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35 Cards in this Set
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What is pressure? |
Pressure is defined as the force acting per unit area. |
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What is the SI unit of pressure? |
The SI unit of pressure is newton per square metre (Nm-2) or Pascal (Pa). |
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What is the formula of pressure? |
Pressure = Force/Area. |
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How to investigate the relationship between force and area? |
To investigate the relationship with force and area: Apparatus; 1.5 litre bottle of soft drink, 2 pieces of soft plasticine Procedure; 1. Place the 1.5 litre plastic bottle of soft drink on a piece of soft plasticine. 2. Remove the bottle and observe the mark formed on the plasticine. 3. Next, turn the bottle upside down and place it on another identical piece of soft plasticine. Support the bottle with your hand to prevent it from toppling. 4. Remove the bottle and again, observe the mark formed on the plasticine. 5. Compare the marks formed on the two pieces of plasticine. |
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Why does a body of liquid exerts pressure on an object placed in it? |
A body of liquid exerts pressure on an object placed in it because of its weight. |
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The deeper it is, the greater the weight of the liquid above it, hence the greater the pressure. |
Learn by heart. |
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State the formula of pressure in liquids. |
p = hpg Pressure due to a liquid column = height of column × density of liquid × gravitational field strength |
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Consider a column of liquid of height h, base area A and density p. Deduce the formula of pressure in liquids from this. |
The volume of the liquid, V= Ah The mass of the liquid, m= pv The weight of the liquid column is W= mg = (pV)g = p(Ah)g The pressure at the base of the liquid column is ; p= F/A = W/A = p(Ah)g/A Therefore, p= hpg. |
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What does the pressure in a liquid depends on? |
The pressure in a liquid increases with the depth and density of the liquid. |
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Does the pressure in a liquid depends on the volume or cross-sectional area of the liquid? |
The pressure in a liquid does not depend on the volume or cross-sectional area of the liquid. |
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Why the pressure at equilibrium, or at any point along the same vertical height h will be the same? |
The pressure at equilibrium, or at any point along the same vertical height h will be the same as pressure in a liquid depends only on the vertical height of the column, not on the volume, cross-sectional area or shape of the container. |
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Why can we float when we smim? |
The reason you can float when you are swimming is due to the up upthrust acting on you. |
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Why is an enclosed body of liquid able to transmit pressure? |
An enclosed body of liquid able to transmit pressure because liquids are incompressible. This means that if pressure is applied to a trapped liquid, the pressure will be transmitted to all other parts of the liquid. |
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Learn pg 131 |
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What does Archimedes' principle states? |
Archimedes' principle: The upthrust on an object immersed in a liquid is equal to the weight of the fluid being displaced by the object. When an object floats in a liquid, it is in equilibrium and there is no resultant force acting onthe object.
When an object floats in a liquid, it is in equilibrium and there is no resultant force acting on the object, I.e all forces are balanced. Thus, the weight of the object is equal to the upward buoyant force, and the weight of the object is equal to the weight of displaced fluid. |
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Give an example of an application of hydraulic pressure. |
An important application of hydraulic pressure is the car hydraulic disc brake system. The system helps a car driver to stop the car and to control speed of the car he is driving. |
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Describe the car hydraulic disc brake system. |
When the driver steps on the brake pedal, the pressure on the brake pedal is transmitted to the large pistons on each side of a large disc on the wheel axle. This pressure causes the pistons to come into contact with the disc to slow down the car by friction. The exact pressure is also applied to other brakes in the car. |
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What will happen if the pistons at the disc have twice the area of the brake pedal? |
If the pistons at the disc have twice the area of the brake pedal, each piston will exert twice the force that the driver applies on the brake pedal. Thus, the force applied by the driver is multiplied by the increased sea of the pistons. |
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State the definition of hydraulic systems? |
Hydraulic systems make use of the transmission of pressure in liquids to do useful work, e.g the hydraulic press. |
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What is the atmosphere? |
The Earth is surrounded by a layer of air called the atmosphere. |
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What is atmospheric pressure? |
