boulde, Depatment of Physics
Physics 2020, Sping 2005 Lab 5 page 8 of 8 POTENTIAL EXAM QUESTIONS: 1. Which of the following statements is tue? a) Two wies caying paallel cuent ae epelled fom each othe because the electons in each wie epel the electons in the othe wie. b) Two wies caying paallel cuent ae attacted to each othe because the magnetic field poduced by each wie attacts the cuent in the othe wie. c) Two wies caying anti-paallel cuent ae attacted to each othe because the magnetic field poduced by each wie attacts the cuent in the othe wie. d) Two wies caying anti-paallel cuent exet no foce on each othe because thei magnetic fields cancel. e) Two wies caying paallel cuent exet no foce on each othe because thei magnetic fields cancel. 2. Shown below ae six configuations of fou cuent-caying wies viewed end on. Each cicle with a dot epesents cuent coming out of the page, and each cicle with a coss epesents cuent flowing into the page. Each wie caies the same amount of cuent. What is the ode of net magnetic field magnitude at the cente of each squae? a) A > D > C = E > F > B b) A = B > D = F > C = E c) D > F > E > C > A = B d) E > D = F > A = B = C e) C = E > D = F > A = B Univesity of Coloado at Boulde, Depatment of Physics