Tiêu đề CHAPTER 13 MAGNETIC EFFECTS OF ELECTRIC CURRENT.doc ✅

1 CHAPTER 2 CBSE X PHYSICS MAGNETIC EFFECTS OF CURRENT `UNIT 13 : MAGNETIC EFFECTS OF ELECTRIC CURRENT PROBLEMS FROM NCERT TEXTBOOK Q1. Why does a compass needle get deflected when brought near a bar magnet? Ans. A compass needle gets deflected due to the forces acting on its poles due to the magnetic field of the bar magnet. Q2. Draw magnetic field lines around a bar magnet. Ans. The magnetic field lines are as shown in the figure below. Q3. List the properties of magnetic lines of force. Ans. Magnetic field lines have the following properties. 1. They travel from the north to the south pole of a magnet outside the magnet and from South to the North Pole inside the magnet. 2. They are continuous closed curves. 3. They emerge out normally from the magnetised surfaces. 4. The tangent drawn at any point of the magnetic lines of force represents the direction of the magnetic field at that point. 5. Two magnetic field lines do not intersect each other. Q4. Why don’t two magnetic field lines intersect each other? Ans. We know that the tangent drawn at any point of the magnetic lines of force represents the direction of the magnetic field at that point. In case, two magnetic field lines intersect each other at a point as shown in Fig. magnetic field at this point will have two directions, shown by two arrows, one drawn to each magnetic field line at that point, which is not possible. Q5. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right hand rule to find out the direction of magnetic field inside and outside the loop. Ans. On applying the right hand rule to the current carrying wire the direction of magnetic field inside and outside the loop is as shown in the figure below. Q6. The magnetic field in a given region is uniform. Draw a diagram to represent it. Ans. For a uniform magnetic field the magnetic field lines are equidistant parallel lines. This is shown in the figure below. Q7. Choose the correct option: The magnetic field inside a long straight solenoid carrying current: (a) is zero. (b) decreases as we move towards its end. (c) increases as we move towards its end. (d) is same at all points. Ans. (b) decreases as we move towards its end. The magnetic field for a point inside a long straight solenoid carrying current is double than for a point situated at one of its ends. Q8. Which of the following properties of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer). (a) mass (b) speed (c) velocity (d) momentum. Ans. (c) and (d) both are correct. This is because when a proton moves in a magnetic field its velocity changes. Since momentum is the product of mass and velocity, therefore momentum also changes. Q9. In the figure given below, how do we think the displacement of rod AB will be affected if (i) current in the rod AB is increased, (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased.
2 CHAPTER 2 CBSE X PHYSICS MAGNETIC EFFECTS OF CURRENT Ans. We know that the force (F) acting on a conductor of length (L) carrying current (I) placed in a magnetic field (B) is given by F=B I L (i) If the current (I in the rod AB is increased, F also increases. (ii) When a stronger horse-shoe magnet is used, B increases. On account of this F also increases. (iii) If the length (L) of the rod is increased, F also increases. Q10. A positively-charged particle (proton) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is (a) towards south (b) towards east (c) downward (d) upward. Ans. The correct option is (d). The direction of motion of proton is the direction of the conventional current (I). The direction of force (F) on the proton is towards north (as the proton is deflected towards north). Applying Fleming’s left hand rule, the direction of magnetic field (B) is upward as shown by in fig below. Q11. State Fleming’s left-hand rule. Ans. Fleming’s left hand rule states that stretch out the fore finger, middle finger and the thumb perpendicular to each other. If the fore finger gives the direction of the magnetic field, the middle finger the direction of current then the thumb will give the direction of the force experienced by the current carrying conductor. Q12. What is the principle of an electric motor? Ans. An electric motor is based on the principle that a current carrying conductor placed in a uniform magnetic field experiences a force. Q13. What is the role of the split-ring in an electric motor? Ans. The split-ring commutator reverses the direction of the current through the coil after every half rotation of the coil. Thus, the direction of rotating couple remains unchanged and the coil continues to rotate in the same direction. Q12. Explain different ways to induce current in a coil. Ans. To induce current in a coil, the magnetic flux linked with it has to be changed. This is achieved by any one of the following; (i) Relative motion between the coil and a magnet. (ii) Relative motion between the coil and a conductor carrying current. (iii) Change in the current in a conductor placed near the coil. Q15. State the principle of an electric generator. Ans. Whenever in a closed circuit (i.e., a coil), the magnetic field lines change, an induced current is produced. Q16. Name some sources of direct current. Ans. A battery, a dc generator, a solar cell. Q17 Which sources produce alternating current? Ans. An ac generator, an oscillator (which converts dc into ac). Q18. Choose the correct option: A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each (a) two revolutions (b) one revolution (c) half revolution (d) one-fourth revolution. Ans. Option (c) is correct. Q19. Name two safety measures commonly used in electric circuits and appliances. Ans. (i) Fuse (for protecting circuits due to short- circuiting or overloading of the circuits). (ii) Earth wire (to ensure that any leakage of current to the metallic body of an electric appliance does not give a severe shock to the user). Q20. An electric oven of a 2 kW power rating is operated in a domestic circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain. Ans. Here, P = 2 kW = 2000 W, V=220 V P2000 I9.09A V220 Since the current rating of the circuit is 5 A therefore the fuse will melt if the current exceeds this value. The current flowing through the electric oven is more than 5 A (9.09 A), therefore the fuse in the circuit melts. As a result, the circuit is broken and the electric oven is saved from getting damaged. Q21. What precaution should be taken to avoid the overloading of domestic electric circuits?
