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Page No 97:

Question 1:

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

Answer:

Parallel and equidistant magnetic field lines represent uniform and constant field strength.

Hence, the correct answer is option C.

Page No 97:

Question 2:

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

(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

Answer:

If the key is taken out then the current will stop and no magnetic field exists due to the conductor. Therefore, at point O, there will be earth’s magnetic field and they will be straight lines parallel to each other.


Hence, the correct answer is option C.

Page No 97:

Question 3:

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

Answer:


When we see from point A, the direction of the current will be anticlockwise and when we see the direction of current from point B then it will be clockwise. It means current is flowing as shown in the figure given below.

By right-hand thumb rule, the magnetic field lines will enter from point B and come out from point A.

We also know that the magnetic field lines emerge from the North Pole and merge at the South Pole outside the magnet.

Hence, in this circuit face, A represents North pole and face B represents south pole.

Hence, the correct answer is option A.



Page No 98:

Question 4:

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

Answer:

Option (a), (b) and (d) are correct. Only option (c) is incorrect because the pattern of magnetic fields due to solenoid and bar magnet is almost the same.

Hence, the correct answer is option C.

Page No 98:

Question 5:

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

(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

Answer:

The direction of the current is taken as opposite to the flow of electron or in the direction of the flow of protons.

In the given figure, the proton and electron are moving in the opposite direction to each other and perpendicular to the direction of the magnetic field. So, current due to both electron and proton are in the same direction. Hence, the forces acting on both will be in the same direction. By Fleming's left-hand rule the direction force is pointing into the plane of the paper.


Hence, the correct answer is option A.

Page No 98:

Question 6:

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

Answer:

An electric motor is a device that converts electric energy into mechanical energy.

An electric motor has an electromagnet instead of a permanent magnet (as a permanent magnet does not produce a strong magnetic field).

A large number of turns of the conducting wire in the current-carrying coil to produce a stronger magnetic field.

It also has a soft iron core on which the coil is wound also known as an armature.

Hence, the correct answer is option C.

Page No 98:

Question 7:

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

(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
(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

Answer:

As the current in the first coil changes, the magnetic field associated with it also changes. Thus the magnetic field lines around the other coil also change. Hence the change in magnetic field lines associated with the secondary coil is the cause of induced electric current in it by the process of electromagnetic induction. Due to this reason, a momentary deflection in the galvanometer will be observed which will die out soon when the current flowing through the first coil attains a steady state.

Due to a similar phenomenon, a momentary deflection is again observed in the opposite direction when the key is removed as in this case current is decreasing in the circuit.


Hence, the correct answer is option D.



Page No 99:

Question 8:

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 150 second

Answer:

Here option (a), (b) and (c) are knowledge-based options and are correct.

Option (d) is incorrect because the frequency of 'AC' in India is 50 cycles/s. So, the time period of AC is 150 s. We know that AC changes its direction after every half time period, i.e., 12×50or1100 s.  So, in India, the AC changes direction after every 1100 s.

Hence, the correct answer is option D.

Page No 99:

Question 9:

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

(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

Answer:

If we observe the direction of the magnetic field by applying right-hand thumb rule then, we will find that the direction of the magnetic field is from North to South below the wire.


Hence, the correct answer is option B.

Page No 99:

Question 10:

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

Answer:

Inside the solenoid, magnetic field lines are parallel to each other hence forming a uniform field strength which indicates that the magnetic field is the same at all points inside the solenoid.



Hence, the correct answer is option C.

Page No 99:

Question 11:

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

Answer:

AC generator has slip rings while the DC generator has a split ring type commutator. To convert an AC generator into DC generator the split ring type commutator must be used with which unidirectional current is obtained.

Hence, the correct answer is option A.

Page No 99:

Question 12:

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

Answer:

The use of electric fuse is the most important method used for protecting home appliances from short-circuiting or overloading.

Generally, thin wire with a low melting point having a specific rating is connected in series with the electric devices. When the current passing through the fuse exceeds a specific value then by the heating effect of current it melts and breaks the circuit.

Hence, the correct answer is option B.



Page No 100:

Question 13:

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?

Answer:

We know that when the magnetic field and the direction of the current are perpendicular to each other, the deflection is maximum. But when they are on the same plane, no deflection takes place. 

If we place the current-carrying conductor in the plane of paper such that it passes through A then, the field produced by it is perpendicular to the plane of paper and parallel to the vertical axis of the compass needle. As a result, there will be no change in the deflection of compass needle.


The deflection in the compass needle will be maximum when the conductor passes through A and is perpendicular to the plane of the paper.

Page No 100:

Question 14:

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.

