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POINTS TO REMEMBER
■ The magnetic field lines emerge from north pole and merge at the south pole.
■ The relative strength of the magnetic field is shown by the degree of closeness of the field lines.
■ The direction of magnetic field associated with a current-carrying conductor is determined by right-hand thumb rule.
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Table of Contents
MAGNETIC EFFECTS OF ELECTRIC CURRENT QUESTIONS
1. What is the direction of magnetic field lines outside a bar magnet?
Ans. From North Pole to South Pole.
2. Define a magnetic field line.
Ans. Magnetic field line is a curve around a magnet such that the tangent at any point on the cure gives the direction of magnetic field at that point.
3. What is meant by magnetic field?
Ans. Magnetic field is the space around a magnet or current-carrying conductor within which its influence can be felt by a magnetic substance like iron.
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4. What is SI unit of magnetic field?
Ans. Tesla (T).
5. A compass needle shows deflection when brought near a current-carrying conductor.
Ans. When a compass needle is brought near a current-carrying conductor, it experiences a magneticforce due to the magnetic field around the conductor. Thus, the needle shows deflection.
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6. How can it be shown that magnetic field exists around a wire through which direct current is passing?
Ans. Place a magnetic compass near the wire. If the needle shows deflection, then a magnetic fieldexists around the wire.
7. How is the strength of the magnetic field around a wire related to the strength of the electric current flowing in the wire?
Ans. The strength of the magnetic field is directly proportional to the electric current flowing in the wire.
8. What constitutes the field of a magnet?
The space around a magnet within which its magnetic effects can be experienced constitutes the magnetic field of the magnet.
9. State the direction of the magnetic field inside the bar magnet.
Ans. The direction of magnetic field inside the bar magnet is from South Pole to North Pole.
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10. A compass needle is placed near a current-carrying wire. State your observation for the following cases and give reason for the same in each case:
(a) Magnitude of electric current in the wire is increased.
(b) The compass needle is displaced away from the wire.
Ans. (a) The deflection of compass needle increases. This is because the strength of the magnetic field is directly proportional to current.
(b) The deflection of compass needle decreases. This is because strength of magnetic field is inversely proportional to the distance from the wire.
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11. A horizontal power line carries current in east to west direction. What is the direction of the magnetic field due to the current in the power line at a point above and at a point below the power line?
Ans. Applying right-hand thumb rule,
(i) The direction of magnetic field at a point above the power line is from south to north
(ii) The direction of magnetic field at a point below the power line is from north to south.
12. State the factors on which the strength of magnetic field at a point due to a current carrying conductor depends. Name the rule which gives the direction of magnetic field
The magnetic field strength at a point due to current-carrying conductor depends on
(i) The strength of electric current flowing in the conductor.
(ii) The distance of the observation point from the conductor.
Right-hand thumb rule.
13. Why does a compass needle get deflected when brought near a bar magnet?
Ans. Compass needle is a small magnet. It experiences a force in the magnetic field of a bar magnet. Due to this force, the needle gets deflected.
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14. List the properties of magnetic lines of force.
(i) Magnetic lines of force are closed continuous curves.
(ii) Magnetic lines of force are crowded in a region of strong magnetic field.
(iii) The tangent at any point on the magnetic line of force gives the direction of the magnetic field at that point.
(iv) No two magnetic lines of force can cross each other.
■ A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
■ A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material, like soft iron, when placed inside the coil. The magnet so formed is called an electromagnet.
15. How is an electromagnet made?
Ans. An electromagnet is made by placing a soft iron bar in a current-carrying solenoid.
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16. What is a solenoid?
Ans. A long helically wound coil of insulated copper wire is a solenoid.
17. State two ways by which the strength of an electromagnet can be increased.
Ans. The strength of an electromagnet can be increased by:
(i) increasing the number of turns in the solenoid.
(ii) increasing the strength of current in the solenoid.
18. Can a freely suspended current-carrying solenoid stay in any direction? Justify your answer. What will happen when the direction of current in the solenoid is reversed Explain.
Ans. A current-carrying solenoid behaves like a magnet. So, a freely suspended current-carrying 14. C solenoid will stay in north-south direction. On reversing current, its polarity will be reversed So, it will turn through 180°.
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19. On what factors does the magnetic field produced by a current-carrying solenoid depend?
Ans. The magnetic field produced by a current-carrying solenoid depends upon the following factors (i) The strength of the current flowing through the solenoid.
(ii) The number of turns of the wire of the solenoid.
(iii) The nature of the material inside the solenoid.