3 CHAPTER 2 CBSE X PHYSICS MAGNETIC EFFECTS OF CURRENT Ans. To avoid overloading, we should not connect too many appliances to a single socket. Q22. Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire. (b) The field consists of straight lines parallel to the wire. (c) The field consists of radial lines originating from the wire. (d) The field consists of concentric circles centred on the wire. Ans. The correct option is (d). Q23. The phenomena, of electromagnetic induction is (a) the process of charging a body, (b) the process of generating magnetic field due to a current passing through a coil, (c) producing induced current in a coil due to relative motion between a magnet and the coil, (d) the process of rotating a coil of an electric motor. Ans. The correct option is (c). Q22. The device used for producing electric current is called a (a) generator (b) galvanometer (c) ammeter (d) motor. Ans. The correct option is (a). Q25. The essential difference between an ac generator and a dc generator is that: (a) ac generator has an electromagnet while a dc generator has permanent magnet, (b) dc generator will generate a higher voltage, (c) ac generator will generate a higher voltage, (d) ac generator has slip rings while the dc generator has a commutator. Ans. The correct option is (d). Q26. At the time of short-circuit, the current in the circuit (a) reduces substantially (b) does not change (c) increases heavily (d) varies continuously. Ans. The correct option is (c). Q27. State whether the following statements are true or false. (Page 220) (a) An electric motor converts mechanical energy into electrical energy. (b) An electric generator works on the principle of electromagnetic induction. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines. (d) A wire with a green insulation is usually the live wire. Ans. (a) False. An electric motor converts electric energy into mechanical energy. (b) True. (c) True. (d) False. A wire with a green insulation is usually the earth wire. Q28. List three sources of magnetic fields. Ans. (i) Natural and artificial magnets (ii) Electromagnets (iii) A conductor, a coil and a solenoid carrying current Q29. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current carrying solenoid with a help of bar magnet? Explain. Ans. A current carrying solenoid behaves like a bar magnet and the polarities of its ends depend upon the direction of current flowing through it. In order to determine the polarities of its ends, place it in a brass hook and suspend it with a long thread so that it moves freely. Bring North Pole of a bar magnet near one of its ends. In case the solenoid moves towards the bar magnet that end of the solenoid is a south pole and in case the solenoid moves away from the magnet, that end of the solenoid is its north pole. Similarly we can determine the polarity of the other end of the solenoid. Q30. When is the force experienced by a current- carrying conductor placed in a magnetic field is largest? Ans. The force experienced by a current-carrying conductor placed in a magnetic field is the largest when the direction of the current is at right angles to the direction of the magnetic field. Q31. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field? Ans. The direction of conventional current is opposite to the direction of electron beam. Therefore the current is towards the back wall. Since the beam is deflected to the right side, the force (F) acting on the beam is towards this side. Applying Fleming’s left rule, it is found that magnetic field (B) is acting vertically downwards (i.e., perpendicular to the plane of the paper and directed inwards). Q32. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split-ring in an electric motor? Ans. Refer to Art. 2.11, fig2.24. Split-ring in an electric motor takes the current from the battery and passes it on to the coil through the brushes after reversing its direction after every half revolution. The reversal of current in the coil reverses the direction of forces acting on the sides of the loop.