Pressure & Pressure Differences in Fluids Flashcards

All 26 cards in this deck

What is a fluid?

A liquid or a gas.

Which equation links pressure ($p$), force normal to a surface ($F$) and area ($A$)?

$p = \dfrac{F}{A}$

In the equation $p = \dfrac{F}{A}$, what are the units of $p$, $F$ and $A$?

$p$ in pascals (Pa), $F$ in newtons (N), $A$ in metres squared (m$^2$).

In which direction does the pressure in a fluid exert a force on a surface?

Normal (at right angles) to the surface.

How many cm$^2$ are there in 1 m$^2$?

$10\,000$ — so divide an area in cm$^2$ by $10\,000$ to get m$^2$.

What are the steps to calculate the area of a surface from the force normal to it and the pressure, giving the answer in standard form?

e.g. force $= 60$ N, pressure $= 120\,000$ Pa

1. Write the equation: $p = \dfrac{F}{A}$
2. Rearrange: $A = \dfrac{F}{p} = \dfrac{60}{120\,000}$
3. Work out and convert: $A = 0.0005 = 5 \times 10^{-4}\ \mathrm{m^2}$

True or false? Pressure is measured in newtons.

False. Pressure is measured in pascals (Pa); $1$ Pa $= 1$ N/m$^2$.

In a simple model, what is the Earth's atmosphere?

A thin layer of air surrounding the Earth — thin compared with the size of the Earth.

What causes atmospheric pressure on a surface?

Air molecules colliding with the surface.

What happens to the density of the atmosphere as altitude increases?

It decreases — the air gets less dense higher up.

As the height of a surface above ground level increases, what happens to the number of air molecules (and so the weight of air) above it?

It decreases, so the atmospheric pressure on the surface decreases.

True or false? Atmospheric pressure at the top of a mountain is the same as at sea level.

False. It is lower, because there is less air above the surface at greater height.

True or false? Atmospheric pressure pushes only downwards on a surface.

False. The pressure of a fluid produces a force normal to any surface, whatever its orientation.

What is the equation for the pressure due to a column of liquid?

$p = h\,\rho\,g$ (height of column $\times$ density of liquid $\times$ gravitational field strength).

In $p = h\rho g$, what are the units of $h$, $\rho$ and $g$?

$h$ in metres (m), $\rho$ in kg/m$^3$, $g$ in N/kg, giving $p$ in pascals (Pa).

What are the steps to calculate the pressure due to a column of liquid?

e.g. depth $2$ m of water, $\rho = 1000$ kg/m$^3$, $g = 10$ N/kg

1. Write the equation: $p = h\rho g$
2. Substitute: $p = 2 \times 1000 \times 10$
3. Answer with unit: $p = 20\,000$ Pa

What are the steps to find the difference in pressure between two depths in a liquid?

e.g. depths $2$ m and $5$ m in water, $\rho = 1000$ kg/m$^3$, $g = 10$ N/kg

1. Find the difference in depth: $5 - 2 = 3$ m
2. Use $p = h\rho g$ with that difference: $3 \times 1000 \times 10$
3. Answer with unit: difference $= 30\,000$ Pa

What happens to the pressure on a submarine as its depth increases, and why?

The pressure increases, because the height of the column of water above it increases, so the weight of water acting on it is greater.

How does the density of a liquid affect the pressure at a given depth in it?

The denser the liquid, the greater the pressure — the column of liquid above the point weighs more.

True or false? The pressure at a point in a liquid depends on the width and shape of the container.

False. It depends only on the depth below the surface, the density of the liquid and $g$.

What is upthrust?

The resultant upward force on an object in a fluid, caused by the pressure on its bottom surface being greater than on its top surface.

Why is the pressure on the bottom surface of a submerged object greater than on its top surface?

The bottom surface is at a greater depth, so there is a greater height of liquid above it.

What is the condition on the forces for an object to float in a fluid?

The upthrust equals the weight of the object (the forces balance).

What happens to an object whose weight is greater than the maximum upthrust the fluid can provide?

It sinks — there is a resultant downward force.

True or false? An object floats if its density is less than the density of the fluid it is placed in.

True. It can displace enough fluid to give an upthrust equal to its weight before it is fully submerged.

True or false? Upthrust acts only on objects that are totally submerged.

False. A partially submerged object also experiences upthrust.