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ICSE Notes 2016 : Physics (Vidya International School (VIS), Yeola)

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Electric Circuits, Resistance and Ohm s Law 1. The charge flowing through a conductor is called conventional current. 2. The electrons in a conductor move in a direction, opposite to the direction of charge. 3. The current due to moving electrons is called electronic current. 4. The unit of electric charge is coulomb. One coulomb is equivalent to 6.25 1018 electrons. 5. The charge on one electron is equivalent to 1.6 10 19C. 6. The rate of flow of charge is called electric current. 7. One ampere is the current when a charge of 1 coulomb flows through a conductor in one second. 8. Electric potential is the condition which determines the direction of the flow of charge. 9. Potential difference between two points in an electric circuit, is the amount of work done to move certain amount of charge from one point to another point. 10. Potential difference (V) = Work done (W) amount of charge (Q). 11. Potential difference is measured in volts. 12. One volt is the potential difference, if one joule of work is done in moving a charge of one coulomb. 13. An electric cell is a device, which maintains a constant potential difference across an electric circuit, by converting chemical energy to electric energy. Class-X 1 Key points 14. Electromotive force is the potential difference across the terminal of a cell, when it is not doing any external work. It is a constant quantity for a given cell. 15. A continuous conducting path, between the terminals of source of electricity (such as cell or battery) is called electric circuit. 16. An electric circuit in which the flow of current stops, because of open switch is called open circuit. 17. An electric circuit in which the current flows continuously, because the switch is closed is called closed electric circuit. 18. The opposition offered by a conductor to the passage of drifting electrons is called electric resistance. 19. A conductor is said to have a resistance of one ohm, if the current of one ampere flows through it, when its ends are at a p.d. of one volt. 20. The reciprocal of electric resistance is called electric conductance. It is measured in mhos. 21. (a)Resistance of conductor is directly proportional to its length. (b) Resistance of conductor is inversely proportional to its area of cross-section. (c) Resistance of conductor depends upon the nature of conductor. (d) Resistance of pure metals increases with the rise in temperature. (e) Resistance of alloys, such as German silver, constantan, Eureka, manganin hardly changes with rise in temperature. Class-X 2 Key points 22. The resistance offered by a conductor of unit length and unit area of crosssection, such that current enters and leaves from its opposite ends is called specific resistance. Or The resistance offered by a unit cube, such that current enters and leaves from its opposite ends is called specific resistance. 23. When a number of resistors are connected end to end, such that tail of one resistor is connected to initial end of other resistor, so as to form closed electrical circuit, then such a circuit is called series circuit. 24. (a) The current in series circuit is a constant quantity. (b) There is a continuous drop in potential all along in series circuit, but potential difference goes on increasing. (c) Total p.d. in series circuit is equal to the sum total of p.d. across the terminals of individual resistors : V = V1 + V2 + V3. 25. When a number of resistors are connected such that they have a common positive and a common negative terminal, then resistors are said to be in parallel circuit. 26. (a) The p.d. in a parallel circuit is a constant quantity. (b) The current divides in parallel circuit in the inverse ratio of resistance. (c) Total current flowing through parallel circuit is equal to the sum total of current flowing through individual resistors. I = I1 + I2 + I3. Class-X 3 Key points 27. Ohm s Law : All physical conditions of a conductor remaining the same, the current flowing through it is directly proportional to potential difference at its ends. 28. Ohm s law can be experimentally verified only : (i) when the temperature of conductor does not change, (ii) the current flowing through conductor is not very large. 29. The resistors which obey Ohm s law are called ohmic resistances. All metals and metallic alloys are ohmic resistances. 30. The resistors which do not obey Ohm s law are called non-ohmic resistances. All non-metals; electronic valves, armatures of fans and motors, electrolytes, arc lamps, etc., are non-ohmic resistances. 31. Electromotive force (emf) : When no current is drawn from a cell or a battery (when the cell or the battery are in open circuit), the potential difference between its terminals is called electro-motive force (emf). 32. Terminal voltage : When the current is drawn from a cell or a battery (when the cell or the battery are in closed circuit), the potential difference between the terminals of the cell is called terminal voltatage. 33. (a) The total resistance in series circuit is equal to sum total of all resistances. (b) The total resistance in series circuit is greater than any of the individual resistance in series. Class-X 4 Key points 34. (a) The reciprocal of total resistance in parallel circuit, is equal to the sum total of the reciprocals of individual resistances in the circuit. (b) The total resistance in parallel circuit is less than any of the individual resistance in parallel circuit. 35. The resistance offered by the electrolyte of a cell is called internal resistance. 36. (a) The internal resistance of cell is inversely proportional to the surface area of electrodes in contact with electrolyte. (b) The internal resistance of cell is inversely proportional to the distance between the electrodes of cell. (c) The internal resistance of cell depends upon : (i) the nature of electrolyte, (ii) the concentration of electrolyte, (iii) temperature of electrolyte. 37. Cells in series can give large current only, if the internal resistance of cells is very small or negligible as compared to external resistance. 38. Cells in parallel can give large current only, if the internal resistance of cells is large as compared to external resistance which may be negligible. Class-X 5 Key points

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