physics : class11 Surface Energy Definition : Surface Energy : Properties of Bulk Matter MCQ Questions and Answers
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Surface Energy Definition
MCQ questions and answers.
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MCQ Questions
1. For a liquid surface at constant temperature, the surface energy is most precisely defined as the:
A.
Work required to increase the surface area by unit area
B.
Force required to move a molecule from the surface into the liquid
C.
Energy required to increase the volume of the liquid by unit volume
D.
Work required to move a liquid molecule along the surface by unit distance
Correct Answer:
Work required to increase the surface area by unit area
Explanation:
Surface energy is the work needed per unit increase in surface area, provided the temperature remains constant. Thus, surface energy is expressed as energy per unit area.
2. A liquid surface has surface energy density γ. If its area increases reversibly by ΔA at constant temperature, the increase in surface energy is:
A.
γΔA
B.
γ/ΔA
C.
ΔA/γ
D.
γ + ΔA
Correct Answer:
γΔA
Explanation:
By definition, γ is the increase in surface energy per unit increase in area. Therefore, the surface-energy increase is ΔU = γΔA.
3. Which pair of dimensions is equivalent to the dimensions of surface energy?
A.
ML²T⁻²
B.
MLT⁻²
C.
ML⁻¹T⁻²
D.
M⁰L²T⁻¹
Correct Answer:
ML²T⁻²
Explanation:
Surface energy is a form of energy, and energy has dimensions ML²T⁻². Although surface energy per unit area has units J m⁻², the surface energy itself has energy dimensions.
4. The SI unit J m⁻² for surface energy density is dimensionally identical to:
A.
N m⁻¹
B.
N m
C.
N m⁻²
D.
kg m⁻¹ s⁻¹
Correct Answer:
N m⁻¹
Explanation:
Since 1 J = 1 N m, J m⁻² = N m m⁻² = N m⁻¹. This represents energy required per unit area.
5. A liquid surface initially has area 0.020 m². Its surface energy density is 0.072 J m⁻². If the area becomes 0.035 m² at constant temperature, the increase in surface energy is:
A.
1.08 × 10⁻³ J
B.
2.52 × 10⁻³ J
C.
3.96 × 10⁻³ J
D.
5.04 × 10⁻³ J
Correct Answer:
1.08 × 10⁻³ J
Explanation:
The area increase is 0.035 − 0.020 = 0.015 m². Hence, ΔU = γΔA = 0.072 × 0.015 = 1.08 × 10⁻³ J.
6. A soap film is stretched so that the area of each of its two surfaces increases by ΔA. If its surface energy density is γ, the total increase in surface energy is:
A.
γΔA
B.
2γΔA
C.
γΔA/2
D.
4γΔA
Correct Answer:
2γΔA
Explanation:
A soap film has two liquid-air surfaces. Each surface contributes γΔA, so the total increase in surface energy is γΔA + γΔA = 2γΔA.
7. A surface has surface energy density 0.050 J m⁻². If its surface energy increases by 2.0 × 10⁻⁴ J, the corresponding increase in surface area is:
A.
2.5 × 10⁻⁶ m²
B.
4.0 × 10⁻⁴ m²
C.
4.0 × 10⁻³ m²
D.
2.5 × 10⁻² m²
Correct Answer:
4.0 × 10⁻³ m²
Explanation:
Using ΔU = γΔA, the area increase is ΔA = ΔU/γ = (2.0 × 10⁻⁴)/0.050 = 4.0 × 10⁻³ m².
8. Two liquid surfaces have the same surface energy density. Surface P increases its area by 3A, while surface Q increases its area by A. The ratio of their increases in surface energy is:
A.
1:3
B.
3:1
C.
1:9
D.
9:1
Correct Answer:
3:1
Explanation:
For the same surface energy density, the increase in surface energy is directly proportional to the increase in area. Therefore, γ(3A):γ(A) = 3:1.
9. Which statement correctly distinguishes surface energy from total surface energy density?
A.
Surface energy is energy per unit area, whereas surface energy density is total energy
B.
Surface energy is the total excess energy associated with a surface, whereas surface energy density is this energy per unit area
C.
Surface energy depends only on volume, whereas surface energy density depends only on mass
D.
Surface energy is measured in newtons, whereas surface energy density is measured in joules
Correct Answer:
Surface energy is the total excess energy associated with a surface, whereas surface energy density is this energy per unit area
Explanation:
Surface energy refers to the excess energy associated with the existence of a surface. Surface energy density γ is the surface energy divided by area, or equivalently the energy required to create unit area.
10. A liquid surface is divided into two separate surfaces without changing the total liquid volume. According to the definition of surface energy, the system's surface energy changes because:
A.
Surface energy is proportional to the total surface area created
B.
Surface energy is proportional only to the liquid's mass
C.
Surface energy decreases whenever the number of surfaces increases
D.
Surface energy is independent of surface area at constant temperature
Correct Answer:
Surface energy is proportional to the total surface area created
Explanation:
At constant temperature, the surface-energy change is γ times the change in total surface area. Creating additional surfaces generally increases the total area and therefore increases the surface energy.
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