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NCERT Solutions for Class 10 Science Magnetic Field






Question 1.Why does a compass needle get deflected when brought near a bar magnet?
Solution
A compass needle is a small magnet itself. It has a north pole and a south pole. When North Pole and south pole of the bar magnets are closer to those of compass needle, they attract or repel as per the properties of poles.
Question 2. List the properties of magnetic lines of force.
Solution
Following are the properties of magnetic lines of force
i)These are close continuous curves originating at north pole and moving to south pole.
ii)The strength of magnetic field is stronger when magnetic field is strong.
iii)No two magnetic lines of force intersect each other.
iv) Magnetic lines of force give the direction in which North Pole of the compass needle would point at that place.
Question 3.  Why don’t two magnetic lines of force intersect each other?
Solution
The direction of magnetic field is taken as the direction in which the north pole of a compass needle moves inside it. If two field lines cross each other, it would mean that a compass needle points towards two directions at the point of intersection, which is not possible.
Question 4.Choose the correct option.
(a) zero   
(b) decreases as we move towards its end
(c) increases as we move towards its end  
(d) the same at all points
Solution (d) Magnetic field inside a solenoid is same at all the points.

Question 5.  Which of the following properties of a proton can change while it moves freely in a magnetic field?
(a) Mass  
(b) Speed          
(c) Velocity    
(d) Momentum
Solution
When a proton i. e. charged particle moves in a magnetic field then magnetic force is applied due to which its velocity and hence, the momentum changes.
Question 6. A positively charged particle 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
Solution  (d) The positive charge particle is moving toward west i. e., the direction of current is towards west (current flows in the direction of the motion of positive charge). The particle is deflected towards north so, the direction of force is toward north. So, from Fleming’s left hand the direction of force obtained will be upward.
Question 7. State Fleming’s left hand rule.
Solution
Fleming’s left hand rule states that, “if thumb, forefinger and middle finger of left hand are stretched perpendicular to each other and first finger points in direction of magnetic field, second finger points in the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.”
Question 8. What is the principal of electric motor?
Solution
An electric motor works on the principle that when a current carrying coil is placed in a magnetic field, a force acts on it and tends to rotate the coil.
Question 9.What is the role of split ring in an electric motor?
Solution
The split ring in an electric motor act as commutator it reverses the direction of current flowing through the armature coil.
Quetsion 10.Explain different ways to induce current in a coil.
Solution
Current is induced in a coil in following ways
a) When a magnet is moved towards or away from coil or there is a relative motion between coil and magnet, a current is induced in the coil circuit.
b) When a current passing through a oil changes, then a current is induced in a coil placed near it.
Quetsion 11.State the principle of an electric generator.
Solution
An electric generator works on the principle of electromagnetic induction. The production of electric current by due to relative motion between a conductor and a magnetic field is called electromagnetic induction. Electric current produced due to these phenomena is called ‘induced current’.
Question 12.Name some sources of direct current.
Solution
Cell or a battery direct current.
Question 13.Which sources produce alternating current?
Solution
Most of the power stations produce alternating current.
Question 14.Choose the correct option.
A rectangular coil of copper wire is rotated in a magnetic field. The direction of induced current changes once in each
(a) two revolutions  
(b) one revolution
(c) half revolution   
(d) 1/4 revolution
Solution (c) Half revolution. After half revolution, the direction of motion of rectangular coil changes, thus the direction of induced current changes.
Question 15. Name two safety measures commonly used in electric circuits and appliances.
Solution
(i) Earth wire (earthing)
(ii) fuse wire are two safety measures commonly used in electric circuits and appliances.
Question 16.An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. what result do you expect? Explain.
Solution
When a high power rating appliance is operated in a 5A current rating circuit, a large amount of heat is produced in the circuit. This leads to possible melting of insulation cover of wires and hence a short – circuit. If a fuse is used in the circuit, the fuse wire melts and the circuit is broken.
Question 17.What precautions should be taken to avoid the overloading of domestic electric circuits?
Solution
To avoid overloading, following precautions must be taken:
a) Never connect too many appliances in a single socket.
b)  Fuse should be used in the circuits.
Question 18.Which of the following correctly describes the magnetic force 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 centered o the wire.
Solution (d) The magnetic field lines due to a straight current carrying wire are concentric circles, i. e., the field consists of concentric circles centered on the wire.
Question 19.The phenomenon of electromagnetic induction is
(a) the process of charging a body.
(b) the process of generating magnetic field due to current passing through coil.
(c) the process of producing induced current in coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.
Solution (c) The process of producing induced current in coil due to relative motion between a magnet and the coil.

