RRB ALP Stage-2 exam: Physics notes- CURRENT ELECTRICITY (PART 5)


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METER BRIDGE

  • The meter bridge consists of a wire of length 1 m.
  • The wire is clamped between two thick metallic strips bent at right angles.
  • The end points where the wire is clamped are connected to a cell through a key. 
  • One end of a galvanometer is connected to the metallic strip midway between the two gaps. The other end of the galvanometer is connected to a ‘jockey’. 
  • The jockey is essentially a metallic rod whose one end has a knife-edge which can slide over the wire to make electrical connection.
  • R is an unknown resistance whose value we want to determine. It is connected across one of the gaps. 
  • Across the other gap, we connect a standard known resistance S.



  • The length AD= l₁ and DC = (100-l₁).
  • Resistance of AD=R cm l₁ where R cm is the resistance of the wire per unit centimeter.
  • Resistance of DC=R cm (100-l₁).
  • The four arms AB, BC, DA and CD [with resistances R, S, R cm l₁ and R cm (100-l₁)] form a Wheatstone bridge with AC as the battery arm and BD the galvanometer arm.
  • When there is no deflection in the galvanometer, the balance condition of meter bridge gives the equation:

POTENTIOMETER

  • It is basically a long piece of uniform wire, sometimes a few meters in length across which a standard cell is connected.
  • A current I flows through the wire which can be varied by a variable resistance (rheostat, R) in the circuit. Since the wire is uniform, the potential difference between A and any point at a distance l from A is
E(l) = ⌽l

where ⌽ is the potential drop per unit length.

An application of the potentiometer is to compare the emf of two cells of emf ε1 and ε2 which is given by the equation:
E1/E2 = l1/l2
  • The potentiometer has the advantage that it draws no current from the voltage source being measured. 
  • Potentiometer is also used to measure internal resistance of a cell. 


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