Science Help

Measure Internal Resistance

 

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Resistance:

Whenwe apply a potential difference between two ends of a conductor, an electric field will be set up inside the conductor and as a result current flows through the conductor.

 

Theapplied potential difference is proportional to the current flowing in the conductor. If V is the potential difference applied and I is the current then

 

          V α I

 

Or     V = RI

 

HereR is the constant of proportionality called resistance. Resistance is characteristic of the material of the conductor. Resistance is given by the equation

 

                 R = V / I

 

Units: Volts / ampere or ohms (Ω)

 

Thevalue of resistance of a conductor depends upon (a) nature of the material, (b) its dimensions (length & cross section area), (c) temperature of the conductor.


Internal Resistance of the battery


 

Theinternal resistance of a cell is due to the electrolyte between the anode and cathode. The magnitude of the internal resistance depends upon

a) Distance between the plates

b) Area and size of the plates

c) Strength of the electrolyte

d) Temperature

 

Theinternal resistance of a cell can be accurately measured using potentiometer. For an ideal cell the internal resistance is zero.

 

    

 

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Ifan external resistance of R is connected across the two terminals of the battery, then the current flows in the circuit through the external resistor and also the internal resistance ‘r’. If ‘I’ is the current through the circuit then

 

     I = V/ R  and  I = ε / (R +r)

 

Internal resistance of the cell is

 

       r = (ε /I ) – R ………(1)

 

Dueto the current flow, there will be potential drop Ir across the cell. The terminal voltage due to the potential drop across the cell is

 

    V = ε – I r

 

The internal resistance r

 

    r = (V – ε ) / I  ………(2)

 

Either from eq. (1) or (2) we get the value the value of internal resistance of the cell.