20. 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. Magnetic field inside the loop is perpendicular to the plane of table and in the downward direction. Outside the loop, magnetic field is perpendicular to the plane of the table and in the upward direction.
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21. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Ans. The pattern of the magnetic field of a current-carrying solenoid is the same as that of the magnetic field of a bar magnet. While one end of the solenoid behaves as north pole, the other end behaves as south pole.
In order to determine the north and south poles of a current-carrying solenoid, suspend the solenoid with a strong thread. Now bring the north pole of a bar magnet towards one end of the solenoid. If the solenoid attracts the magnet, then that face of the solenoid is south pole. If the bar magnet moves away from the solenoid, then that face of the solenoid is the north pole
■ When a charge q moves with velocity in a magnetic field B, a force F acts on it such that F = B q sin .
Where is the angle between velocity and magnetic field B.
■ The direction of the force experienced by a current-carrying conductor in a uniform magnetic field is determined by Fleming’s left-hand rule.
■ An electric motor is a rotating device that converts electrical energy to mechanical energy. The induced current is found to be the highest when the direction of motion of the coil is at right angles to the magnetic field.
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22. State the rule to determine the direction of a force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular.
Ans. Fleming’s left hand rule. Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of the force acting on the conductor.
23. What is electric motor?
Ans. Electric motor is a device which converts electrical energy into mechanical energy.
24. Define electromagnetic induction.
Ans. Electromagnetic induction is the phenomenon of producing induced current in a closed due to the changing magnetic field.
25. On what factors does the force experienced by a current-carrying conductor placed in a uniform magnetic field depend?
Ans. The force depends upon the following factors:
(i) The strength of the magnetic field.
(ii) The strength of the current.
(iii) The length of the conductor.
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26. Explain the meaning of the words “electromagnetic” and “induction” in the term electromagnetic induction.
Ans. Electromagnetic” means production of induced current in a closed coil due to changing magnetic field.
“Induction” means the induced current is produced without the actual contact of closed coil and moving magnet.
27. A coil made of insulated copper wire is connected to a galvanometer. What will happen to the deflection of the galvanometer if this coil is moved towards a stationary bar magnet and then moved away from it? Give reason for your answer and name the phenomenon involved.
Ans. When coil is moved towards a stationary bar magnet, the magnetic field linked with coil increases. So, an induced current is produced in the coil. Due to this current, there is deflection of the needle of galvanometer. When coil is moved away from a stationary bar magnet, the magnetic field linked with the coil decreases. So, an induced current is again produced in the coil but in the opposite direction. Thus, needle of galvanometer is deflected but in opposite direction. The phenomenon is electromagnetic induction.
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28. What is the principle of electric motor?
Ans. Electric motor works on the principle that a current-carrying conductor placed perpendicular to a magnetic field experiences a force.
29. What is the role of the split-ring in an electric motor?
Ans. The split-ring in an electric motor reverses the direction of current in the armature coil. So, the direction of the force acting on the two arms of the coil is also reversed. Thus, the coil of d.c. motor continues to rotate in the same direction.
■ The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.
■ Electric fuse is an important component of all domestic circuits.
■ Overloading is caused by connecting too many appliances to a single socket.
30. What is electric generator?
Ans. Electric generator is a device which converts mechanical energy into electrical energy.
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31. What is the frequency of direct current?
Ans. Zero.
32. What is frequency of AC in India?
Ans. 50 Hz.
33. Name the material generally used to make electric fuse.
Ans. Alloy of lead and tin or copper.
34. What is short-circuiting in an electric supply?
Ans. Short-circuiting is said to occur when the live wire touches the neutral wire.
35. Differentiate overloading and short-circuiting.
Ans. Overloading is connecting too many devices to a circuit. Short-circuiting is the coming to gather of the live and the neutral wire.
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36. When does an electric short circuit occur?
An electric short circuit is said to occur when live wire and neutral wire touch each other. In this situation, the resistance of the circuit becomes small and therefore large amount of current flows through it. Due to this, large amount of heat is produced and the circuit catches fire.
37. What is the function of a large wire? Why is it necessary to earth metallic casings of electric appliances?
Ans. Earth wire acts as a safety measure. When the live wire touches the metallic casing of an electric appliance, the current flows from the casing to the earth through the copper wire. The earth offers practically no resistance to the flow of current. So, large current passes through the copper wire instead of human body. As a result of this, the circuit is heated and the fuse in the circuit melts. The circuit is automatically switched off. Thus, the human body suffers no electric shock and the electric appliance is saved from burning.
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