1
JEE Main 2021 (Online) 31st August Morning Shift
MCQ (Single Correct Answer)
+4
-1
Out of Syllabus
Change Language
A reversible engine has an efficiency of $${1 \over 4}$$. If the temperature of the sink is reduced by 58$$^\circ$$C, its efficiency becomes double. Calculate the temperature of the sink :
A
174$$^\circ$$C
B
280$$^\circ$$C
C
180.4$$^\circ$$C
D
382$$^\circ$$C
2
JEE Main 2021 (Online) 31st August Morning Shift
MCQ (Single Correct Answer)
+4
-1
Change Language
An object is placed at the focus of concave lens having focal length f. What is the magnification and distance of the image from the optical centre of the lens?
A
1, $$\infty$$
B
Very high, $$\infty$$
C
$${1 \over 2}$$, $${f \over 2}$$
D
$${1 \over 4}$$, $${f \over 4}$$
3
JEE Main 2021 (Online) 31st August Morning Shift
MCQ (Single Correct Answer)
+4
-1
Out of Syllabus
Change Language
A sample of a radioactive nucleus A disintegrates to another radioactive nucleus B, which in turn disintegrates to some other stable nucleus C. Plot of a graph showing the variation of number of atoms of nucleus B versus time is :

(Assume that at t = 0, there are no B atoms in the sample)
A
JEE Main 2021 (Online) 31st August Morning Shift Physics - Atoms and Nuclei Question 103 English Option 1
B
JEE Main 2021 (Online) 31st August Morning Shift Physics - Atoms and Nuclei Question 103 English Option 2
C
JEE Main 2021 (Online) 31st August Morning Shift Physics - Atoms and Nuclei Question 103 English Option 3
D
JEE Main 2021 (Online) 31st August Morning Shift Physics - Atoms and Nuclei Question 103 English Option 4
4
JEE Main 2021 (Online) 31st August Morning Shift
MCQ (Single Correct Answer)
+4
-1
Change Language
A coil having N turns is wound tightly in the form of a spiral with inner and outer radii 'a' and 'b' respectively. Find the magnetic field at centre, when a current I passes through coil:
A
$${{{\mu _0}IN} \over {2(b - a)}}{\log _e}\left( {{b \over a}} \right)$$
B
$${{{\mu _0}I} \over 8}\left[ {{{a + b} \over {a - b}}} \right]$$
C
$${{{\mu _0}I} \over {4(a - b)}}\left[ {{1 \over a} - {1 \over b}} \right]$$
D
$${{{\mu _0}I} \over 8}\left( {{{a - b} \over {a + b}}} \right)$$
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