Lesson: Magnetic Effects of Electric Current

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

c

Question: 2

If the key in the arrangement in the figure below 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.

c

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

a

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 in correct statement is:

(a) The field lines inside the solenoid are in the form of straight lines which indicate 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

c

Question: 5

A uniform magnetic field exists in the plane of paper pointing from left to right as shown in figure. 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

a

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

c

Question: 7

In the arrangement shown in the figure, 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

d

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 $\frac{1}{50}$ second

d

Question: 9

A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in figure. 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

b

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

c

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

a

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

b

Question: 13

A magnetic compass needle is placed in the plane of paper near point A as shown in figure. 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?

Solution:

When the magnetic field and the current-carrying conductor are in the same plane, the deflection in the conductor will be zero.

When the magnetic field and the direction of current are in mutually perpendicular directions to each other, the deflection in the conductor is maximum.

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.

Solution:

Electric current can be used to make temporary magnets called electromagnets. An electromagnet consists of a long coil of insulated copper wrapped inside around a soft iron core.

This iron core is magnetized when current passes through the coil.

Permanent electromagnet is obtained when:

1.The current flowing through the solenoid is direct current.

2. Magnetic material like steel is used instead of the soft iron core.

Question: 15

AB is a current carrying conductor in the plane of the paper as shown in figure. What are the directions of magnetic fields produced by it at points P and Q? Given ${r}_{1}>{r}_{2}$, where will the strength of the magnetic field be larger?

Solution:

The direction of the magnetic force at P and Q are anti-clockwise. At P, it is into the plane of paper and at Q, it is out of it.

The strength of the magnetic field reduces with distance. Since Q is closer than P, the strength of the magnetic field is larger at Q.

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.

Solution:

The strength of magnetic field around a current-carrying conductor is directly proportional to the magnitude of current passing through it. If the current in the wire is increased, the deflection of the compass too increases.

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.

Solution:

i) Alpha particles: Alpha particles are positively charged. So, a magnetic field would be created around its path.

ii) Neutrons: Neutrons are neutrally charged. So, no magnetic field would be created around its path.

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?

Solution:

In right-hand thumb rule, the thumb indicates the direction of current.

The right-hand thumb rule is used to find the direction of a magnetic field when the current passes through a conducting wire.

Right-hand thumb rule: If a current-carrying straight conductor is held with the right hand, such that the thumb points towards the direction of current, then the curl of the fingers points in the direction of the field lines of the magnetic field.

The Fleming’s left-hand rule is used to find the force on a current-carrying conductor in a magnetic field.

Fleming’s left-hand rule: If we stretch our thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular and if the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, then the thumb points in the direction of motion or the force acting on the conductor.

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?

Solution:

The magnetic field lines of a current-carrying circular loop are shown below.

As the distance from the current-carrying conductor increases, the strength of the magnetic field decreases. This is indicated by the reduced density of field lines.

The magnetic field around the wire is stronger and is indicated by more dense concentric circles around the wire than at the centre. The concentric circles representing the magnetic field become bigger as we move away from the wire. By the time we reach the centre of the circular loop, the arcs of these big circles would appear as straight lines.

Thus, when Meena moves away from the centre of the circular loop she observes that the lines keep on diverging from the centre.

Question: 20

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

Solution:

The divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicates that the strength of the field inside the solenoid is more compared to its strength near the ends of it.

Question: 21

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?

Solution:

Four appliances wherein an electric motor is used as an important component are:

·         Electric fans

·         Mixers

·         Washing machines

·         Vacuum cleaners

A motor converts electrical energy into mechanical energy. A generator, on the other hand, converts mechanical energy into electrical energy.

Question: 22

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

Solution:

The two conducting stationary brushes in a simple electric motor draw current from the battery and supply it to the armature of the motor.

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?

Solution:

If the current flows in one direction only, it is called a direct current. If the current reverses its direction periodically, it is called alternating current. An alternating current reverses its direction in every $\frac{1}{{100}^{th}}$ second. Therefore, AC changes direction 100 times in one second.

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?

Solution:

A fuse is a safety device that protects a device or a circuit from excessive flow of current through it or current overloading. It blows off when a current, more than the rated value, flows through it.

A fuse is used in series connection with an appliance and is connected before the appliance in the circuit. So, the entire current passes through it before passing through the appliance.

A fuse with defined rating should not be replaced by the one with a larger rating, as a rating of fuse is done based on the capacity of the appliance to handle a load or current supplied to it. A larger rating would let overloading or excessive flow of current in the circuit. This may damage the appliances.

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.

Solution:

A magnetic compass needle points to North and South in the absence of a nearby magnet due to the earth’s magnetic field.

The magnetic compass is deflected when a bar magnet or a current carrying loop is brought near it, as the magnetic field of the bar magnets or the current-carrying loop acts on the compass needle.

Salient features of magnetic field lines are given below.

1.   Magnetic field lines emerging from the North Pole converge to the South Pole.

2.   Magnetic field lines do not intersect one another.

3.   Closer field lines indicate stronger magnetic field and vice-versa.

4.   Magnetic field lines are closer near the poles.

5.   Inside the magnet, the direction of field lines is from its south pole to its north pole.

6.   Magnetic field has a direction and a magnitude at a particular point around a magnet.

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?

Solution:

Right-hand thumb rule is useful to find the direction of the magnetic field around a current-carrying conductor.

