Simple Electric Circuits

Transcription

1 Simple Eletri Ciruits Gol: To uild nd oserve the opertion of simple eletri iruits nd to lern mesurement methods for eletri urrent nd voltge using mmeters nd voltmeters. L Preprtion Eletri hrges move through eletril ondutors in response to potentil differene or voltge. The eletri urrent is the rte tht these hrges move through the iruit. Eletri urrent is mesured in mperes () or mps. Current diretion is defined s the diretion positive hrges will flow (opposite to the eletrons in the ondutors). The eletri iruits in this l re uilt with tteries, light uls, swithes, nd wires. The tteries provide potentil differene or voltge. The hrteristis nd rrngement of the light uls will determine how muh urrent flows. Series iruits re ones where ll devies re onneted end-to-end, forming single pth for eletrons to flow. The sme urrent exists lmost immeditely through ll the devies nd lso in the ttery tht is providing the voltge. Prllel iruits hve t lest two or more devies onneted in suh wy tht the sme voltge ts ross eh one nd ny single one ompletes the iruit independently of the other devies. Equipment mmeters. Eletri urrent is mesured with n mmeter. In order for the meter to mesure the urrent, the eletri hrges must flow through the meter, tht is, they enter through one terminl (the red of positive terminl) nd exit from nother terminl (usully indited y lk, or leled negtive). In iruit, n mmeter should e hooked up in series. Voltmeters. Mesuring potentil differene or voltge is done with voltmeter (whih mesures in volts). Unlike mmeters mesuring urrent t single point in the iruit, voltmeters ompre the eletri potentil t two different points in iruit. Voltmeters re hooked up in prllel in iruit. There re mny eletril omponents used in this l. Btteries, light uls, nd swithes re ll used. The shemti symols of the omponents used in this l re shown elow. V Bttery Bul Wire Swith (open) mmeter Voltmeter The wire exmple shows "juntion," where eletrilly onneted wires re indited y the solid irle. 1

2 Proedure I. Qulittive oservtions.. Without using ny meters yet, egin y uilding the simplest of iruits using the ttery pk, one light ul, nd the swith, long with neessry wires to mke the onnetions, shown in Figure 2. Ple your omponents s shown in the shemti s losely s possile. The swith is normlly open. When the swith is pushed to ring the two sides into ontt, it is referred to s eing losed. Close the swith nd oserve wht hppens nd reord you oservtion. Figure 2 B. Continue y uilding the series iruit in in Figure 3. Close the swith nd oserve wht hppens. Figure 3 Mke tle similr to Tle 1 elow nd rnk the rightness of the uls for your iruit. Let 3 represent the rightest nd 1 represent the dimmest. Bul 1 Bul 2 Bul 3 1 Bul - Series Buls - Series - 3 Buls - Series 2 Buls - Prllel - 3 Buls - Prllel Comined Tle 1 2

3 C. Build the series iruit in Figure 4. Close the swith, oserve wht hppens, nd rnk the rightness of the uls in your tle. B3 Figure 4 D. Return to the strting iruit (Figure 2) nd dd seond ul in prllel with the originl s shown in Figure 5. Close the swith, oserve wht hppens, nd rnk the rightness of the uls in your tle. Figure 5 E. dd third ul in prllel s shown in Figure 6. Close the swith, oserve wht hppens, nd rnk the rightness of the uls in your tle. B3 Figure 6 F. Lstly, uild the three-ul omined iruit shown in Figure 7. gin, lose the swith, oserve wht hppens, nd rnk the rightness of the uls in your tle. B3 Figure 7 3

4 II. Mesuring eletri urrent. *mmeters re esily dmged or destroyed y llowing urrents to flow tht re lrger thn the full-sle vlue for given setting. They re the lest durle of eletril instruments you will use in l. lwys doule hek the wiring nd rnge seletion efore operting your iruits. When using mmeters lwys egin mesuring using the lrgest urrent rnge ville (5 ). If the meter produes smll reding or defletion, then hnge the rnge to the next more sensitive setting (.5 ). Do not use rnge tht will send the meter eyond the full-sle vlue.. One gin uild the iruit in Figure 2. The mmeter will e put in the iruit t 3 different ples:,, & s shown in Figure 8. To mesure the urrent t point, the iruit must e dis-ssemled it so tht the mmeter in the iruit is in series with the light ul s shown in Figure 8. Be sure to onnet the mmeter so tht urrent enters (one of) the + or red terminls nd exits the or lk terminl to the rest of the iruit. I I + - Figure 8 Figure 8 Mesure I, the urrent leving the ttery nd entering the mmeter nd reord on your dt worksheet. Tke the mmeter out of the iruit nd ssemle it t point to mesure I nd ssemle it gin t point to mesure I. How re I, I, nd I relted in this iruit? B. Mke nd reord urrent mesurements for the series iruit S2 t the lettered points in Figure 9. Figure 9 4

