INVESTIGATORY PROJECT Aim: To To study various factors on which the internal resistance/EMF of a cell depends.
Concepts: Internal Resistance of a cell 1. When a cell connected in a circuit is made to send current through the circuit by closing the key, current ows from the positive pole to the negative pole of the cell in the e!ternal part of circuit and ows from the negative pole to the th e positive pole through the electrolyte inside the cell. "uring the passage of the current through the interior of the cell, electrolyte o#ers some resistance to the ow of current. The resistance ofered by the electrodes and the electrolyte to the passage o current through the interior o the cell is called the internal resistance o the cell. It is donated by the symbol ‘r’. E.M.F. o a cell (E). The maimum potential diference that 2. E.M.F. e!ists between the terminals of a cell, when cell is in open circuit i.e. when it is not sending any current through the circuit is called e.m.f. e.m.f. of the cell. $t is donated by the symbol %E&. Terminal potential diference (!). The (!). The potential di#erence that e!ists across the terminal of a cell when the cell is sending current in the circuit, is called the terminal potential di#erence of the cell. $t is donated by the symbol %'&.
3. "elation
#et$een E and !. When a cell of terminal resistance %r& is sending current $ through a circuit, then E(')$.r
Factors on $hich the Internal "esistance o a cell depends. . Factors on $nternal resistance of a cell depends upon the following factors* i. distance bet$een electrodes ii. common area o electrodes inside the electrolyte iii. nature o the electrolyte iv. iv. amount o current dra$n rom the cell.
Nat!re of "epen#ence on: i.
ii.
iii.
iv.. iv
"istance $et%een Electro#e s. $nternal resistance of a cell is directly proportional to the distance between the electrodes i.e. the length of the electrolyte through the current passes through the cell. Common Area of Electro#es. $t is inversely proportional to the common area of electrodes +or plates dipping in the electrolyte. Nat!re of Electrol&te. $nternal resistance of a cell is inversely proportional to the speci-c conductivity of the electrolyte. peci-c conductivity is reciprocal of speci-c resistivity or speci-c resistance of the electrolyte. Amo!nt of C!rrent "ra%n from a Cell. $nternal resistance of a cell also depends upon the amount of current drawn from the cell. eyond a certain critical value of current drawn from a cell, its cell, its internal resistance
increases with the increase in the value of current drawn from it. There is, however, no de-nite mathematical relation between the two.
Apparat!s 0n improvised simple voltaic cell, a multimeter, a resistance bo! +1 2 314 range, a plug key, beakers of 311 m5 and 611 m5 capacity, high resistance voltmeter. voltmeter.
"escription of t'e impro(ise# Primar& Cell 7. T 7. Take ake a beaker beaker of capacity 311 m5 and paste on it a vertical strip of a cm graph paper such that the lower edge of strip touches the bottom and upper edge touches the top of the beaker. beaker. Mark on the graph strip, the distances in centimeter from bottom to the top +Fig. +Fig. "2 7.7. 6. Electro#es: Take Take two plates of si8e 3 cm 9 76 cm 9 7 mm each cut out of sheets of copper and 8inc. :ne threaded bolt of length ; cm and diameter about < mm is soldered in the middle of each of the plates. Each of the bolts is provided with a pair of tightly -tting nuts and a connecting terminal at the top. <. 0 thin lid of wooden sheet +ply with a rectangular slot of width slightly more than the diameter of bolts +i.e., about <.3mm and length about 76 cm, is taken taken and metallic plates with their parallel faces facing each other, are -tted into the slot of the lid and tightened by the nuts. 7 molar solution of =>:? is -lled in the beaker up to +@ th of its height and the metallic plates +called electrodes are dipped in the solution without
touching the bottom of the th e beaker. beaker. This arrangement +shown in Fig. "27.7 is your improvised cell needed for the activity.
T'eor&: The di#erence between the e.m.f. e.m.f. +E +E of a cell and its terminal p.d. +! is governed by the relation, E%!&I . r where r is the internal resistance of the cell. T'e increase in t'e #i)erence *E+V, for t'e same c!rrent s'o%s an increase in internal resistance an# (ice (ersa.
