SOURCES OF EMF AND KIRCHHOFF S LAWS

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Bennett Turner
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1 SOURCES OF EMF AND KIRCHHOFF S LAWS VERY SHORT ANSWER QUESTIONS 1. What is the SI unit of (i) emf (ii) terminal potential difference? 2. When an ammeter is put in series in a circuit, does it read slightly less or more than the actual current in the original circuit? 3. When a voltmeter is put across a part of the circuit, does it read slightly less or more than the original voltage drop? 4. Can Kirchhoff s laws be applied both to direct and alternating currents? 5. It is unsafe to turn light-switch on or off while taking bath. Why? 6. Why the slider (jockey) should not be rubbed against the potentiometer wire? 7. Under what circumstances can the terminal potential difference of a battery exceed its emf? 8. Why is it not advisable to use copper wire in a potentiometer? 9. Suppose balance point is not obtained on the potentiometer wire. Give one possible cause for this? 10. It is possible to generate a 1,00,000 volt potential difference by rubbing a pocket comb with wool. Why is this voltage not dangerous when the much lower voltage provided by ordinary electric outlet is very dangerous? 11. Why is it easier to start a car engine on a warm day than on a chilly day? 12. Why light from a bathroom bulb gets dimmer for a moment when the geyser is switched on? 13. Of which material is a potentiometer wire normally made and why? 14. Write the condition under which the potential difference between the terminals of a battery and its emf are equal. 15. What do you understand by electromotive force? State its SI unit. 1

2 SHORT ANSWER QUESTIONS 1. While determining an unknown resistance using a metre bridge, readings for the balance point are taken (i) using both direct and reverse currents and (ii) also with the positions of the known and unknown resistances interchanged. Which type of errors are minimised by each of the two processes? 2. If the resistance connected in series with a battery is halved, then current is not exactly doubled. Justify this statement. 3. Does the direction of emf depend on the direction of current flow through a battery? 4. An ammeter with internal resistance 90 Ω reads 1.85 A when connected in a circuit containing a battery and two resistors 700 Ω and 410 Ω in series. The actual current will be 1.85 A. Is this statement true of false? Give reason in support of your answer. 5. Why is a potentiometer of longer wire considered more accurate than a potentiometer of shorter wire? 6. A discharged battery is being charged by an external emf source. Is the terminal voltage of the battery during charging greater or less than its emf? 7. Why is voltmeter less accurate in measuring emf than a potentiometer? 8. What is the resistance between the points A and B in the circuit shown in Fig? 9. Deduce dimensional formula for potential difference. 10. In the circuit shown in Fig, each battery is 5 V and has an internal resistance of 0.2 Ω. The reading in the ideal voltmeter is. Fill in the blank. 11. Why do we prefer a potentiometer with a longer bridge wire? 12. Why is the terminal voltage across a battery more than the emf during recharging? 13. What is the magnitude of the resistance X in the circuit shown in Fig, when no current flows through the 5 Ω resistor? 2

3 14. Calculate the resistance between A and B of the given network [Fig ]. 15. A voltage of 200 V is applied across a colour coded carbon resistor with first, second and third rings of blue, black and yellow colours. What is the current flowing through the resistor? 16. What are the factors on which the emf of a cell depends? 17. The variation of potential difference V with length in the case of two potentiometers X and Y is as shown in Fig. Which one of these two will you prefer for comparing emfs of two cells and why? 18. Draw a circuit diagram using a meter bridge and write the necessary mathematical relation used to determine the value of an unknown resistance. Why cannot such an arrangement be used for measuring very low resistances? 19. State Kirchhoff s rules of current distribution in an electrical network. SHORT ANSWER QUESTIONS 1. Give five points of difference between potentiometer and voltmeter. 2. What should be the value of R so that there is no potential difference between B and D in the circuit shown in Fig? 3