Answer:

The following conditions are required to obtain permanent electromagnet when a current-carrying solenoid is used:

1. Rod inside the solenoid should be made of a magnetic material like steel which should retain magnetic properties for a long time after magnetization.

2. The current through the solenoid should be direct current.

3. The number of turns in the solenoid should be large and closely packed so that a strong uniform magnetic field inside it is produced.

Page No 100:

Question 15:

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?

Answer:

By applying Right-hand thumb rule, the direction of the Magnetic field is into the plane of the paper at P and coming out from the plane of paper at point Q.

The magnitude of magnetic field B is inversely proportional to distance r.

Now, r1 > r2,  or point Q is closer than point P, so the magnetic field is stronger at point Q and weaker at point P.

Page No 100:

Question 16:

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.

Answer:

Magnetic field (B) produced by current-carrying wire is directly proportional to the current (I) flowing through the wire.

If the current is increased then the magnetic field produced is stronger and vice-versa. So, deflection in the compass will be more when the current flowing through the wire will be increased.

Page No 100:

Question 17:

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.

Answer:

Magnetic field is produced by a current-carrying conductor due to the motion of charged particles or electrons.

(i) As alpha particles are positively charged so their movement will also produce a magnetic field.

(ii) Neutrons do not carry any charge so their motion will not produce any magnetic field

Page No 100:

Question 18:

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?

Answer:

According to the right-hand thumb rule, the thumb shows the direction of electric current as shown in the figure given below.

The curled fingers holding the conductor give the direction of magnetic lines.

Fleming's left-hand rule gives the direction of force experienced by a current-carrying straight conductor placed in an external magnetic field and here thumb represents force on the conductor.

Page No 100:

Question 19:

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.

Answer:

We know that the magnetic field produced by a current-carrying straight wire depends inversely on the distance from it. Similarly, at every point of a current-carrying circular loop, the concentric circles representing the magnetic field around it would become larger and larger as we move away from the wire. By the time we reach the center of the circular loop, the arcs of these big circles would appear as straight lines. Every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the center of the loop.

Due to this reason, as she moves away from the center of the circular loop she observes that the lines keep on diverging.

Page No 100:

Question 20:

What does the divergence of magnetic field lines near the ends of a current carrying straight solenoid indicate?

Answer:

Divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicates a decrease in the strength of the magnetic field.

Page No 100:

Question 21:

Name four appliances where in 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?

Answer:

The four appliances are electric juicer, electric fan, walkman, washing machine.

The motor is a device that converts the electric energy into mechanical energy, whereas the generator converts mechanical energy into electrical energy.

Page No 100:

Question 22:

What is the role of the two conducting stationary brushes in a simple electric motor?

Answer:

The role of carbon brushes is to make contact with the rotating rings of the commutator and through them to supply current to the coil.

Page No 100:

Question 23:

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?

Answer:

The direction of the AC (alternating current) reverses periodically whereas the direction of the DC (direct current) remains constant with time. The frequency of AC in India is about 50 Hz whereas of DC it is zero.
As the frequency of AC in India is
 f=50 cycles/sTherefore, Time Period of AC = T= 1f=150sDirections of AC reverses after every half time period,T=150×2=1100s

Therefore, in India AC change its direction after every 1/100s So, in one second AC changes its direction 100 times.



Page No 101:

Question 24:

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?

Answer:

Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.

The use of an electric fuse prevents the electric circuit and the appliance from possible damage by stopping the flow of unduly high electric current. The Joule heating that takes place in the fuse melts it to break the electric circuit. So, a fuse is always connected in series with an appliance. If it is connected in parallel then it will not be able to break the circuit and current keeps on flowing.

When a fuse with a defined rating for a particular appliance is replaced by one with a larger rating, then it will not melt as the current exceeds in the wire and short-circuiting or overloading may occur.

Page No 101:

Question 25:

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.

Answer:

In the absence of an external magnetic field, only Earth’s magnetic field acts due to which magnetic compass needle pointing north and south direction.

When a bar magnet or a current-carrying loop is brought near it, then it produces a magnetic field, due to which magnetic fields of the compass needle and the bar magnet (or current-carrying loop) interact with each other causing a deflection in the needle.

The salient features of magnetic field lines are

Magnetic lines form closed and continuous curve
Magnetic lines emerge from north-pole to south-pole outside the magnet
Magnetic field lines never intersect each other
Closer the magnetic lines represent a stronger magnetic field and vice-versa.

Page No 101:

Question 26:

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?

Answer:

A labeled circuit diagram due to a current-carrying straight long conducting wire is shown below.

When current is passed through the conductor then concentric magnetic field lines will be produced. The strength of the magnetic field keeps on decreasing as we away from the wire.

