Magnetic Effects Of Electric Current

Multiple Choice Questions

Question 1.
Choose the incorrect statement from the following regarding magnetic lines of eld
A. The direction of magnetic eld at a point is taken to be the direction in which the
north pole of a magnetic compass needle points

B. Magnetic eld lines are closed curves

C. If magnetic eld lines are parallel and equidistant, they represent zero eld
strength

D. Relative strength of magnetic eld is shown by the degree of closeness of the

eld lines

Answer:
This statement is false about the magnetic eld lines that if magnetic eld lines are
parallel and equidistant then they represent zero eld strength because if they are
parallel and are at equal distance then they have uniform magnetic eld and
doesn’t have zero magnetic eld strength.

Question 2.
If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open)
and magnetic eld lines are drawn over the horizontal plane ABCD, the lines are
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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:
When the above circuit is open and the magnetic eld lines are drawn over the
horizontal plane ABCD, then the magnetic eld lines will be in the form of concentric
circles with centre at the axis of the conductor because since the circuit is open,
there will be no ow of current and therefore there will be no magnetic eld due to
the conductor. So, at point O. only Earth’s magnetic eld will be present so the
magnetic eld lines will be in the form of concentric circles

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 eld lines point from B to A. The N-pole of the resultant magnet is on
the face close to
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 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:
On applying Right hand thumb rule, the magnetic eld lines will be from point B to
point A. As we know that magnetic eld lines moves from North pole to South pole
outside the magnet. So, A will represent North and B will represent south pole. So,
The N-pole of the resultant magnet is on the face close to point 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 incorrect statement is
A. The eld lines inside the solenoid are in the form of straight lines which indicates
that the magnetic eld is the same at all points inside the solenoid

B. The strong magnetic eld produced inside the solenoid can be used to
magnetize a piece of magnetic material like soft iron, when placed inside the coil

C. The pattern of the magnetic eld associated with the solenoid is different from
the pattern of the magnetic eld around a bar magnet
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D. The N- and S-poles exchange position when the direction of current through the
solenoid is reversed

Answer:
Statement given in option ‘c’ is not correct because the pattern of magnetic eld
lines associated with the solenoid is same as the pattern of magnetic eld lines
around a bar magnet.

Question 5.
A uniform magnetic eld exists in the plane of paper pointing from left to right as
shown in Figure 13.3. In the eld, 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 eld
respectively

Answer:
The electron and proton experiences forces pointing into the plane of the paper and
out of the plane of the paper because the direction of current will be opposite to the
direction of electrons and in the direction of protons. As shown in the gure, the
direction of electrons and protons is in opposite directions and perpendicular to the
direction of the magnetic eld. Therefore, the forces acting on electron and proton
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 will be in same direction. Fleming’s left-hand rule can be applied and the direction
will be into the plane of paper.

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:
Commercial electric motors don’t use a permanent magnet to rotate the armature
instead commercial electric motors uses an electromagnet for rotating the
armature.

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 de ection in the galvanometer remains zero throughout

B. There is a momentary de ection in the galvanometer but it dies out shortly and
there is no effect when the key is removed

C. There are momentary galvanometer de ections that die out shortly; the
de ections are in the same direction

D. There are momentary galvanometer de ections that die out shortly; the
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 de ections are in opposite directions

Answer:
In the given gure, when electric current through rst coil changes then due to
change in magnetic eld lines, there will be an EMF (electromotive force) induced in
the another coil. When the key is inserted in the circuit and when it is removed, the
magnetic eld lines will be rst increase and then decreases. So, the current will be
in opposite directions in the two cases, therefore, there are momentary
galvanometer de ections in opposite directions.

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 nd the direction of magnetic elds due to
current carrying conductors

C. The difference between the direct and alternating currents is that the direct
current always ows in one direction, whereas the alternating current reverses its
direction periodically

D. In India, the AC changes direction after every 1/50 second

Answer:
The frequency of Alternating current in India = 50 cycles/ second

Therefore, time period of alternating current =1/50s

Since, AC changes the direction of current after every half time period. Therefore Ac
will change current in 1/100 seconds.

So, statement given in option d is not correct.

Question 9.
A constant current ows in a horizontal wire in the plane of the paper from east to
west as shown in Figure 13.5. The direction of magnetic eld 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

Answer:
The direction of magnetic eld at a point will be from North to South will be directly
below the wire. On applying Fleming’s Right hand rule, the direction of magnetic
eld will be from North to South below the wire.

Question 10.
The strength of magnetic eld inside a long current carrying straight solenoid is
A. More at the ends than at the center

B. Minimum in the middle

C. Same at all points

D. Found to increase from one end to the other

Answer:
Since magnetic eld lines are parallel to each other inside a long current carrying
straight solenoid which means that magnetic eld is same at all points.

