6 — pressure Flashcards
Pressure
Pressure is defined as the force acting per unit area
SI: pressure
1N/m^2
Density
Density is defined as mass per unit volume of a substance
Atmospheric pressure
Weight of a layer of air that acts on the Earth per unit area
Density of mercury
13600kg/m^3
Approximate density of air
1.28kg/m^3
Atmospheric pressure
P0 = 1.0 x 10^5 N/m^2
= 1.0 x 10^5 Pa
= 76cmHg
= 1 bar
= 1 atmosphere
Characteristics of liquid pressure
- Pressure in liquid acts in all directions
- Pressure at the same depth is the same
- Pressure depends on depth
Properties of liquid relevant to Pascal’s Principle
- Liquids r incompressible
- If pressure is applied to a trapped liquid, the same pressure is transmitted to all parts of the liquid.
Barometer
- Instrument to measure atmospheric pressure
- mercury barometer
- aneroid barometer - Length of a mercury column is used to obtain readings to measure atmospheric pressure
- Shape of tube/angle of tilt and size does not affect the height of the mercury column. Pressure only depends on the vertical height of the liquid.
Manometer
- Instrument to measure gas pressure
- U-shaped tube containing a liquid, can be mercury, water or oil
- Pressure of gas>P0 = h cmHg + 76cmHg
- Pressure of gas <P0 = 76cm Hg - h cmHg
- Constant atmospheric pressure -> constant difference in height between 2 points of the manometer
Lifting heavy weights w small pressure
Liquid is incompressible, pressure exerted on oil by small piston is transmitted uniformly throughout the oil to the larger pistons. Due to larger area, a greater force is exerted on the larger piston.
How leak in system affects pressure to lift weight.
The force exerted on object will be smaller due to loss in pressure in the oil by the leak. Hence, pressure transmitted from smaller piston to larger piston is decreased.
The smaller piston moves 25cm to the left. State and explain whether large piston moves >25cm, = 25cm or <25cm.
Smaller than 25cm. Since work done is equal, a larger force will result in a smaller distance moved by the larger piston.
State one modification to the system to lift a heavier load for the same applied force.
Decrease area of small piston/ increase area of larger piston
If A and B are mid-points of liquid C and water column respectively, would pressure at both points be identical?
Yes pressure will be identical. Since difference in height representing atmospheric pressure is constant and heights are halved equally, pressure between the points will be identical.
When the U-tube is brought up a high mountain, state and explain what would happen to levels of X and Y.
Difference between the levels remain the same. Decrease in atmospheric pressure will be the same on both arms.
If liquid L in point X of the U-tube is replaced by another liquid of the same volume twice as dense, state what would happen to levels of X and Y.
Level at X will be lower than that at Y. New liquid will exert greater pressure and push Y up.
(Take reference from when gas w pressure >P0)
Consideration of designing U-tube for set-up.
Length must be long enough for water measuring atmospheric pressure.
Crushed can by atmospheric pressure
- Some water was heated in an open can until the water boils.
- The can is removed from the heat and the cap is screwed on.
- The can is now placed under running water.
- The metal can suddenly crumples.
a. Steam produced remove some air from the can.
b. Prevents air from entering back to the can
c. To condense the steam in the can, removing more gas from can.
d. Force due to atmospheric pressure acting on can > inner pressure exerted on the walls of the can thus can is crushed.
Why are we not crushed by atmospheric pressure.
Pressure inside our body balances atmospheric pressure.
Uses of atmospheric pressure
Rubber sucker sticking onto kitchen tiles.
Drinking water from straw.
Uses of liquid pressure
Hydraulic jack
- 2 cylinders of different diameters joined tgt by a pipe. Liquid used is usually oil.
Hydraulic car brake
Why can’t air be present in the mercury barometer. (Why vacuum?)
If air is present, there is air pressure is acting on mercury -> presenting inaccurate value of atmospheric pressure.
Suggest why the hydraulic jack cannot work properly if it contains bubbles of air. [1]
Pressure is not able to transmit uniformly as air is compressible and some of the work done is used to compress the air bubble, resulting in a loss in pressure from the reduction in the force exerted by piston Q.
