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InterviewSolution
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Explain cell, emf and internal resistance. Derive relation between potential difference emf and internal resistance. |
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Answer» <P> Solution :`rArr` As shown in figure in a glass vessel electrolyte is field in. `rArr` In this electrolyte tw" electrode are partially DIPPED. One electrode is positive electrode (P) and second electrolyte is negative (N) . `rArr` Electrodes dipped in electronde exchange electric charge in form of positive and negative ion produced asresult of positive and negative ion produced as a result of chaemical reaction. `rArr ` Potential difference between electrode A and electrolyte is`(V_(+) V_(+) gt0) ` and potential difference between electrolyte and negative electrode is `V_(-)V_(-)LT 0. ` `rArr` When current is not flowing in the circuit potential drift between two point P and N. ` V_(+) - (V_(-))` = `V_(+)+ V_(-)` `rArr` Potential difference between `V_(+) and V_(-) ` is called electromotive force (emf) of the cell, it is denoted by `epsilon`. `epsilon = V_(+) + V_(-) gt 0 `.... (1) `rArr ` Definition of emf of cell : " When unit positive charge moves from negative to positive terminal of cell due to non electric force then energy gained by charge is called emf of cell. " `. epsilon. ` is not force force but it is potential difference. `rArr`Consider resistor R connected to cell as shown in figure. `rArr `In resistor R, current I flows from C to D. `rArr ` In electrolyte steady current flows from negative terminal (N) to positive tenninal (P) and in resistance current Hows from P to N. `rArr` If resistance R is infinite then, `I = (V)/(R) = (V)/("infinite" ) = 0 ` where, V is potential difference between P and N. `therefore V = `[p.d. between p and N [ + [ p.d. between A and B ] + [ p.d. between B and N] = `V_(+) + (-Ir) + V_(-)` `= V_(+) + V_(-) - Ir` `epsilon = V_(+) + V_(-)` In open circuit condition, I = 0 `therefore V = epsilon` `rArr"" ` Thus, `epsilon` is potential difference between positive and negative electrode. `rArr` If R be finite resistance of the circuit then potential difference betweenP and N , V = `epsilon - 1r"" ` ...(2) `rArr` Practically If `epsilon gt `Ir then internal resistance of cell can be neglected. `rArr ` For different cell internal resistance is of different value . `rArr` Internal resistance of dry cell is large compared to internal resistance of electrolytic cell. `rArr` When current I flows through R, then p.d. between two END of resistance, V = IR `""` ... (3) from equation (2) and (3) IR = `epsilon`- Ir or I = `(epsilon)/(r + R)` when R = 0 MAXIMUM current tlow through cell. `therefore I_("max") = (epsilon)/(R)` Remember : (1) When cell is not in use then, V = `epsilon ` (2) In case of open circuit condition, ` V = epsilon ` (3)When cell is in use , ` ( V lt epsilon)` (4) When cell is being charged, `(V gt epsilon)` `therefore V = epsilon + Ir ` |
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