Choose the incorrect statement from the following regarding magnetic lines of field
- (a)The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points
- (b)Magnetic field lines are closed curves
- (c)If magnetic field lines are parallel and equidistant, they represent zero field strength
- (d)Relative strength of magnetic field is shown by the degree of closeness of the field lines
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(c) If magnetic field lines are parallel and equidistant, they represent zero field strength
If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are
This question refers to a figure in the original PDF.
- (a)concentric circles
- (b)elliptical in shape
- (c)straight lines parallel to each other
- (d)concentric circles near the point O but of elliptical shapes as we go away from it
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(c) straight lines parallel to each other
Hint: Hint— Only earth’s magnetic field will be present.
A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The current as seen from points A and B (in the plane of paper and on the axis of the coil) is anti clockwise and clockwise respectively. The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to
- (a)A
- (b)B
- (c)A if the current is small, and B if the current is large
- (d)B if the current is small and A if the current is large
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(a) A
Hint: Hint— Misconception is that magnetic field lines point from north to south pole. The fact is that they emerge out of North pole and entre into South pole.
For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is
- (a)The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid
- (b)The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil
- (c)The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
- (d)The N- and S-poles exchange position when the direction of current through the solenoid is reversed
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(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
A uniform magnetic field exists in the plane of paper pointing from left to right as shown in Figure 13.3. In the field an electron and a proton move as shown. The electron and the proton experience
This question refers to a figure in the original PDF.
- (a)forces both pointing into the plane of paper
- (b)forces both pointing out of the plane of paper
- (c)forces pointing into the plane of paper and out of the plane of paper, respectively
- (d)force pointing opposite and along the direction of the uniform magnetic field respectively
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(a) forces both pointing into the plane of paper
Commercial electric motors do not use
- (a)an electromagnet to rotate the armature
- (b)effectively large number of turns of conducting wire in the current carrying coil
- (c)a permanent magnet to rotate the armature
- (d)a soft iron core on which the coil is wound
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(c) a permanent magnet to rotate the armature
In the arrangement shown in Figure 13.4 there are two coils wound on a non-conducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then
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- (a)the deflection in the galvanometer remains zero throughout
- (b)there is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed 98 E XEMPLAR P ROBLEMS – SCIENCE
- (c)there are momentary galvanometer deflections that die out shortly; the deflections are in the same direction
- (d)there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions
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(d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions
Choose the incorrect statement
- (a)Fleming’s right-hand rule is a simple rule to know the direction of induced current
- (b)The right-hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors
- (c)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
- (d)In India, the AC changes direction after every 50 second
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(d) In India, the AC changes direction after every 50 second
A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in Figure 13.5. The direction of magnetic field at a point will be North to South
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- (a)directly above the wire
- (b)directly below the wire
- (c)at a point located in the plane of the paper, on the north side of the wire
- (d)at a point located in the plane of the paper, on the south side of the wire
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(b) directly below the wire
The strength of magnetic field inside a long current carrying straight solenoid is
- (a)more at the ends than at the centre
- (b)minimum in the middle
- (c)same at all points
- (d)found to increase from one end to the other
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To convert an AC generator into DC generator
- (a)split-ring type commutator must be used
- (b)slip rings and brushes must be used
- (c)a stronger magnetic field has to be used
- (d)a rectangular wire loop has to be used
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(a) split-ring type commutator must be used
The most important safety method used for protecting home appliances from short circuiting or overloading is
- (a)earthing
- (b)use of fuse
- (c)use of stabilizers
- (d)use of electric meter MAGNETIC EFFECTS OF ELECTRIC CURRENT 99 5JQTV#PUYGT3WGUVKQPU
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A magnetic compass needle is placed in the plane of paper near point A as shown in Figure 13.6. In which plane should a straight current carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?
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In the plane of the paper itself. The axis of the compass is vertical and the field due to the conductor is also vertical. It could result in a dip of compass needle which is not possible in this case (dips result only if axis of compass is horizontal). The deflection is maximum when the conductor through A is perpendicular to the plane of paper and the field due to it is maximum in the plane of the paper.
Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.
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Hint— (i) The current through the solenoid should be direct current. (ii) The rod inside is made of a magnetic material such as steel.
AB is a current carrying conductor in the plane of the paper as shown in Figure 13.7. What are the directions of magnetic fields produced by it at points P and Q? Given r1 > r2, where will the strength of the magnetic field be larger?
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Into the plane of paper at P and out of it at Q. The strength of the magnetic field is larger at the point located closer i.e. at Q.
A magnetic compass shows a deflection when placed near a current carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Support your answer with a reason.
