Liquids Flashcards
The blood pressure in humans is greater at the feet than at the brain
The height of the blood column in the human body is more at feet than at the brain. That is why the blood exerts more pressure at the feet than at the brain.
Atmospheric pressure at a height of about 6 km decreases to nearly half of
its value at the sea level, though the height of the atmosphere is more than
100 km
Density of air decreases very rapidly with an increase in height and reduces to nearly half its value at the sea level at a height of about 6 km. After
km, the density of air decreases but rather very slowly.
Hydrostatic pressure is a scalar quantity even though pressure is force
divided by area
When force is applied on a liquid, then according to Pascal’s Law, the pressure in the liquid is transmitted in all directions. Hence, hydrostatic pressure does not have a fixed direction and it is a scalar physical quantity.
The angle of contact of mercury with glass is obtuse, while that of water
with glass is acute.
mercury and glass: - adhesive force is less than cohesive force.
water and glass: - adhesive force is greater than cohesive force.
Water on a clean glass surface tends to spread out while mercury on the
same surface tends to form drops. (Put differently, water wets glass while
mercury does not.)
Water on a clean glass surface tends to spread out i.e., water wets glass because force of cohesion of water is much less than the force of adhesion due to glass. In case of mercury force of cohesion due to mercury molecules is quite strong as compared to adhesion force due to glass.
Surface tension of a liquid is independent of the area of the surface
Surface tension is defined as the force acting per unit length on either side of a imaginary line drawn tangentially on the surface of a liquid. As the force is independent of area of liquid surface taken, so is the surface tension.
Water with detergent dissolved in it should have small angles of contact.
The cloth has narrow spaces in the form of capillares. The rise of liquid in a capillary tube is directly proportional to cos θ. If θ is small cos θ will be large. Hence capillary rise will be more so that the detergent will penetrate more in cloth and hence gets dissolved.
A drop of liquid under no external forces is always spherical in shape
A liquid tends to acquire the minimum surface area because of the presence of surface tension. The surface area of a sphere is the minimum for a given volume. Hence, under no external forces, liquid drops always take a spherical shape.
To keep a piece of paper horizontal, you should blow over, not under, it
When we blow over the piece of paper, then the velocity of air increases. As a result From the Bernoulli’ theorem, the pressure over it decreases whereas the atmospheric pressure below remains the same. Thus, to keep the paper horizontal, we should blow over the paper not under it.
When we try to close a water tap with our fingers, fast jets of water gush
through the openings between our fingers
When we try to close a tap of water with our fingers, fast jets of water gush through the openings between our fingers. This is because very small openings are left for the water to flow out of the pipe. Hence, area and velocity are inversely proportional to each other. Therefore velocity of water increases.
The size of the needle of a syringe controls flow rate better than the thumb
pressure exerted by a doctor while administering an injection
Because of the extremely small size of the opening of a needle, its size can control the flow with more precision than the thumbs of a doctor. According to the equation of continuity area * velocity= constant. if the area is very small the velocity must be large. Thus if the area is small flow becomes smooth
A fluid flowing out of a small hole in a vessel results in a backward thrust on
the vessel
When a fluid is flowing out of a small hole in a vessel, it acquires a large velocity and hence possesses large momentum. Since, no external force is acting on the system, a backward velocity must be attained by the vessel (according to law of conservation of momentum) is experienced by the vessel.
A spinning cricket ball in air does not follow a parabolic trajectory
Magnus effect
A spinning ball displaces air. The ball moves formed and relative to it, the air moves backward. Hence, the velocity of air above the ball relative to it is larger and below it is smaller. This difference in the velocities of air results in a pressure difference between the lower and upper faces leading to a net upward force on the ball. Therefore the ball does not follow a parabolic trajectory.
