Direct Current Theory 5
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The document explains concepts related to direct current theory, including electromotive force (emf), terminal potential difference, and internal resistance of a cell. It details how to calculate the potential difference in a circuit when a key is either open or closed, as well as the relationship between emf, terminal potential difference, and the current flowing through the cell and external resistance. The content is aimed at providing a thorough understanding of these electrical principles through theoretical explanations and formulas.
Direct Current Theory 5
- 1. Physics Helpline L K Satapathy Direct Current Theory - 5 E M F Terminal Potential Difference Internal Resistance r R E K V CBA I
- 2. Physics Helpline L K Satapathy Cells in Parallel Direct Current Theory - 5 EMF (E) of a Cell : It is the potential difference between the two poles of a cell in an open circuit (when no current is drawn from the cell). It is denoted by E . Terminal Potential Difference (V) of a Cell : It is the potential difference between the two poles of a cell in a closed circuit (when current is drawn from the cell). It is denoted by V. Internal Resistance (r) of a Cell : The resistance offered by the electrolyte of the cell when electric current flows through it is called internal resistance of the cell. It is denoted by r.
- 3. Physics Helpline L K Satapathy Cells in Parallel Direct Current Theory - 5 r R E K V CBA I Consider a cell of emf E and internal resistance r connected between points A and C in a circuit containing an external resistance R. When the key is closed (2nd fig) , a current I flows from left to right through R and from right to left through r and also through the cell. Hence r is shown outside the cell without any loss of accuracy. r R E K V CBA A voltmeter V is connected across A and C , which reads the potential difference across the cell. Potential Difference across the cell = ( )A CV V Understanding the relations between E , V , r , R and I :
- 4. Physics Helpline L K Satapathy Cells in Parallel Direct Current Theory - 5 When the key is open , current in the circuit = 0 Potential Difference across the cell = E , ( ) ( ) . . . (1)A C A B B CNow V V V V V V r R E K V CBA When I = 0 , potential drop across r = 0 ( ) 0B CV V (1) ( ) . . . (2)A C A BV V V V E
- 5. Physics Helpline L K Satapathy Cells in Parallel Direct Current Theory - 5 When the key is closed , current in the circuit = I r R E K V CBA I Potential Difference across the cell = V A is connected to the +ve and B to the –ve of the cell ( ) . . . (3)A BV V E Current through r is from C to B ( ) . . . (4)B CV V I r Current through R is from A to C ( ) . . . (5)C AV V I R (1) ( ) ( )A C A B B CV V V V V V E I r . . . (6)V E I r Terminal Potential Difference
- 6. Physics Helpline L K Satapathy Cells in Parallel Direct Current Theory - 5 r R E K V CBA I ( ) ( ) ( ) 0 . . . (7)A B B C C AV V V V V V In the closed loop ABCA , we have Putting the values from (3) , (4) and (5) , we get 0 ( ) . . . (8)E I r I R E I R r Again Potential Difference across external resistance R = IR , (8) . . . (9)Now I R E I r V Terminal Potential Difference = Potential drop across external resistance
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