4 3. Show graphically the changes in potential in the circuit shown in Fig. In the circuit, the internal resistance and the emf have been shown separately although actually they occupy the same region of space. 4. Show graphically the changes of potential in the circuit shown in Fig, assuming that potential at the point a is zero. 5. The circuit shown in Fig shows use of a potentiometer to measure the internal resistance of a cell. (i) When the key is open, how does the balance point change, if the current from the driver cell decreases? (ii) When the key K is closed, how does the balance point change if R is increased, keeping the current from the driver cell constant? 4

5 6. In the circuit shown in Fig, AB is a resistance wire of uniform cross-section in which a potential gradient of 0.01 V cm -1 exists. (i) If the galvanometer G shows zero deflection. What is the emf E 1 of the cell used? (ii) if the internal resistance of the driver cell increases on some account, how will it change the balance point in the experiment? 7. In the potentiometer circuit shown in Fig, the balance (null) point is at X. State with reason, where the balance point will be shifted when (i) resistance R is increased, keeping all parameters unchanged. (ii) resistance S is increased, keeping R constant. (iii) cell P is replaced by another cell whose emf is lower than that of cell Q. 8. For the potentiometer circuit shown in the given figure, points X and Y represent the two terminals of an unknown emf E. A student observed that when the jockey is moved from the end A to the end B of the potentiometer wire, the deflection in the galvanometer remains in the same direction. What may be the two possible faults in the circuit that could result in this observation? If the galvanometer deflection at the end B is (i) more, (ii) less, than that at the end A, which of the two faults, listed above, would be there in the circuit? Give reasons in support of your answer in each case. 5

6 CONCEPTUAL PROBLEMS 1. In potentiometer experiment, it is advised to obtain the null point in the middle of the wire. Why? 2. The emf of a cell is greater than potential difference. But there is one situation where this is not true. What is that situation? 3. Why are the connections between resistors in a Wheatstone or metre bridge made of thick copper strips? 4. Why the light of a motor car becomes slightly dim when the car is started? 5. What will be the effect on the accuracy of the results if we replace a single-wire potentiometer by a potentiometer having 12 wires, the length of each wire being 1 m? 6. (a) A battery of emf 2 volt and internal resistance 0.1 Ω is being charged with a current of 5 ampere. In what direction will the current flow inside the battery? (b) What is the potential difference between the two terminals of the battery? 7. In the circuit shown in Fig, the resistance P and R are unequal. If the reading of the galvanometer remains the same with switch S open or closed, then what is the value of current through galvanometer? 8. R 1, R 2 and R 3 are different values of R, A, B and C are the null points obtained corresponding to R 1 R 2 and R 3 respectively. For which resistor, the value of X will be the most accurate and why? LONG ANSWER QUESTIONS 6

7 1. State the principle of potentiometer. Draw a circuit diagram used to compare the emfs of two primary cells. Write the formula used. How can the sensitivity of a potentiometer be increased? 2. State the principle of potentiometer. With the help of circuit diagram, describe a method to find the internal resistance of a primary cell. 3. Obtain the condition for maximum current through a resistor, When a number of cells are connected : (i) in series (ii) in parallel. 4. With the help of a circuit diagram, explain how the internal resistance of a cell can be determined using a potentiometer. Write the formula used. 5. What is emf of a cell? State the factors on which its value depends. Derive a relation between emf E, contact potential V, internal resistance r of a cell and external resistance R. Prove that emf is more than potential difference. 6. State Kirchhoff s laws of electric circuits and deduce Wheatstone bridge principle form these laws. 7. Write the principle of potentiometer. How will you compare emf of two cells using a potentiometer? Explain with a circuit diagram. Write two precautions to be taken during this experiment. 8. Define internal resistance of a cell. Prove that the internal resistance of a cell, where E=emf of the cell, V=Terminal potential difference, R=External resistance used in the circuit. 9. The given figure shows a network of resistances R 1, R 2, R 3 and R 4. Using Kirchhoff s laws, establish the balance condition for the network. 7

INDIAN SCHOOL MUSCAT

INDIAN SCHOOL MUSCAT Department of Physics Class:XII Physics Worksheet-3 (2018-2019) Chapter 3: Current Electricity Section A Conceptual and Application type Questions 1 Two wires of equal length, one