Liquids Flashcards
Pressure in a liquid
The force divided by the area over which the force is exerted
- liquids practically incompressible
- except for small changes produced by temperature, the density of a particular liquid is practically the same at all depths
Pressure = force / area
Pressure depends on
Depth and density of liquid
- NOT volume
- water dams depend on average depth, not volume
Liquid pressure = density x depth
When liquid presses against a surface
There is a net force that is perpendicular to the surface
- this is why water spurting from a hole in a bucket exits in direction at right angles to the surface of the bucket, then curves downward due to gravity
Buoyancy
- loss of weight experienced by objects submerged in a liquid
- consequence of pressure increasing with depth
- when submerged, the water exerts an upward force on the object that is exactly opposite to the direction of gravity’s pull
Buoyancy cont
- upward forces against the bottom are greater than downward forces against the top, producing a net upward force
- a completely submerged object always displaces a volume of liquid equal to its own volume
Archimede’s principle
An immersed object is buoyed up by a force equal to the weight of the fluid it displaces
Immersed = completely or partially submerged
Archimede’s principle rules
- the container can displace no greater volum of water than its own volume
- and the weight of the displaced water (not that of the object!) is equal to the buoyant force
- the apparent weight of a submerged object is its usual weight in air minus the buoyant force
Archimede’s principle cont
- it makes no difference how deep the object is placed because, although the pressures are greater with increased depths, the difference between the pressure up against the bottom of the object and the pressure down against the top of the object is the same at any depth
- whatever the shape of the submerged body, the buoyant force is equal to the weight of the fluid displaced
Sink or float
- the volume of the submerged object—not its weight—that determines the buoyant force
- whether an object will sink or float depends on how the buoyant force compares with the object’s weight
3 sink or float rules
- An object more dense that the fluid in which it is immersed will sink
- An object less dense that the fluid in which it is immersed will float
- An object having a density equal to the density of the fluid will remain in the position in which it was released
Principle of flotation
A floating object displaces a weight of fluid equal to its own weight
- a denser fluid exerts a greater buoyant force than a less dense fluid
Pascals’ principle
A change in pressure at any point in a enclosed fluid at rest is transmitted undiminished to all points in the fluid
Surface tension
Contractive tendency of the surface of liquids is called surface tension
- it accounts for the spherical shape of liquid drops
- molecular attractions tend to pull the molecule from the surface into the liquid, which then minimized surface area. The surface acts as if it were elastic film
Surface tension cont
- water has stronger surface tension than soapy water
- oil has less surface tension that cold water
- hot water has less surface tension than cold water
Capillarity
The rise of liquid in a fine, hollow tube or in a narrow space
- the film on the inner surface of a narrow tue will contract and raise water with it until adhesive force is balanced bu the weight of the water lifted