5 From your oservtions of the urrents in these series iruits, nswer the following questions. 1. How is the rightness of the ul relted to the urrent flowing? 2. Wht n you sy out the urrent flowing through eh ul in series iruit? C. Mke nd reord urrent mesurements for the prllel iruit P2 t the lettered points in Figure 10. Hooking up the mmeter t points nd n e little triky so you might wnt to hek with your T to mke sure the mmeter is hooked up orretly. d Figure 10 From your oservtions of the urrents in this prllel iruit, nswer the following questions. 1. How does the urrent through Bul 1 ompre to the urrent through Bul 2? 2. Wht is the likely mthemtil reltionship etween I, I, nd I? D. Mke nd reord urrent mesurements for the omined iruit t the lettered points in Figure 11. Figure 11 From your oservtions of the urrents in this omined iruit, nswer the following questions. 1. If ul is not glowing, does tht men there must neessrily e no urrent flowing? Explin. 2. Test your mthemtil reltionship etween I, I, nd I tht ws developed for prt C. 5

6 III. Mesuring voltge. Unlike hooking up n mmeter, voltmeter n e used without disssemling the iruit. The voltmeter leds or proes n e tthed (or touhed) t two different points in the iruit. The voltmeter reports the potentil differene, ΔV, etween the two proe lotions, V + - V - or V red V lk. This differene is usully lled simply the voltge or the voltge drop etween the points. It is good ide to use olor-oded proes or leds with the voltmeter to keep the proper sense of polrity in the mesurements. Let s represent these differenes y simple nottion: V B = V V B, so tht V B would e mesured y pling the red (+, or high) proe t point nd the lk (- or low) proe t point B. The rel voltmeter used here is digitl multimeter tht n e used to mesure voltge, urrent, nd resistne. Be sure to selet the pproprite funtion DC voltge mesurement, not C nd suitle rnge for the mesurements t hnd. sk your l instrutor for ssistne in interpreting the vrious ions on the meter if neessry. Be sure not to use urrent-mesuring sle!. Begin with the simple one-ul iruit gin nd with the swith losed mesure nd reord the voltge drop V BC ross the light ul s in Figure 12. B V D C Figure 12 lso mesure the voltge drops long the wire from the + ttery terminl to the light ul, V B, nd from the ul k to - terminl of the ttery, V CD. To finish, mesure V D, nd V D (reversing the roles of red nd lk leds). Now mesure the ove vlues when the swith is open. List ll mesurements (swith losed nd open) in tle. From your oservtions of the voltges in this simple iruit, nswer the following questions. 1. How re V D nd V D relted? 2. How does V B + V BC + V CD ompre to V D? 3. Wht do you get if you sum up the voltge drops one round the iruit? (V B + V BC + V CD + V D =?) 4. With the swith losed does it pper resonle to neglet the voltge drops V B nd V CD long the wire segments? 6

7 B. For the series iruit S2 mesure the voltge drops ross eh light ul nd the ttery gin. Refer to Figure 13 nd mesure V B, V BC, nd V C round the iruit with the swith losed. Wht is the sum of the voltge drops round the iruit? B C Figure 13 C. For the prllel iruit P2 (Fig. 5 or Fig. 10) mesure the voltge drop ross eh ul nd the voltge drop ross the ttery nd reord. How do the voltge drops ompre? D. Mke voltge drop mesurements ross ll uls for the omined iruit, V B, V BD, V CD, nd V D (Figure 14). B C D Figure 14 From your oservtions nd mesurements of voltges in the omined iruit nswer the following questions. 1. Find the sum of the voltge drops long pth -B-D (V B + V BD ). Compre this sum to the voltge supplied y the ttery, V D. 2. Find the sum of the voltge drops long pth -B-C-D (V B + V CD ). Compre this sum to the voltge supplied y the ttery, V D. *When finished with your l len up your l sttion. Mke sure you put ll of your wires wy. Homework 1. Wht n you sy out the eletri urrent tht flows through two or more uls onneted one fter nother in series iruit? 2. Wht n you sy out the urrent flowing into juntion of 3 wires ompred to the urrents tht flow out from the juntion? Stte Kirhhoff's juntion rule nd ompre. 3. Stte Kirhhoff's Loop rule. How does this relte to the omined iruit in prt III D? 7