Question 11.
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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 eld has to be used

D. A rectangular wire loop has to be used

Answer:
An AC generator consists of slip rings whereas a DC generator consists of split
ring. So, to convert an AC generator into DC generator, split ring type commutator
should be used.

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:
An electric fuse is wire which is made of a material of high resistance and low
melting point. Using an electric fuse prevents the ow of unduly high electric current
and protects the appliances from the damage. Due to Joule heating, the fuse melts
to break the electric circuit. So, use of fuse is the most important method used for
protecting home appliances from short circuiting or overloading.

Short Answer Questions

Question 1.
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 de ection of the
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compass? Under what condition is the de ection maximum and why?

Answer:
Since the de ection in the compass will be maximum when magnetic eld and
electric current are perpendicular to each other. So, to obtain no de ection in the
magnetic compass, the current carrying conductor should be placed in the same
plane.

Question 2.
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:

To make a permanent electromagnet, a soft iron core is placed inside the solenoid
and the soft-iron should be made up of ferromagnetic material which increases the
magnetic properties of solenoid.
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Question 3.
AB is a current carrying conductor in the plane of the paper as shown in Figure
13.7. What are the directions of magnetic elds produced by it at points P and Q?
Given r1 > r2, where will the strength of the magnetic eld be larger?

Answer:
At point P: By applying Fleming’s right hand rule, magnetic eld will be in
anticlockwise direction around the current direction. The magnetic eld will be to
point P and towards the plane of paper.

At point Q: At this point, the direction of the current is away from the conductor and
away from the plane of paper.

Magnetic eld will be stronger at point Q than at point P because the strength of
magnetic eld is stronger when it is near to the conductor and weak when away
from the conductor.

Question 4.
A magnetic compass shows a de ection when placed near a current carrying wire.
How will the de ection of the compass get affected if the current in the wire is
increased? Support your answer with a reason.

Answer:
If the current in the wire is increased, then the magnetic eld will also increase
because the magnetic eld is directly proportional to the applied current.
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Question 5.
It is established that an electric current through a metallic conductor produces a
magnetic eld around it. Is there a similar magnetic eld produced around a thin
beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.

Answer:
When an electric current through a metallic conductor is established and produces
a magnetic eld around it. Alpha particles are positively charged particles, so
magnetic eld will be created along the path of the alpha particles but neutrons are
the particles which have no charge on them. So, there will be no magnetic eld
along the path of neutrons.

Question 6.
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:
In right hand thumb rule, the direction of thumb indicates the direction of electric
current and the direction of ngers wrapping shows the direction of magnetic eld.

This rule is different from Fleming’s left-hand rule because Fleming left hand rule
explains the effects of magnetic eld on a current carrying conductor whereas Right
hand thumb rule explains the magnetic eld produced due to a current carrying
conductor.

Question 7.
Meena draws magnetic eld lines of eld close to the axis of a current carrying
circular loop. As she moves away from the center of the circular loop she observes
that the lines keep on diverging. How will you explain her observation?

Answer:
Since magnetic eld is stronger near the current carrying conductor and weak when
away from the conductor. In current carrying circular loop, the magnetic eld lines
are stronger at the periphery of the loop but as we move away i.e. towards the
center of the loop, the magnetic eld becomes weak there. As a result, the
magnetic eld lines looks like straight lines at the center and as we move near to
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the conductor i.e. towards periphery, the magnetic eld lines seems like diverging
so as to maintain the circular shape around the wire of the circular loop.

Question 8.
What does the divergence of magnetic eld lines near the ends of a current
carrying straight solenoid indicate?

Answer:
The divergence of the magnetic eld lines near the ends of a current carrying
straight solenoid indicates that that the solenoid behaves like a magnet. As like
magnetic eld lines makes loop around a magnet similarly the divergence of
magnetic eld lines near the ends of current carrying straight solenoid indicates that
solenoid behaves like magnet. The divergence of magnetic eld lines shows that
the magnetic eld is strongest at the poles/ends of the solenoid.

Question 9.
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?

Answer:
The four appliances where an electric motor, a rotating device that converts
electrical energy to mechanical energy are as follows:

1. CD/DVD Player

2. Table Fan

3. Vacuum cleaner

4. Hand Blender

Electric motors are different from electric generators because electric motors works
on the principle of Fleming’s left hand rule whereas Electric generators works on
the principle of Fleming’s right hand rule.
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 Question 10.
What is the role of the two conducting stationary brushes in a simple electric motor?

Answer:
The two conducting stationary brushes are used in a simple electric motor in order
to keep the two poles of the armature in contact with the power supply.

