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Physics Assertion (A) & Reason (B) MCQ

Magnetism And Matter · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 20 questions.

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Assertion (A) & Reason (B) MCQ

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Question 11 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): The true geographic north direction is found by using a compass needle.
Reason (R): The magnetic meridian of the earth is along the axis of rotation of the earth.
Answer
  1. A is false and R is also false.
Explanation:
From the compass we are able to know the poles.the north of compass points towards the magnetic south pole.

If we know the magnetic declination at that particular place (which is angle between geographic meridian and magnetic meridian) we can easily find out the true geographic north-south direction. Imaginary lines drawn along the earth's surface in the direction of the horizontal component of the magnetic field of the earth at all points passing through the north and south magnetic poles. This is similar to the longitudes of the earth, which pass through the geographic north and south poles.
 
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Question 21 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): The ends of a magnet suspended freely point out always along north south.
Reason (R): Earth behaves as a huge magnet.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:
Earth's magnetic field can be represented as the field of a huge bar magnet. If the magnet is freely suspended its north-pole points towards geographic north pole (really a south magnet pole of earth)
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Question 31 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): Earth's magnetic field does not affect the working of a moving coil galvanometer.
Reason (R): The earth's magnetic field is quite weak as compared to magnetic field produced in the moving coil galvanometer.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:
The field magnet used in a moving coil galvanometer is very strong. The earth's magnetic field is quite weak as compared to the magnetic field produced by the field magnet. Practically the coil rotates under the effect of the strong magnetic field due to the field magnet and the weak magnetic field due to the earth does not affect the working of the moving coil galvanometer.
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Question 41 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: To protect any instrument from external magnetic field, it is put inside an iron box
Reason: Iron is a ferromagnetic substance
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true and R is not correct explanation of A.
  3. A is true, But R is false.
  4. A is false, But R is true.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:
To protect any instrument from external magnetic field it is put inside an iron box as iron is a ferromagnetic substance it will not get magnetize by the magnetic field due which there will not be any effect of magnetic field on the instrument.
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Question 51 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: Basic difference between an electric lines of force and magnetic lines of force is that former is discontinuous and the latter is continuous or endless.
Reason: No electric lines of force exist inside a charged body but magnetic lines do exist inside a magnet.
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Explanation:
Electric lines of current is nothing but flow which is from one electron to another within a closed path.
But magnetic field lines are representation magnetic force which extends everywhere with decrease in value with increase in distance.
It flows inside the magnet as well.
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Question 61 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: A magnet remains stable, If it aligns itself with the field
Reason: The P.E. of a bar magnet is minimum, if it is parallel to magnetic field.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true and R is not correct explanation of A.
  3. A is true, But R is false.
  4. A is false, But R is true.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:
The magnet remains stable,if it align itself with the field as  its potential will be minimum in that position.
$\mu=-\overrightarrow{\text{m}}.\overrightarrow{\text{B}}=-\text{MB}\cos\theta$
The potential energy will be minimum if $\theta=0$ i.e. −MB
$\therefore$ If it is parallel to magnet field.
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Question 71 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: Magnetic force is always perpendicular to the magnetic field.
Reason: Electric force is along the direction of electric field.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true and R is not correct explanation of A.
  3. A is true, But R is false.
  4. A is false, But R is true.
Answer
  1. A is true, But R is false.
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Question 81 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: It is more difficult to push a magnet into a coil with more loops.
Reason: Emf induced in the current loop resists the motion of the magnet.
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Explanation:
According to Lenz's law, the emf induced in a current loop is such that it resists the motion of the magnet As emf in each turn opposes the motion of the magnet, when the number of turns is increased the force opposing the motion also increases.
Thus pushing a magnet into a coil with more number of loops is more difficult.
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Question 91 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): At neutral point, a compass needle point out in any arbitrary direction.
Reason (R): Magnetic field of earth is balanced by field due to magnets at the neutral points.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:
A neutral point in the magnetic field of a bar magnet is that point, where the field due to magnet is completely neutralised by the horizontal component of earth's magnetic field. The net horizontal field is zero at such a point. If a compass needle is placed at such a point, it can stay in any position.
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Question 101 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: We cannot think of a magnetic field configuration with three poles.
Reason: A bar magnet does exert a torque on itself due to its own field.
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are incorrect.
Explanation:
The bar magnet or any other magnet does not exert a torque on itself in its own magnetic field.
Torque $\tau\propto\overrightarrow{\text{M}}\times\overrightarrow{\text{B}}.$ As the magnetic field B is due to the magnetic moment M the value of cross product will be zero.
$\because\theta=0$
Magnetic poles exist in pairs. However one can visualize, theoretically, magnetic field due to three poles formed by placing the north poles of the two magnets in contact and the south poles separated by some distance. 
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Question 111 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: The ferro magnetic substances do not obey curie's law.