Question 20.The device used for producing electric current is called
(a) generator    
(b) galvanometer
(c) ammeter    
(d) motor
Solution(a) Generator
Question 21.The essential difference between AC generator and DC generator is that:
(a) AC generator has electromagnet while DC generator has permanent magnet.
(b) DC generator higher voltage.
(c) AC generates higher voltage
(d) Ac generator has slip rings while DC generator has commutator.
Solution (d) AC generator has slip rings while DC generator has commutator. Due to slip ring rings, the current produced by AC generator flows in both directions while current produced by DC generator flows sin single direction.
Question 22.State whether the following statements are true of false.
a) An electric motor converts mechanical energy into electrical energy.
b) An electric generator works on the principle of electromagnetic induction.
c) Field at the centre of a long circular coil carrying current will be parallel.
d) Wire with a green insulation is usually the live wire of an electric supply.
Solution 
a. False, electric motor converts electric energy into mechanical energy.
b. True
c. True
d. False, the wire with green insulation is the earth wire not the live wire.
Question 23.  List three methods of producing magnetic fields.
Solution
Following are the methods of producing magnetic field:
a) Passing electric current through a straight conductor/circuit.
b) Passing electric current through a circular loop.
c) Passing electric current through a solenoid.
Question 24.How does a solenoid behave like a magnet? Can you determine the north and south poles of a current carrying solenoid using a bar magnet? Explain.
Solution
A solenoid behaves like a magnet when electric current passes through it. Such a solenoid is called electromagnet. One end of a solenoid behaves as a north pole and the other end behaves as a south pole. We can use a bar magnet to determine the north and south poles of a current carrying solenoid by using property of like poles to repel and unlike poles to attract each other. The end of solenoid which attracts North Pole of a bar magnet is magnetic South Pole in a solenoid. The end which repels the north pole of bar magnet is the magnetic north pole of solenoid.
Question 25.
When the force experienced by a current carrying conductor placed in magnetic is fielded largest?
Solution
The force experienced by a current carrying conductor placed in a magnetic field is the largest when conductor is kept perpendicular to the direction of the magnetic field.
Force acting on current carrying conductor F = Bli sin θ, F will be maximum whey sinθ is maximum. Sin θ max = 1 = sin 900 i. e., conductor is perpendicular to the field.
Question 26.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 magnetic  field?
Solution
According to Fleming’s left hand rule, direction of magnetic field is upwards.
Question 27.Name some devices in which electric motors are used.
Solution
Electric motors are used in electric fans, refrigerator, mixtures, washing machines, computers, MP3 players etc.
Question 28.A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil and (iii) held stationary inside the coil?
Solution
(i) The galvanometer shows a deflection, which means current is induced in the coil. Current is induced in the coil due to the relative motion between coil and magnet.
(ii)The galvanometer shows a deflection in opposite direction, which means current is induced in opposite direction. In this case, the direction of motion is in the opposite direction w r.t. coil.
(iii) There is no deflection in galvanometer as no current is induced in the coil. There is no relative motion between coil and current, so on current will be induced in the coil.
Question 29. Two circular coils A and B are placed close to each other. If the current in coil A to changed, will some current be induced in the coil? Give reason.
Solution
Yes, current will be induced in coil B. wjhen the current in coil A is changed, the magnetic field around it changes. So, the changing, magnetic filed lines around coil B induce a current in coil B.
Question 30.State the rule to determine the direction of a
(i) magnetic field produced around a straight conductor carrying current.
(ii) force experienced by a current carrying straight conductor placed in a magnetic field.
(iii) current induced in a coil due to its rotation in magnetic field.
Solution 
(i) Ampere’s right hand thumb rule: If current is passing through a conductor in the direction of thumb of the right hand, then the direction of magnetic field is given by that of curvature of fingers.
(ii)Fleming’s left hand rule: If the forefinger, centre finger and thumb of left hand are starched mutually perpendicular and forefinger points in the direction of magnetic field, centre finger points in the direction of flow of electric current, then thumb points in the direction of force acting on the conductor.
(iii)Fleming’s right hand rule: If the forefinger, centre finger and thumb of right hand are stretched mutually perpendicular and forefinger points in direction of magnetic field, thumb points in the direction of motion of conductor, then centre finger points in the direction of induced current.
Question 31.When does an electric short circuit occur?
Solution 
When the insulation of wires is damaged or there is a fault in the appliance, due to which the live wire and neutral wire come in direct contact, the current in the circuit increases abruptly. This is called short – circuiting.
Question 32.What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Solution
The earth wire is connected to a metallic plate deep inside the earth. In this way, the metallic body of the appliance is connected to the earth, which provides a low resistance conducting path for electric current. Hence, any leakage of current to the metallic body of appliance keeps the potential to that of earth. The user might not get a severe electric shiock on touching such an appliance.
 
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