Question 11.
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 difference between Alternating current and Direct current are:

Alternating current reverses the direction of the current periodically as re ected in

the name ‘Alternating’ which means changing current whereas the direct current
ows only in one direction and don’t reverses the direction of current. D.C can be
transmitted over long distances without loss of energy but Alternating current can’ t
be transmitted to long distances.

AC used in India changes its direction at rate of 100 times in a second.

Question 12.
What is the role of fuse, used in series with any electrical appliance? Why should a
fuse with de ned rating not be replaced by one with a larger rating?

Answer:
An electric fuse is wire which is made of a material of high resistance and low
melting point. Using an electric fuse prevents the ow of unduly high electric current
and protects the appliances from the damage. Due to Joule heating, the fuse melts
to break the electric circuit.

A fuse with de ned rating should not be replaced by the one with a higher rating
because a fuse wire works due to its low melting point which is due to its rating and
if a fuse with higher rating is used than it might not melt and thus failing in its
purpose of using it.
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 Long Answer Questions

Question 1.
Why does a magnetic compass needle pointing North and South in the absence of
a nearby magnet get de ected when a bar magnet or a current carrying loop is
brought near it. Describe some salient features of magnetic lines of eld concept.

Answer:
Due to interaction between the magnetic elds of the compass needle and the
magnetic eld of bar magnet, a magnetic compass needle shows de ection when a
bar magnet is brought close to it.

The various features of magnetic eld lines are:

1. The direction of magnetic eld lines is from North to South pole.

2. Magnetic eld lines are closed curves.

3. The more the lines are closer, the more will be the magnetic eld.

4. No two eld lines can cross each other.

5. Strength of magnetic eld near the poles.

Question 2.
With the help of a labelled circuit diagram illustrate the pattern of eld lines of the
magnetic eld around a current carrying straight long conducting wire. How is the
right hand thumb rule useful to nd direction of magnetic eld associated with a
current carrying conductor?

Answer:
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 The pattern of the magnetic eld lines of the magnetic eld around a current
carrying straight long conducting wire are in circular pattern in form of concentric
circles as shown in the below diagram:

As depicted in the diagram , the direction of magnetic eld can be nd out by using
the right hand thumb rule which says that if we are holding a current carrying
conductor in the right hand such that the thumb will point towards the direction of
the current. The ngers will wrap around the conductor in the direction of the eld
lines of the magnetic eld.

Question 3.
Explain with the help of a labelled diagram the distribution of magnetic eld due to a
current through a circular loop. Why is it that if a current carrying coil has n turns
the eld produced at any point is n times as large as that produced by a single
turn?

Answer:
The magnetic eld lines due to a current carrying circular loop can be shown with
the help of following diagram:

Magnetic eld lines are in the form of concentric circles in a circular current carrying
conductor. Magnetic eld lines are strong near the periphery because magnetic
eld lines remain close when they are near to the conductor
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When we move towards the centre of the conductor, the magnetic eld lines distant
each other and at the centre, the arc appear of big circles.

On increasing the number of coils in the circular loop, the magnitude of the
magnetic eld increases and is equal to n times with n number of coils in the loop.

Question 4.
Describe the activity that shows that a current-carrying conductor experiences a
force perpendicular to its length and the external magnetic eld. How does
Fleming’s left-hand rule help us to nd the direction of the force acting on the
current carrying conductor?

Answer:
The activity to demonstrate that a current carrying conductor experiences a force
perpendicular to its length and the external magnetic eld can be explained as
follows:

Activity: To show the effect of magnetic eld on current-carrying conductor

Materials Required: For this, we need to take a small aluminum rod, a horse-shoe
magnet, battery, plug key, wires and a stand.
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1. Suspend an aluminum rod horizontally from the stand and two wires at the ends
of it are tied. The wires are connected to a Rheostat, battery and a key so that a
circuit is completed,

2. Place a horse shoe magnet in such a manner that the aluminum rod is between
the poles of magnet.

Assume that the above the aluminum rod is South pole of the magnet and below,
north pole of the magnet. Insert the plug key and current is supplied to the rod.

Observation: the aluminum rod is de ected towards the left direction

On changing the direction of current, the rod is de ection towards the right
direction.

Hence, it demonstrates that a currentcarrying conductor experiences a force

perpendicular to its length and the external magnetic eld

The direction of the magnetic eld can be nd out with the help of Fleming’s left-
hand rule. Let current is moving in anticlockwise direction, then the direction of
magnetic eld will be in clockwise direction i.e. at the top of the loop whereas vice-
versa in case of clockwise direction of current.

Question 5.
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:
Working of Electric Motor:-

An electric motor is a rotating device which converts electrical energy into the
mechanical energy.

