potentiometer

NAMDEO PHYSICS Potentiometer Principle When a constant current is passed through a wire of uniform area of cross-section, the potential drop across any portion of the wire is directly proportional to the length of that portion. Construction Consists of a number of segments of wire of uniform area of cross-section Small vertical portions are made of thick metal strip connecting the various sections of wire. A rheostat is connected to the circuit, which can vary the amount of current flowing in the wire. Applications of Potentiometer Comparison of emf of two cells E1, E2 are the emf of the two cells. 1, 2, 3 form a two way key. When 1 and 3 are connected, E1 is connected to the galvanometer (G). Jokey is moved to N1, which is at a distance l1 from A, to find the balancing length. Applying loop rule to AN1G31A, Φ l1 + 0 − E1 = 0 (1) Where, Φ is the potential drop per unit length Similarly, for E2 balanced against l2 (AN2), Φ l2 + 0 −E2 = 0 (2) From equations (1) and (2), Measures internal resistance of a cell The cell of emf E (internal resistance r) is connected across a resistance box (R) through key K2. K2 − open, balance length is obtained at length AN1 = l1 E= Φ l1 (3) K2 − closed Let V be the terminal potential difference of cell and the balance is obtained at AN2 = l2 ∴ V = Φ l2 (4) From equations (3) and (4), From (5) and (6),