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The deflection increases. The strength of magnetic field is directly proportional to the magnitude of current passing through the straight conductor.
It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving
- (i)alpha particles,
- (ii)neutrons? Justify your answer.
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Hint— (i) Yes, Alpha particles being positively charged constitutes a current in the direction of motion. (ii) No. The neutrons being electrically neutral constitute no current.
What does the direction of thumb indicate in the right-hand thumb rule. In what way this rule is different from Fleming’s left-hand rule?
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The thumb indicates the direction of current in the straight conductor held by curled fingers, whereas the Fleming’s left-hand rule gives the direction of force experienced by current carrying conductor placed in an external magnetic field.
Meena draws magnetic field lines of field close to the axis of a current carrying circular loop. As she moves away from the centre of the circular loop she observes that the lines keep on diverging. How will you explain her observation.
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Strength of the magnetic field falls as distance increases. This is indicated by the decrease in degree of closeness of the lines of field.
What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?
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The divergence, that is, the falling degree of closeness of magnetic field lines indicates the fall in strength of magnetic field near and beyond the ends of the solenoid.
Name four appliances wherein an electric motor, a rotating device that converts electrical energy to mechanical energy, is used as an important component. In what respect motors are different from generators?
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Electric fans, mixers, washing machines, computer drives, etc. Motors convert electrical energy into mechanical energy whereas generators convert mechanical energy into electrical energy.
What is the role of the two conducting stationary brushes in a simple electric motor?
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The brushes are connected to the battery and touch the outer side of two halves of the split ring whose inner sides are insulated and attached to the axle.
What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second? 100 E XEMPLAR P ROBLEMS – SCIENCE
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Direct current always flows in one direction but the alternating current reverses its direction periodically. The frequency of AC in India is 50 Hz and in each cycle it alters direction twice. Therefore AC changes direction 2 × 50 = 100 times in one second.
What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating? .QPI#PUYGT3WGUVKQPU
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Fuse is used for protecting appliances due to short-circuiting or overloading. The fuse is rated for a certain maximum current and blows off when a current more than the rated value flows through it. If a fuse is replaced by one with larger ratings, the appliances may get damaged while the protecting fuse does not burn off. This practice of using fuse of improper rating should always be avoided. .QPI#PUYGT3WGUVKQPU
Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept.
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Current carrying loops behave like bar magnets and both have their associated lines of field. This modifies the already existing earth’s magnetic field and a deflection results. Magnetic field has both direction and magnitude. Magnetic field lines emerge from N-pole and enter S- pole. The magnetic field strength is represented diagrammatically by the degree of closeness of the field lines. Field lines cannot cross each other as two values of net field at a single point cannot exist. Only one value, a unique net value, can exist. If in a given region, lines of field are shown to be parallel and equi- spaced, the field is understood to be uniform.
With the help of a labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire. How is the right hand thumb rule useful to find direction of magnetic field associated with a current carrying conductor?
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Right hand thumb rule states that if a current carrying straight conductor is supposedly held in the right hand with the thumb pointing towards the direction of current, then the fingers will wrap around the conductor in the direction of the field lines of the magnetic field. 168 E XEMPLAR P ROBLEMS – SCIENCE
Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current carrying coil has n turns the field produced at any point is n times as large as that produced by a single turn?
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Hint— The magnetic field at a point is the addition of the field produced by each turn.
Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current carrying conductor?
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Hint— Explain the activity with the help of the diagram. According to Fleming’s left hand rule, stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular. If the fore finger points in the direction of magnetic field and the central in the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.
Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are diffferent from commercial motors?
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Hint— Explain working with the help of the diagram. Commercial motors use an electromagnet in place of a permanent magnet, a large number of turns of conducting wire in the current carrying coil and a soft iron core on which the coil is wound. ANSWERS 169
Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.
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Hint— The process by which a changing magnetic field in a conductor induces a current in another conductor is called electromagnetic induction. Explain the working of the set up with the help of the diagram.
Describe the working of an AC generator with the help of a labelled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?
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Explain working with the help of the diagram. To get a direct current a split ring type commutator must be used in place of slip ring type commutator.
Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt out fuse should be replaced by another fuse of identical rating? MAGNETIC EFFECTS OF ELECTRIC CURRENT 101
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Hint— A fuse in a circuit prevents damage to the appliances and the circuit due to overloading. Otherwise the appliances or the circuit may get damaged. 170 E XEMPLAR P ROBLEMS – SCIENCE %*#26'4 ANSWERS /WNVKRNG%JQKEG3WGUVKQPU