Construction of an electric motor:

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 An electric motor consists of a rectangular coil of insulated wire. The rectangular
coil is placed between the two poles of magnetic eld such that the arm AB and CD
are perpendicular to the direction of the magnetic eld. The ends of the coil are
connected through two halves of a split ring. The inner sides of the halves are
insulated and attached to an axle. The external edges of the conducting halves
touch two conducting stationary brushes X and Y.

Working of an electric motor:

Current owing in the rectangular coil ABCD from the battery source through
conducting brush X and the current ows back to the rectangular coil through brush
Y. The current in arm AB of the coil ows from A to B and in arm CD, it ows from C
to D i.e. opposite to the direction of the arm AB. The direction of current can be nd
out using Fleming’s left hand rule. The force acting on arm AB pushes it downwards
and while force acting on CD pushes it downwards. Therefore, the coil and the axle
are free to turn about an axis and can rotate in anticlockwise direction. At half
rotation, Q makes contact with brush X and P with brush Y. So, now the current is
reversed and ows along the path DCBA. The split ring acts as a commutator in the
electric motor. On reversing the current, the direction of force acting on arms AB
and CD also gets reversed. Now AB will be pushed in upward direction and CD will
be pushed in downward direction. So, now the coil and axle rotate a half turn more
in the same direction. The current reverses in every half rotation of the coil to the
axle.
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 Commercial electric motors don’t use a permanent magnet to rotate the armature
instead commercial electric motors uses an electromagnet for rotating the
armature.

Question 6.
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 eld
passing through the loop increases or decreases.

Answer:
Electromagnetic Induction is de ned as the process by which a changing magnetic
eld in a conductor induces a current in another conductor, is known as
electromagnetic Induction.

An experiment to show that a current is set up in a closed loop when an external

magnetic eld passing through the loop increases or decreases is as follows:

1. Insert a coil over a hollow tube of a cardboard. With the wire connect the ends of
the coil with a galvanometer.

2. Move the north pole of the magnet towards the end B of the coil

Observation: The galvanometer shows a de ection in the needle towards right

direction

3. When Galvanometer is moved away from the coil

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Observation: there is de ection in the galvanometer needle towards left direction.

4. When galvanometer is kept static

Observation: there is no de ection in the galvanometer.

This proves that when the coil and the magnet are brought in contact, a current is
induced in the coil.

Question 7.
Describe the working of an AC generator with the help of a labeled circuit diagram.
What changes must be made in the arrangement to convert it to a DC generator?

Answer:

An electric generator consists of a rotating rectangular coil ABCD placed between

two poles of permanent magnet. The coil ends are connected to the two rings
R1 and R2 . The inner sides of the ring are insulated. The stationary brushes
B1 and B2 are kept pressed separately on rings R1 and R2. The rings are internally
attached to the axle. The axle is mechanically rotated from outside to rotate the coil
inside the magnetic eld. The outer ends of the rings are connected to the
galvanometer.
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 Working of Electric generator:

When the axle connected to the two rings is rotated, then the arm AB moves
upward and CD moves downward in the magnetic eld produced by the permanent
magnet. Let the coil AB is rotated in clockwise direction. Using Fleming’s right-hand
rule, the induced currents are set up in directions along AB and CD and an induced
current ow in ABCD. If there are large number of turns in the coil, then the current
adds up in each current to give large current. This means that the current in the
external current ows from B2 to B1. After half turn, arm AB moves in downward
direction and CD moves in upward direction. Now the direction of induced current is
reversed and now current ows in DCBA direction. The current now ows from
B1 to B2. Thus, after every half rotation, the direction of the current reverses. This
current is known as alternating current which changes the direction of current
periodically.

AC generator can be converted into a DC generator with the help of a split ring
commutator.

Question 8.
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:
The diagram of domestic circuit is:
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The electric power is supplied through electric poles by underground cables. In this
supply, there are three wires: Live wire, insulation wire and the Earth wire. Live wire
is of Red color, insulation wire is of black color and the Earth wire is of green color.
At the meter board in the house, there electric wires are passed through main fuse
into electric meters. Through main switch, they are connected to line wire in the
house. There are two kinds of circuits used in the house which are: one is of 15A
current for appliances with high power rating, the other is of 5A for low power
devices. The earth wire is usually connected to metal plate deep in the Earth near
the house to minimize shock due to short circuit effects. In each circuit, the different
circuits can be connected across live and neutral wires. Each appliance is
connected in parallel to each other so that each appliance has an equal potential
difference.

An electric fuse is wire which is made of a material of high resistance and low
melting point. Using an electric fuse prevents the ow of unduly high electric current
and protects the appliances from the damage. Due to Joule heating, the fuse melts
to break the electric circuit

A fuse with de ned rating should not be replaced by the one with a higher rating
because a fuse wire works due to its low melting point which is due to its rating and
if a fuse with higher rating is used than it might not melt and thus failing in its
purpose of using it.
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