We use the right hand thumb rule to find the direction of the magnetic field.

Right-Hand Thumb Rule: If a current-carrying conductor is held by the right hand; keeping the thumb straight and if the direction of the electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of the magnetic field.

Page No 101:

Question 27:

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?

Answer:

Suppose a straight conducting wire is bent in the form of a circular loop and a current is passed through it.

We know that the magnetic field produced by a current-carrying straight wire depends inversely on the distance from it.

Similarly, at every point of a current-carrying circular loop, the concentric circles representing the magnetic field around it would become larger and larger as we move away from the wire as shown in the figure.

By the time we reach the center of the circular loop, the arcs of these big circles would appear as straight lines.

Every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the center of the loop.

By applying the right-hand thumb rule, it is easy to check that every section of the wire contributes to the magnetic field lines in the same direction within the loop.

Now, the magnetic field produced by a current-carrying wire at a given point depends directly on the current passing through it. Therefore, if there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.

Page No 101:

Question 28:

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?

Answer:

The activity is as follows:

Take a small aluminum rod AB. Using two connecting wires suspend it horizontally from a stand, as shown in the figure given below.

Place a strong horseshoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. For this put the North Pole of the magnet vertically below and the South Pole vertically above the aluminium rod.

Connect the aluminium rod in series with a battery, a key and a rheostat.

Now pass a current through the aluminium rod from end B to end A.

It is observed that the rod is displaced towards the left and the rod gets displaced.

Reverse the direction of current flowing through the rod and observe the direction of its displacement. It is now towards the right.

Page No 101:

Question 29:

Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors?

Answer:

An electric motor is a rotating device that converts electrical energy into mechanical energy. The circuit diagram of a simple electric motor is given below.

An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively, as shown in the figure.

Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.

Now the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB.

On applying Fleming’s left-hand rule for the direction of the force on a current-carrying conductor in a magnetic field as shown in the figure above.

We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O mounted free to turn about an axis, rotate anti-clockwise.

At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.

The reversal of current also reverses the direction of the force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down.

Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Commercial motors are different from simple electric motors in the following ways

In commercial motor, an electromagnet is used in place of a permanent magnet.

Commercial motors have more number of turns of conducting wire in the current-carrying coil.

Commercial motors have a soft iron core on which the coil is wound.

Page No 101:

Question 30:

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.

Answer:

The process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction. In practice, we can induce the current in a coil either by moving it in a magnetic field or by changing the magnetic field around it.

Experiment

Take two different coils of copper wire having a large number of turns (say 40 and 90 turns respectively). Insert them over a non-conducting cylindrical roll, as shown in the figure given below. Connect the coil-1, having a larger number of turns, in series with a battery and a plug key.

Also, connect the other coil-2 with a galvanometer as shown in the figure. Plugin the key. Observe the galvanometer. We will observe that the needle of the galvanometer instantly jumps to one side and just as quickly returns to zero, indicating a momentary current in coil-2.

Disconnect coil-1 from the battery. You will observe that the needle momentarily moves but to the opposite side. It means that now the current flows in the opposite direction in coil-2.

Page No 101:

Question 31:

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?

Answer:

The labeled diagram of the AC generator is as follows

The working of the AC generator is as follows:

An electric generator (as shown in the figure above), consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings is made insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively.

The two rings R1 and R2 are internally attached to an axle.

The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in the figure. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. This means that the current in the external circuit flows from B2 to B1.

After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus, after every half rotation, the polarity of the current in the respective arms changes and an alternating current is produced. 

Page No 101:

Question 32:

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?

Answer:

Schematic diagram of common domestic circuits is shown below:


Fuse is a safety device in a circuit that prevents damage to the appliances and the circuit due to overloading. A fuse works on the Joule heating effect and is connected in series with the electric devices. Every fuse has a specific rating, it means that a specific fuse will allow only a specific amount of current through itself. When the current exceeds the limit, then the fuse will heat up and melt.

Overloading occurs when the live wire and the neutral wire come into direct contact. In such a situation, the current in the circuit abruptly increases. This is called short-circuiting. The use of an electric fuse prevents the electric circuit and the appliance from possible damage by stopping the flow of unduly high electric current. The Joule heating that takes place in the fuse melts it to break the electric circuit. Overloading can also occur due to an accidental hike in the supply voltage. Sometimes overloading is caused by connecting too many appliances to a single socket.

The burnt-out fuse should be replaced by another fuse of identical rating because if a fuse of less rating is used then it will blow and electric appliances will not work. If the fuse of a higher rating is used then in case of short circuit and overloading the fuse will not blow up and the excess amount of current may harm the appliances.



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