The atmosphere(layer of air) is the result of the Earth's gravitational pull on the air molecules near its surface. Although we cannot see or feel the air molecules, they take up space and have weight. Thus, this layer of air actually weighs down on the Earth's surface, including is! As a result of this weight, the layer of air exerts a pressure on the Earth's surface. This pressure is known as atmospheric pressure. |
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State the pressure of air exerted by this layer of air at sea level. |
The pressure of air exerted by this layer of air at sea level is 1.013 × 10^5 Pa. This value is commonly known as 1 atmosphere. |
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If the pressure of the atmosphere is so great, why is it that we cannot feel its effects? |
The natural pressure within our bodies is also about 1 atmosphere. As such, the internal pressure of our bodies is able to balance out the atmospheric pressure that acts on us. If this natural pressure was not present within our bodies, we would be crushed to death by the atmosphere. |
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What would happen if the changes in atmospheric pressure is too great? |
Our bodies are unable to adjust quickly to changes in atmospheric pressure, and we may suffer injuries or altitude sickness if the change is too great. |
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How the passengers on an aeroplane is safeguarded? |
The air pressure at high altitudes is much lower than 1 atmosphere. Hence, to safeguard the passengers on an aeroplane, the pressure in the aircraft is gradually increased as the plane ascends. This way, the people on board will not experience a great pressure change. |
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Describe 3 daily applications of atmospheric pressure. |
Daily applications of atmospheric pressure:
1. Drinking with a straw When we suck through a straw, we are in fact decreasing the pressure in the straw. This causes a pressure difference to be set up between the straw and the atmosphere. As the atmospheric pressure is greater,it pushes on the surface of the liquid, forcing the liquid to rise up the straw into our mouths.
2. Syringe To fill the syringe with liquid, we pull the plunger upward. This decreases the pressure within the barrel. The atmospheric pressure acting on the liquid surface, which is greater than the pressure in the barrel, pushes the liquid in the barrel.
3. Suction caps Sunction caps are commonly used for hanging clothes and other things at home. When a suction cap is pressed against the wall, most of the air is squeezed out from under the sucker. This causes a pressure difference between the atmosphere and the trapped space under the suction cap. The higher atmospheric pressure outside the suction cap thus holds it firmly against the wall. |
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What is the instrument used to measure atmospheric pressure? |
The barometer is an instrument used to measure atmospheric pressure. |
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Note: the vertical height of the mercury is dependent only on the atmospheric pressure outside the tube. Even if the column is tilted or the cross-sectional area of the tube varied, the height of the column remains the same. |
Learn |
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Very often, atmospheric pressure isn't expressed in terms of Pascal but as the height of the mercury column in the barometer. |
To convert mm Hg to Pascals, we simply convert mm Hg to 10^-3 m Hg and multiply by the density of mercury P Hg in kg m^-3 , and the gravitational field strength ,g , in N kg^-1 . Atmospheric pressure = 760 × 10^-3 × P Hg × g |
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What is the instrument used to measure the differences in pressure of gasses or liquids? |
The manometer is the instrument used to measure the differences in pressure of gasses or liquids |
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How does the manometer measure the pressure of a gas supply? |
When the manometer is not connected to any gas supply, only atmospheric pressure acts on both surfaces of the liquid at A and B. The liquid will thus settle at a common level in both arms.
On connecting one end to a gas supply, the gas would exert a pressure on the surface of the liquid at B. If this pressure is greater than atmospheric pressure, it will push the liquid level at B to equalise the pressure difference. At equilibrium, the pressure at B and C must be equal since they are at the same horizontal level.
Thus, the gas pressure at B is given by : Pressure at B = atmospheric pressure + pressure due to liquid column AC Pressure at B = atmospheric pressure + hpg
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What does Boyle's law states? |
Boyle's law states that for a fixed mass of gas at constant temperature, the volume occupied by the gas is inversely proportional to its pressure. P= K/V |
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Draw the graph of pressure against volume. |
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Descrube what happens as we push on the end of a syringe. |
As the volume of the Container decreases on compressing a gas, the number of molecules of gas per unit volume increases, I.e the molecules of te gas are confined to a smaller space. This causes an increase in the frequency of collisions of the molecules with the walls of the Container and thus the pressure of the gas increases. |
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Draw the graph of pressure against 1/v . |
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