physics : class11 Surface Energy of a Soap Bubble : Surface Energy : Properties of Bulk Matter MCQ Questions and Answers
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Surface Energy of a Soap Bubble
MCQ questions and answers.
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MCQ Questions
1. A soap bubble has two liquid surfaces. If its radius is r and the surface tension is T, what is its total surface energy?
A.
4πr²T
B.
8πr²T
C.
2πrT
D.
8πrT
Correct Answer:
8πr²T
Explanation:
A soap bubble has two spherical surfaces, and each surface has area 4πr². Therefore, total surface energy is T × 2(4πr²) = 8πr²T.
2. Why is the surface energy of a soap bubble twice that of a single spherical liquid surface of the same radius?
A.
The bubble contains twice the volume
B.
The bubble has two surfaces
C.
The bubble has twice the radius
D.
The bubble has twice the density
Correct Answer:
The bubble has two surfaces
Explanation:
A soap bubble has an inner surface and an outer surface. Both surfaces possess surface energy, so the total surface energy is twice that of one spherical surface.
3. The surface energy of a soap bubble is directly proportional to which quantity when surface tension is constant?
A.
Its radius
B.
The square of its radius
C.
The cube of its radius
D.
The inverse of its radius
Correct Answer:
The square of its radius
Explanation:
The surface energy of a soap bubble is E = 8πr²T. Thus, for constant surface tension, E is directly proportional to r².
4. If the radius of a soap bubble is doubled without changing its surface tension, its surface energy becomes:
A.
Two times
B.
Four times
C.
Half
D.
Unchanged
Correct Answer:
Four times
Explanation:
Since surface energy E is proportional to r², doubling the radius gives E' = (2r)²/r² × E = 4E.
5. A soap bubble of radius r has surface tension T. Which expression represents the area responsible for its total surface energy?
A.
4πr²
B.
8πr²
C.
2πr²
D.
πr²
Correct Answer:
8πr²
Explanation:
The bubble has two spherical surfaces, each of area 4πr². Hence, the total area contributing to surface energy is 2 × 4πr² = 8πr².
6. The SI unit of surface energy of a soap bubble is:
A.
N
B.
J
C.
Pa
D.
m²
Correct Answer:
J
Explanation:
Surface energy is the energy required to create the surfaces of the bubble, so its SI unit is the joule (J).
7. For a soap bubble of fixed radius, its surface energy increases when:
A.
Surface tension decreases
B.
Surface tension increases
C.
The number of surfaces decreases
D.
Its volume becomes zero
Correct Answer:
Surface tension increases
Explanation:
The surface energy is E = 8πr²T. For fixed radius, E increases directly with surface tension T.
8. A soap bubble has radius 1 cm. If its radius is increased to 3 cm while surface tension remains constant, its surface energy becomes:
A.
Three times
B.
Six times
C.
Nine times
D.
One-third
Correct Answer:
Nine times
Explanation:
Surface energy is proportional to the square of the radius. Therefore, E' / E = (3/1)² = 9.
9. The work required to form a soap bubble of radius r and surface tension T is equal to:
A.
2πr²T
B.
4πr²T
C.
8πr²T
D.
16πr²T
Correct Answer:
8πr²T
Explanation:
The work required to form the bubble equals its surface energy. Since a soap bubble has two surfaces, the work is W = 8πr²T.
10. If the surface tension of a soap solution is doubled while the bubble radius remains unchanged, the surface energy of the bubble becomes:
A.
Half
B.
Unchanged
C.
Twice
D.
Four times
Correct Answer:
Twice
Explanation:
For a bubble of fixed radius, surface energy E = 8πr²T is directly proportional to surface tension. Doubling T therefore doubles E.
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Surface Energy of a Soap Bubble
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