Mechanical Properties of Fluids - Online Test
Q1. According to Stokes’ law if η is the coefficient of viscosity, 'R' the radius of the sphere and it is moving with a velocity v the viscous drag force F is given by
Answer : Option B
Explaination / Solution:
The force of viscosity on a small sphere moving through a viscous fluid is given by:
Where:
- Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particle
- η is the dynamic viscosity
- R is the radius of the spherical object
- v is the flow velocity relative to the object.
In SI units, Fd is given in Newtons, η in Pa·s, R in meters, and v in m/s.
Q2. If is the density of the fluid flowing with a speed v, D stands for the dimension of the pipe, and is the viscosity of the fluid Reynolds number is given by
Answer : Option A
Explaination / Solution:
It is a dimension less number comprised of the physical characteristics of the flow. An increasing Reynolds number indicates an increasing turbulence of flow.
It is defined as:
where;
v is the flow velocity
D is a characteristic linear dimension
ρ fluid density(kg/m3)
is dynamic viscosity (Pa.s)
ν kinematic viscosity (m2/s),
Q3. The molecules on the surface,are-
Answer : Option A
Explaination / Solution:
The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.A molecule at the surface of a liquid experiences only net inward cohesive forces.The unbalanced attraction of molecules at the surface of a liquid tends to pull the molecules back into the bulk liquid leaving the minimum number of molecules on the surface. It required energy to increase the surface area of a liquid because a larger surface area contains more molecules in the unbalanced situation and this may results as the surface being stretched.
Q4. If a film of width L is stretched in the longitudinal direction a distance d by force F, surface tension is given by
Answer : Option C
Explaination / Solution:
Let L be the width of the film. Since the film has two surfaces, the total length along which the surface force acts on the slider is 2L. The surface tension S in the film is defined as the ratio of the surface force F to the length d (perpendicular to the force) along which the force acts
S=F/d
Hence, in the case, d = 2L
S=F/2L
Q5. Capillary Rise of water is explained by
Answer : Option C
Explaination / Solution:
Capillary action(sometimes capillarity or wicking) is the ability of a liquid to flow in narrow spaces without the assistance of,or even in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a cell. It occurs because of intermolecular forces between the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to propel the liquid.
Q6. An iron needle floats on the surface of water. This phenomenon is attributed to
Answer : Option C
Explaination / Solution:
The answer lies in the surface tension of the water, As the cohesive forces between the molecules is responsible for the surface tension. When the molecules on the glass or container do not have a surface to stick to, they cohere more strongly to the molecules they are associated with. Since the molecules on top have no neighbours on the top to cohere to, they associate with their other neighbours more strongly. This creates a strong bond between the top layer of molecules. The molecules inward face no net force because they all cancel out each other. However, the surface molecules face a net inward force because there is no force for them on the surface to cancel it. This causes the molecules on the surface to 'contract' and resist being broken or stretched.
When a small body like a needle is placed onto the water carefully, the surface tension will not break due to the small mass of the object.
As long as the surface tension is undisturbed, the needle will continue to float. Agitating the water will cause the surface tension to break and hence the needle will then sink
As long as the surface tension is undisturbed, the needle will continue to float. Agitating the water will cause the surface tension to break and hence the needle will then sink
Q7. The excess pressure across the surface of a soap bubble of radius r(with σ as the surface tension of soap solution)is
Answer : Option B
Explaination / Solution:
No Explaination.
Q8. Two soap bubbles are blown simultaneously .In the first soap bubble, the excess pressure is 4 times of second soap bubble. The ratio of radii of first to second soap bubble is
Answer : Option B
Explaination / Solution:
No Explaination.
Q9. A drop of oil is placed on the surface of water. Which of the following statements is correct?
Answer : Option D
Explaination / Solution:
when any liquid is poured onto another liquid there are different phenomena that can take place depending on the properties of liquids:In case of crude oil and water -
The two liquids are non-miscible and the first is lighter than the second. The first liquid then spreads on the surface of the second liquid forming a thin layer. The thickness of this layer depends on the properties of the two liquids. This is the case for most hydrocarbons, including both refined products and crude oil of API gravity greater than 10.
Q10. Two small drops of mercury, each of radius r, form a single large drop. The ratio of surface energy before and after this change is
Answer : Option A
Explaination / Solution:
