1. What is the excess pressure inside a soap bubble of radius r and surface tension T?
A.
T/r
B.
2T/r
C.
4T/r
D.
T/4r
Correct Answer:
4T/r
Explanation:
A soap bubble has two liquid surfaces, inner and outer. Therefore, its excess pressure is ΔP = 4T/r.
2. Why is the excess pressure inside a soap bubble greater than that inside a liquid drop of the same radius and surface tension?
A.
A soap bubble has two surfaces
B.
A soap bubble has no surface tension
C.
A liquid drop contains more liquid
D.
A soap bubble has a larger radius
Correct Answer:
A soap bubble has two surfaces
Explanation:
A soap bubble has inner and outer surfaces, so the excess pressure is 4T/r, whereas for a liquid drop it is 2T/r.
3. If the radius of a soap bubble is doubled while its surface tension remains constant, its excess pressure becomes:
A.
Twice as large
B.
Half as large
C.
Four times as large
D.
Unchanged
Correct Answer:
Half as large
Explanation:
For a soap bubble, ΔP = 4T/r. Thus, excess pressure is inversely proportional to radius.
4. A soap bubble has surface tension 0.03 N m−1 and radius 0.01 m. What is its excess pressure?
A.
3 Pa
B.
6 Pa
C.
12 Pa
D.
24 Pa
Correct Answer:
12 Pa
Explanation:
Using ΔP = 4T/r, ΔP = 4 × 0.03/0.01 = 12 Pa.
5. For two soap bubbles made of the same liquid, which bubble has greater excess pressure?
A.
The bubble with larger radius
B.
The bubble with smaller radius
C.
Both always have equal pressure
D.
The bubble with greater volume only
Correct Answer:
The bubble with smaller radius
Explanation:
Since ΔP = 4T/r, excess pressure increases when the radius decreases, provided the surface tension is the same.
6. The excess pressure inside a soap bubble is directed in which way relative to the surrounding atmosphere?
A.
The pressure inside is greater than outside
B.
The pressure inside is less than outside
C.
The pressure inside is always equal to outside
D.
There is no pressure inside the bubble
Correct Answer:
The pressure inside is greater than outside
Explanation:
Surface tension tends to contract the bubble, producing an excess pressure inside it compared with the outside pressure.
7. If the surface tension of the liquid film of a soap bubble is increased while its radius remains constant, the excess pressure will:
A.
Decrease
B.
Increase
C.
Become zero
D.
Remain unchanged
Correct Answer:
Increase
Explanation:
From ΔP = 4T/r, excess pressure is directly proportional to surface tension when the radius is constant.
8. A soap bubble has radius 0.02 m and surface tension 0.04 N m−1. Its excess pressure is:
A.
2 Pa
B.
4 Pa
C.
8 Pa
D.
16 Pa
Correct Answer:
8 Pa
Explanation:
Using ΔP = 4T/r, ΔP = 4 × 0.04/0.02 = 8 Pa.
9. What happens to the excess pressure of a soap bubble when its radius becomes very small?
A.
It becomes very large
B.
It becomes zero
C.
It remains constant
D.
It becomes equal to surface tension
Correct Answer:
It becomes very large
Explanation:
The relation ΔP = 4T/r shows that excess pressure increases greatly as the radius becomes smaller.
10. For a soap bubble of radius r, the factor 4 in the formula ΔP = 4T/r arises because:
A.
The bubble has two surfaces and each contributes 2T/r
B.
The bubble has one surface contributing 4T/r
C.
The bubble contains four layers of liquid
D.
The radius is measured in four directions
Correct Answer:
The bubble has two surfaces and each contributes 2T/r
Explanation:
Each surface of the soap bubble contributes 2T/r, and the inner and outer surfaces together give ΔP = 4T/r.
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