MEASUREMENTS OF ARYING OLTAGES Measuremens of varying volages are commonly done wih an oscilloscope. The oscilloscope displays a plo (graph) of volage versus imes. This is done by deflecing a sream of elecrons (which has been acceleraed from he back of he insrumen o he fron) boh horizonally and verically before i srikes a fluorescen screen. The amoun of verical deflecion is deermined by he volage under sudy; he horizonal deflecion is proporional o he ime elapsed since he sweep began. The oscilloscope always responds o volage, bu hese volages are ofen characerisic of oher phenomena. For example, a microphone changes he variaions in air pressure associaed wih he sound o changes in volage ha can hen be observed wih he oscilloscope. The oscilloscopes you will use in his lab all have common feaures. Each is a riggered dual race D.C. coupled scope. Dual race means ha wo races, or beams, can be viewed simulaneously. Each inpu volage goes ino is own amplifier and is displayed on he screen independenly of he oher. Triggered means ha he elecrons sweeping across he screen begin heir journey from he lef side only when "old o" or "riggered." There are several ways o rigger he beam. For many, bu no all, of our uses, he beam will be riggered by he inpu volage. If he riggering adjusmen is no se correcly, he paern will drif across he screen and will no be "locked in." (You will probably find his adjusmen a bi of a mysery. Don' worry ask your insrucor if adjusmens are needed.) D.C. coupled means ha any inpu volage, including a consan volage, causes a deflecion. (In A.C. coupling, only variaions cause a deflecion; he average of he display is zero). Abou he only hing you can do o hur a scope is o leave a brigh do a one place on he screen for a long ime. Also, avoid puing a sharp pencil or pen on he screen as his scraches he surface. 1. Spend some ime sudying he fron panel of he scope. There is a myriad of swiches and knobs. Locae he following imporan ones: a) On Off (or A.C. on) b) Inensiy or brighness c) Focus d) erical Gain, in vols/div (one for each channel) e) Horizonal Sweep rae, in ime/div f) D.C. A.C. inpu swich Be sure you undersand he marking on he verical and horizonal conrols. Each mus be se o he CAL posiion before he marked values will be accurae. 2. Turn on he scope. Afer a minue or so, you should have a focused line cenered on he screen. Turn he sweep rae o is slowes value. Wach he do move across he screen. Now sep by sep, increase he speed of he elecron beam's ravel across he screen. The spo will develop a come like ail which finally seems o urn ino a coninuous line. Remember ha his line is an illusion caused by he persisen glowing of he phosphor afer being hi by he elecrons. There is sill a do of elecrons moving across, now very rapidly. Coninue o increase he sweep rae. The line will ge very dim a he highes raes because he ime spen a any place on he screen decreases. 3. Se he verical gain o.5/div and he inpu o D.C. Read he volage of a 1.5 vol baery on he oscilloscope. Try i wih he opposie polariy of he baery by reversing he leads. Wha would happen if he gain were changed o 1/Div? Try i and find ou if wha you expec indeed happens. Now se he INPUT o AC and ry o measure he baery. Wha happens?
4. A signal generaor produces a volage which varies wih ime. The ype of variaion is characerized by a plo of he volage versus imes. The plo may look like a sine wave a square wave a riangular wave or some oher form. The frequency of he signal is he repeiion rae of he regular paern (frequency is he inverse of he period, which is he ime beween adjacen peaks). The ampliude of he signal is he maximum volage of he signal (half he peak o peak value). (See he figure below). period ampliude peak o peak 2
a) Turn on he signal generaor. Make sure i is se for sine oupu. Se he scope inpu o A.C. Connec he oscilloscope cable o he oupu of he generaor. You may have o vary he sweep speed (horizonal on he scope) o spread he paern enough o make a frequency measuremen and you may need o vary he verical gain on he scope o make i fi nicely verically on he screen. If you have rouble geing a sable paern, see your insrucor. As you are making hese changes in gain and sweep speed, he paern will change; are he characerisics of he inpu signal changing? Based on your measuremens wih he scope, wha are he period, frequency, ampliude and p p volage of he signal you have? b) Now connec a small speaker o he oupu of he signal generaor and leave he scope also conneced o he oupu. Now he same signal ha he oscilloscope looks a is driving he speaker, convering i o sound waves of he same frequency. Do you hear he one? (If you don', don' panic ye). c) Find he conrols on he signal generaor which would change he frequency of he signal. You should be able o observe on he oscilloscope when you have found he righ conrol. Now find he conrol on he signal generaor which would vary he ampliude of he signal. Again, you should be able o observe on he scope when you have he righ conrol. Adjus