Pressure and pressure differences in fluids Flashcards
What is meant by the term ‘fluid’?
Something that can flow. A liquid or a gas.
In any fluid, at what angles do the forces due to pressure act on a given surface?
At right angles to the surface.
State the equation used to calculate pressure. Give appropriate units
Pressure = force / area
Pressure (pascals or N/m²), force (newtons), area (m²).
What happens to the density of the atmosphere with increasing altitude?
The atmosphere becomes less dense as altitude increases.
Explain why atmospheric pressure decreases with an increase in height
- Air molecules colliding with a surface create atmospheric pressure.
- As height increases, the number of air molecules (and so weight) above a surface decreases.
- So as height increases, there is less air above a surface than their is at a lower height. Therefore lower collisions with surfaces.
- This means atmospheric pressure decreases with an increase in height.
What is the earth’s atmosphere?
A thin (relative to the magnitude of the earth) layer of gas surrounding the earth.
How does pressure in fluids increase with depth?
As the depth increases, the mass of liquid above that depth also increases. This means that the force (and weight) due to the mass increases. Since the force has increased whilst the area has remained constant, the pressure will increase.
Why does pressure in fluids increase with density?
As the density of a fluid increases, the number of particles in a given volume increases. Consequently, the weight of the fluid is greater. This means that the force from the fluid above a certain point is larger. Since the force has increased, the pressure also increases.
What equation shows the magnitude of pressure in liquids at different depths?
Pressure due to column of liquid = column height x density x gravitational field strength
Pressure (pascal/Pa), column height (m), density (kg/m³), gravitational field strength (N/kg).
Why are objects in a fluid subject to an upthrust?
When an object is submerged in a fluid, it has a higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top. This leads to an upwards force called upthrust. You float or sink depending on whether the upthrust is more or less than your weight.
What is upthrust always equal to?
The weight of the fluid that the object displaces.
What factors influence whether an object will sink or float?
- Upthrust.
- Weight.
- Density of fluid.
Explain why an object with a density greater than that of water can never float
Upthrust is equal to the weight of the displaced fluid. If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object’s weight. This means that it will sink.
