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The device which converts chemical energy into electrical energy is known as electric cell. Cell is a source of constant emf but not constant current.
Emf of cell (E) The potential difference across the terminals of a cell when it is not supplying any current is called its emf.
Potential difference (V) The voltage across the terminals of a cell when it is supplying current to external resistance is called potential difference or terminal voltage. Potential difference is equal to the product of current and resistance of that given part, i.e., V = iR.
Internal resistance (r) In case of a cell the opposition of electrolyte to the flow of current through it is called internal resistance of the cell. The internal resistance of a cell depends on the distance between electrodes (r d), area of electrodes [r (1/A)] and nature, concentration (r ∝ C) and temperature of electrolyte [r (1/temp.)].
A cell is said to be ideal, if it has zero internal resistance.

Cell in various positions

Closed circuit Cell supplies a constant current in the circuit.
  • Current given by the cell, 70734.png.
    Fig. 16
  • Potential difference across the resistance, V = iR
  • Potential drop inside the cell = ir
  • Equation of cell E = V + ir (E > V)
  • Internal resistance of the cell, 70742.png
  • Power dissipated in external resistance (load)
    Power delivered will be maximum when R = r. So 70757.png
    Fig. 17
    This statement in generalized form is called “maximum power transfer theorem.”
  • When the cell is being charged, i.e., current is given to the cell then E = Vir and E < V.
Open circuit When no current is taken from the cell it is said to be in open circuit (Fig. 18).
Fig. 18
  • Current through the circuit, i = 0
  • Potential difference between A and B, VAB = E.
  • Potential difference between C and D, VCD = 0.
Short circuit If two terminals of cell are joined together by a thick conducting wire,
Fig. 19
  • Maximum current (called short circuit current) flows momentarily, isc = E /r.
  • Potential difference, V = 0.

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