Proce#!re: A. E)ect of c'an-e of #istance $et%een t'e plates: 7. Aeep eep a dista istanc nce e of abo about 71 cm in betwe etween en the the plates, dip then completely in the solution and -! them in position with the help of nuts BC and B>. 6. T 6. Take ake out a suitable resistance of @ ohms from the resistance bo! +D... lug in the key A and measure the terminal potential di#erence (!) with (!) with the help of a high resistance voltmeter of a multimeter.< <. :pen the circuit by taking out the plug from the key A and again measure the drop of potential across the terminals of the cell in the open circuit. This p.d. gives the e.m.f. (E) of the cell. @. Bow change the distance between the plates to 3 cm and repeat the steps 6 and < taking out a suitable
resistance from D.. such that the current is same as in step 6. 3. T 3. Take ake three more sets of observations, keeping the separation between plates as cm, @ cm and 6 cm. . Decord Decord your observations as detailed below *
O$ser(ations it' c'an-e of "istance $et%een Electro#es Ta$le 1.1 No. of O$s.
7. 6. <. @. 3.
"istance $et%een electro#es *cm, 71.1 ;.1 .1 @.1 6.1
V *(olt,
E *(olts,
"i)erence *E+V, *(olt,
Inference
/. "epe "epen# n#en ence ce on on Comm Common on Are Area a of Electro#es
Proce#!ral Steps 0. Aeep a -!ed distance between the electrodes +or plates say 3 cm and keep top edges of the plates Gust immersed in the electrolyte. . Take out a suitable resistance from the resistance bo! and measure the terminal p.d. (!) by the voltmeter. 2. :pen the circuit by taking out the plug from key A and measure e.m.f. e.m.f. of the cell (E) by multimeter or voltmeter. voltmeter. 13. ull both the plates out of the electrolyte by 6 cm so that the common dipped area in the solution decreases. Bow repeat the steps ; and H. ake two two more more sets sets of obse observa rvati tion ons s by furthe furtherr pull pullin ing g 11. Take electro electrodes des out of the electro electrolyt lyte e suitably suitably and repeat repeat steps steps ; and H again.
14. 14. Decord your observations as given below*
5or #epen#ence area of plates electrol&te.
on common insi#e t'e
Ta$le a$le +1.4 +1.4 No. of O$s.
1. 4.
6en-t' of electro#es insi#e electrol&te 7 *cm,
V *(olt,
E *(olt,
"i)erence *E+V, *(olt,
Inference
8. . 70 decrease in length of electrode dipped in electrolyte will decrease the e#ective area of electrodes inside the electrolyte. electrolyte.
C. "epe "epen# n#en ence ce on t'e Conc Concen entr trat ation ion of Electrol&te 18. 5ower down the plates suitably in the solution and set the gap between the plates about 3 cm. 5et the concentration of the electrolyte elect rolyte solution be 7 molar +$M II to start with. $f it is not so, check up the concentration of the acid solution and prepare it accordingly. 1. Take out suitable resistance from the resistant bo! and measure the terminal potential di#erence ‘!’ and e.m.f. ‘E’ of the cell as e!plained earlier. 19. our out solution of the cell in a bigger graduated cylinder and add calculated Juantity of distilled water to increase the volume of the solution so that the concentration reduces to 1.; M. and E of of the cell. 1. Depeat step +7@ for measuring ! and E Take two more sets of observations with concentration 10. Take of 1. M and 1.@ M, of the electrolyte. 1. Decord your observations as detailed below*
O$ser(ations: it' c'an-e of concentration of electrol&te Ta$le "+1.8 No. of O$s. 1. 4. 8. .
Concentratio n *molar,
V *(olt,
E *(olt,
Inference
Concl!sion 7.
$nter $nterna nall resi resist stan ance ce of a prim primary ary cell cell incr increas eases es with with the the increase in distance between electrodes. 6. $nternal resistance increases with decrease in common area of electrodes dipped in the electrolyte. <. $nternal resistance increases with decrease in concentration of the electrolyte.
Preca!tions 1. 0ll connections in the circuit should be neat and tight. 0ll plugs in resistance bo! should also be tight. 4. ositive terminal of the voltmeter should be connected to the positive terminal of the cell. 8. To decrease the common area of the electrodes dipped in the electrolyte, the plates.