hese unil you can hear a one; large enough ampliude (loud enough) and a frequency you are sensiive o. Change he signal frequency and noice he effec i has on he display and he pich of he one. Make sure you undersand how he display changes as you go from low o high frequency and vice versa. In going over he range of audible frequencies, you may have o change he sweep rae of he oscilloscope. When you do, make sure you undersand ha he display looks differen because of a scale change and no necessarily a change in frequency of he signal. Find he highes and lowes frequencies you can hear. d) Now ry o use your speaker as a microphone. In (b) and (c) you supplied a varying volage o he speaker and ha caused he speaker o vibrae. Suppose you cause he speaker o vibrae (by apping i or "hiing" i wih sound waves). Would i oupu a volage? Disconnec he speaker from he signal generaor so ha only he speaker is now conneced o he inpu of he oscilloscope. See if you can deec any volage oupu from he speaker as you ap i or alk ino i. (You may need o increase he sensiive by changing he verical gain of he scope.) Aciviy 2: RC Circuis Exponenial Decay. This aciviy uses a circui similar o he following: PS C R Single pole double hrow swich (SPDT) The swich is firs hrown so ha he power supply is aached o he capacior. The capacior is charged unil he volage across i equals ha of he source. The swich is hrown he oher way so ha he power supply is disconneced and he resisor is aached. This gives a pah for he capacior o discharge. Since he rae a which he charge leaves he capacior is proporional o
Page 4 of 5 he amoun of charge on he capacior, his discharge is exponenial. Theory gives he proporionaliy consan as 1/RC. (Refer o your ex book for he heory.) 1. Wih he circui assembled, connec a mulimeer across he capacior. Flip he swich o he power supply and observe he charging of he capacior. When fully charged, flip he swich o he resisor and observe he decay in he poenial difference across he capacior. 2. To faciliae he collecion of daa of he decay, we will read he volage across he capacior wih he compuer inerface and save he values a regular ime inervals. a. Turn on he inerface box. b. Connec he leads of he volage probe across he capacior. (Leave he mulimeer aached, oo.) c. Sar he Science Workshop program. (Double click is icon on he deskop) d. Selec File and Open. Open he file RCLAB locaed in folder PHY152 under Sciwkshp. e. Click on MON and observe he volage reading in he digial display on he screen as you flip he swich firs o he power supply and hen o he resisor and a graph of he volage as a funcion of ime. STOP before he daa on he graph reaches he maximum ime shown on he graph. The values on he screen should be abou he same as hose shown on he mulimeer. 3. You are now ready o record real daa. a. Flip he swich o he power supply and charge he capacior. b. Flip he swich o he resisor and click on record immediaely (simulaneously, if possible, bu afer he swich is hrown)! The value of he volage across he capacior is recorded every 0.5 seconds. c. Click on sop jus before 15 seconds (he end of he graph). d. Repea if necessary. 4. Copy he daa o a spreadshee using he following procedure: a. There is also a able display of your daa. Click on ha o make i acive. The second column in he able is your daa. (If he daa displayed in he able is no ha of your bes Run, hen click on he small Daa buon in he heading of he column o selec he run wih your bes daa.) Then click in an open area of he column heading and he enire column will be highlighed. b. Go o EDIT and click on Copy. c. Minimize he Science Workshop by clicking on lef buon in he upper righ corner of he screen. d. Make EXCEL acive by clicking on is buon on he askbar a he boom. Click on File and selec New, hen OK. e. In he Spreadshee, click on cell A4. Then in EDIT click on Pase. Your daa of ime and volage should now be in he spreadshee. f. Save he Spreadshee as RCXXX in he direcory c:\152lab, wih your lab period for XXX (WED, THR) as before. 5. Analyze he daa: a. Make a linear scaer (x y) graph of your ime and volage daa. (NOTE: When you selec he wo columns of daa, DO NOT highligh he column headings; only highligh 4
Page 5 of 5 he numbers.) b. Fi an exponenial rendline o he daa and display he equaion. Wrie he equaion below and hen rewrie i in erms of your physical variables, and : c. From he equaion, deermine he value of he capacior. (Hin: You can measure he value of he resisor wih your mulimeer!) d. Change he graph display o a logarihmic scale for he verical. Add minor gridlines by clicking on he graph area (plo area) and hen clicking he righ mouse buon. In he menu selec Char Opions, hen he gridline ab and hen check minor gridlines. Now reading from your graph, wha is he ime i akes for he volage o be decreased by onehalf (a half life )? e. How does his value compare o he heoreical value? (Recall ha doubling or half imes are given by T =.693/ k, where k is he proporionaliy consan.) f. Prin your spreadshee. 5