Reason: At curie point ferromagnetic substances start behaving as a paramagnetic substances.
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are incorrect.
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Question 121 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: Basic difference between an electric line and magnetic line of force is that former is discontinuous and the later is continuous or endless.
Reason: No electric lines of force exist inside a charged body but magnetic lines do exist inside a magnet.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true and R is not correct explanation of A.
  3. A is true, But R is false.
  4. A is false, But R is true.
Answer
  1. Both A and R are true and R is the correct explanation of A.
Explanation:
In case of the electric field of an electric dipole, the electric lines of force originate from positive charge and, end at negative charge.
Since, isolated magnetic lines are closed continuous loops extending throughout the body of magnet, hence they form endless curves.
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MCQ 131 Mark
Two statements are given$-$one labelled Assertion$(A) a$nd the other labelled Reason $(R).$ Select the correct answer to these questions from the codes$(a), (b), (c)$ and $(d)$ as given below.
Assertion $(A):$ Magnetic moment is measured in joule/ tesla or amp $m^2.$
Reason $(R):$ Joule/ tesla is equivalent to amp $m^2.$
  • ✓
    Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
  • B
    Both $A$ and $R$ are true but $R$ is $\text{NOT}$ the correct explanation of $A.$
  • C
    $A$ is true but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
Answer
Correct option: A.
Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
Magnetic moment $=\frac{\text{joule}}{\text{tesla}}=\frac{\text{W}}{\text{B}}=\frac{\text{W}}{\text{F\ qv}}$
$=\frac{\text{Wqv}}{\text{F}}=\frac{[\text{MF}^2\text{T}^{-2}][\text{AT}[\text{LT}^{-1}]]}{[\text{MLT}^{-2}]}$
$=[\text{AL}^2]=\text{amp m}^2$
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Question 141 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets.
Reason: Permanent magnet can be made by placing a ferromagnetic rod in a solenoid and passing current through it.
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
Explanation:
The substances that can retain their magnetic properties at the room temperature for a longer time are termed as the permanent magnets.
The permanent magnets can be obtained by keeping the ferromagnetic rod in a solenoid and the magnetic field of the solenoid magnetizes the material.
Therefore, the assertion and reason both are correct but reason is not the correct explanation of the assertion.
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Question 151 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): There is only one neutral points on a horizontal board when a magnet is held vertically on the board.
Reason (R): At the neutral point the net magnetic field due to the magnetic and magnetic field of the earth is zero.
Answer
  1. Both A and Rare true but R is NOT the correct explanation of A.
Explanation:
There will be only one neutral point on the horizontal board. This is because field of earth magnetic field is from south to north; and the field of pole on the board is radially outwards. At any point towards south of magnetic pole, field of earth and field of pole will cancel out to give a neutral point.
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Question 161 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): A clinic lines on the magnetic map represents lines of equal dip.
Reason (R): When the horizontal and vertical components of the earth's magnetic field are equal, the angle of dip is 60º.
Answer
  1. A is false and R is also false.
Explanation:
The angle of dip is the angle between the axis of the dip needle in the magnetic meridian and the horizontal direction.
$\tan\theta=\frac{\text{Vertical component of the earth's magnetic field}}{\text{Horizontal component of the earth's magnetic field}}$
$=\frac{\text{B}_\text{V}}{\text{B}_\text{H}}$ When, $\text{B}_\text{V}=\text{B}_\text{H},\tan\theta=1\Rightarrow\theta=45^\circ$
Lines joining points of zero dip are called Aclinic lines. Lines joining points of equal dip are called isoclinic lines.
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Question 171 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): Magnetic moment of an atom is due to both, the orbital motion and spin motion of every electron.
Reason (R): A charged particle produces a magnetic field. 
Answer
  1. A is true but R is false.
Explanation:
In an atom, electrons revolve around the nucleus and as such the circular orbits of electrons may be considered as the small current loops. In addition to orbital motion, an electron has got spin motion also. So the total magnetic moment of electron is the vector sum of its magnetic moments due to orbital and spin motion.
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Question 181 Mark
Directions: In the following questions, the Assertions (A) and Reason(s) (R) have been put forward. Read both the statements carefully and choose the correct alternative from the following:
Assertion: χ - T graph for a diamagnetic material is a straight line parallel to T axis.
Reason: This is because susceptibility of a diamagnetic material is not affected by temperature
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
  2. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
  3. Assertion is correct but Reason is incorrect.
  4. Both Assertion and Reason are incorrect.
Answer
  1. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
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Question 191 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): The earth's magnetic field is due to iron present in its core.
Reason (R): At a low temperature magnet losses its magnetic property or magnetism.
Answer
  1. A is false and R is also false.
Explanation:
The temperature inside the earth is so high that it is impossible for iron core to behave as a magnet and act as a source of magnetic field. The magnetic field of earth is considered to be due to circulating electric current in the iron (in molten state) and other conducting materials inside the earth.
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Question 201 Mark
Two statements are given-one labelled Assertion(A) and the other labelled Reason (R). Select the correct answer to these questions from the codes(a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): A compass needle when placed on the magnetic north pole of the earth rotates in vertical direction.
Reason (R): The earth has only horizontal component of its magnetic field at the north poles.
Answer
  1. A is false and R is also false.
Explanation:
The earth has only vertical component of its magnetic field at the magnetic poles. Since compass needle is only free to rotate in horizontal plane. At north pole the vertical component of earth's field will exert torque on the magnetic needle so as to align it along its direction. As the compass needle can not rotate in vertical plane, it will rest horizontally, when placed on the magnetic north pole of the earth.
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