If we imagine holding a current-carrying straight conductor in our right hand, such that the thumb points towards the direction of current, then our fingers wrapped around the conductor indicates the direction of the field lines of the magnetic field.

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?

Solution:

The magnetic field produced by a current-carrying straight wire depends inversely on the distance of the field line from it.

At every point of a current-carrying circular loop, the concentric circles represent the magnetic field around it. These circles become larger and larger as we move away from the wire and appear as straight lines as we reach the centre of the loop. Thus, every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the centre of the loop.

Number of turns of coil

The magnetic field at a point is the sum of fields produced by each turn of the coil. Therefore, if there are 'n' turns of coil, the magnitude of the magnetic field will increase ‘n’ times that of the magnetic field produced by a single turn of coil.

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?

Solution:

Let’s take a small aluminium rod (AB), a horse-shoe magnet, battery, plug key, wires and a stand as shown in the given figure.

Let’s insert the plug key to initiate current supply to the rod. It is observed that the aluminium rod deflected towards the left. When the direction of the current was reversed the aluminium rod deflected towards the right.

Explanation:

The displacement of the rod in the above activity suggests:

a)   A force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field.

b)   The direction of force is also reversed when the direction of current through the conductor is reversed.

Fleming’s left-hand rule: The Fleming’s left-hand rule is used to find the direction of the force on a current carrying conductor in the magnetic field. According to this rule, if we stretch our thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular and if the first finger points in the direction of the magnetic field and the second finger in the direction of the current, then the thumb will point in the direction of motion or the force acting on the conductor.

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?

Solution:

Rectangular coil: An electric motor consists of a rectangular coil; let’s say ABCD, of insulated copper wire.

Magnetic poles: The coil is placed between the two poles of a magnetic field such a way that the arm AB and CD are perpendicular to the direction of the magnetic field.

Split-Rings: The ends of the coil are connected to the two halves P and Q of a split-ring.

Axle: The inner sides of the split-rings are insulated and attached to an axle.

External conducting edges: The external conducting edges say P and Q touch two conducting stationary brushes say X and Y, respectively.

How it works:

a)   When a current is allowed to flow through the coil ABCD, due to magnetic forces, an upward force acts on length CD and a downward force acts on length AB simultaneously. As a result, the coil starts rotating anti-clockwise.

b)   When the coil completes half rotation, the position of DC and BA interchange. The half-ring D comes in contact with brush X and half-ring Y gets reversed.

c)   Now the direction of current is through the coil DCBA. It reverses after every half rotation. As a result, the coil rotates in the same direction.

d)   The split-rings help to reverse the direction of current in the circuit.

A commercial motor uses:

a)   an electromagnet whereas an electric motor uses a permanent magnet.

b)   large number of turns of the conducting wire in the current-carrying coil.

c)   a soft iron core on which the coil is wound.

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.

Solution:

Electromagnetic induction:

The process by which the motion of a magnet with respect to the current-carrying conductor produces an induced potential difference, thereby setting up an induced electric current in the circuit is called electromagnetic induction.

Experimental set up:

Let’s take a coil of wire AB having a large number of turns and connect the ends of the coil to a galvanometer.

Let’s take a strong bar magnet and move its north pole towards the end B of the coil, stop for a moment and then withdraw the north pole of the magnet away from the coil.

Observation: We will observe a momentary deflection of the needle of the galvanometer. This indicates the presence of a current in the coil AB. There is no deflection, the moment the motion of the magnet stops. The galvanometer is again deflected when the North Pole is withdrawn.

Conclusion: A moving magnet induces electric current in a current conducting wire.

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?

Solution:

Construction of an AC generator:

a)    An electric generator consists of a rotating rectangular coil say ABCD placed between the two poles of a permanent magnet.

b)   The ends of the coil are connected to the two rings say R1 and R2 insulated from each other.

c)    There are two conducting stationary brushes say B1 and B2. These are kept pressed separately on the rings , respectively.

d)   R1 and R2 are internally attached to an axle.

e)    The axle is mechanically rotated from outside to rotate the coil inside the magnetic field.

f)    The outer ends of the two brushes are connected to the galvanometer.

g)   The galvanometer shows the flow of current in the given external circuit.

Working of an AC generator:

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, it induces electric current along the directions AB and CD. Thus, an induced current flows in the direction ABCD.

The current in the external circuit flows from .

After half a rotation, arm CD starts moving up and AB moving down. These results induced currents in the direction DCBA.  The current in the external circuit now flows from B1 to B2.

There is a change in direction of the induced current after every half rotation in equal interval of time. The current produced is called AC current.

Converting AC current to DC current:

DC current does not change its direction after a fixed interval of time. To convert AC current to DC current, we can use a split-ring commutator in place of slip-rings. This will ensure no change in the direction of the induced current.

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?

Solution:

A fuse is a safety device that protects a device or a circuit from excessive flow of current through it or overloading. It blows off when a current, more than the rated value, flows through it. A fuse is used in series connection with an appliance so that the entire current passing through the appliance also passes through it.

A fuse with defined rating should be replaced by another with identical rating as the rating of fuse is done based on the capacity of the appliance to handle the load or current supplied to it.

So, a larger rating would let overloading or excessive flow of current in the circuit. This may damage the appliances.

Similarly, a smaller rating would not let the appliance in the circuit work as it will burn before the required amount of current flows through the circuit.