P11: Force And Pressure (Y10 - Spring 1) Flashcards
🟠 Equations for Pressure
Pressure (N/m^2) = Force (N) / Area (m)
(P = F/A)
Pressure (N/m^2) = Density (kg/m^3) x Gravity (m/s^2) x Height (m)
(P = ϼ x g x h)
🟢 If you ‘capped’ a syringe with your finger over the end containing a gas, Air. What would you feel if you pressed the plunger down?
You would feel air the inside the syringe pushing against your finger. This is because you will be decreasing the volume in the syringe and therefore increasing the pressure of the air inside it which you will feel on your finger.
🟢 How Is Air Pressure formed
Air pressure is caused by the force of air gas particles bumping into things. This can be accelerated when the particles are heated up, meaning they’d have more kinetic energy.
🟢 What is the Atmosphere
The atmosphere is a thin layer of air around the Earth. The density at different altitudes can be assumed to be constant and therefore its pressure. The weight of air going upwards from the a level will decrease and therefore so will the atmospheric pressure experienced.
The concentration of oxygen in the atmosphere decreases with increasing altitude.
🟢 How is Air Pressure Increased in a Ballon?
Air is forced into a balloon under pressure. As the number of air particles and collisions increase, the pressure inside the balloon increases, and eventually it will explode.
🟢 How does the Collapsing Can Experiment work
- Usually, the air pressure inside an open can is the same as the pressure outside.
- Heating the can with a small amount of water means the air is driven out of the can and replaced by water vapour.
- When the water vapour condenses quickly from
cooling, the pressure inside the can becomes much less than the air pressure outside. - Then the atmospheric pressure outside crushes the can.
🟢 What are the Magdeburg Hemispheres and how do they work?
The Magdeburg hemispheres are a pair of large metal hemispheres, with matching rims. The rims are sealed with grease and the air is pumped out to form a vacuum inside.
It works by The pump sucks out any air/gas particles from inside the two hemispheres. Therefore, the only air particles that are hitting the hemispheres are on the outside, keeping the hemispheres together.
🟢 What is Pressure?
Pressure is caused when objects exert forces on each other, or when a fluid (which can be either a liquid or a gas) exerts a force on an object in contact with the fluid. The pressure caused by any force depends on the area of contact which the force acts, as well as on the size of the force.
🟢 How can Pressure in Liquids Change?
The pressure along a horizontal line in a liquid is constant. You can see this for yourselfby making several holes around the bottle at the same depth, and the jets from these holes are at the same pressure.
The pressure in a liquid depends on the density if the liquid. Suppose water is poured into ine side of a U-shaped tube, and then oil is carefully poured into the other side; when the liqid settles, the oil level is higher than the water level on the side side. This is because the oil is less dense than water, so a greater depth of oil is needed to create the same pressure.
🟢 What is Atmospheric Pressure?
Atmospheric pressure is due to air molecules colliding with surfaces. Each impact exerts a tiny force on a surface, but the number if molecules that collide with the surface each second is very larbe. The density of the atmosphere decreases with increasing height avove the ground. This is because the weight of air pressing down on a horizontal surface at any altitude decreases with altitude. This means there are fewer moleucles per cubic metre and therefore fewer impacts per second at a higher altitude, so atmospheric pressure decreases with increasing altitude. In effect, the weight of air above any given altitude exerts pressure on the air below. At increased altitude, there is less weight of air above, so atmospheric pressure deceases with increasing altitude.
The Earth’s atmosphere extends more than 100km into space. The means pressure of the atmosphere at sea level is about 100kPa. Atmospheric pressure changes slightly from day to day, changing with the local weather conditions. Fine clear weather is usually associated with high pressure.
🟢 How can Atmospheric Pressure be used in certain Everyday Products like a Rubber Suction Cap, or a Drinking Straw?
A rubber suction cap pressed onto a wall tile stays on the tile and doesn’t fall off. This is because atmospheric pressure acts on the outside of the cap, but not on the inside between the cap and the wall. The action of pressing the cap onto the wal squeezes out any tiny air trapped between the cap and the wall. The force due to atnospheric pressure acts on the outside surface of the cap only and keeps the cap on the wall.
A drinking straw only works when the air in the straw is sucked out. Without any air in the straw, atmospheric pressure acting on the liquid surface outside the straw pushed liquid up the straw.
🟢 Why are Dams built thicker at the bottom amd why the turbines are placed at the bottom?
The dam is thicker at the bottom than at the top because the water pressure is much higher at the bottom of the water. This means if the dam was built straight, then the resistance of the dam’s wall would not equal out the increasing pressure of the water the further down it goes.
As a result of this, the turbine is placed at the bottom of the dam rather than at the top because if the water pressure is larger at the bottom, then the water will be travelling through the generator at a much higher velocity, resulting in more power goimg through the gnerator, making it more efficient.
🟢 How does Depth affect liquid and gas Pressure
The pressure of the liquid/gas increases with the depth. The further down that you go either in water, or the atmosphere, the greater that atmospheric pressure at that point, and as a result, the force at that specific point too.
🟢 What is a Manometer and What Does It Do?
A manometer is a device that measures air pressure using a container with a “U”-shaped tube open at one or both ends. In a closed manometer, a sample of gas is introduced into one end, which is then capped. Then, a fluid of known density is poured into the other end. The fluid will stop moving when the pressure of the gas trapped between the cap and the fluid together with the pressure at the bottom of the fluid column on that side matches the pressure of air plus the pressure of the fluid column on the open side.
The height of the fluid on the open side will be higher on that side when air pressure is less than the gas pressure and lower on the open side when the air pressure exceeds the gas pressure. You can use this height difference to calculate the gas pressure.
(Since P = F/A = mg/A, m = ϼV and V = Ah for a cylindrical tube (i.e., volume = area times height), it can be shown that the pressure created by a vertical column of fluid is ρgh, where h = height in meters. This pressure represents the positive or negative difference between the gas pressure and atmospheric pressure.)
🟢 What is a Barometer
A barometer is a scientific instrument used to measure atmospheric pressure, also called barometric pressure. The atmosphere is the layers of air wrapped around the Earth. That air has a weight and presses against everything it touches as gravity pulls it to Earth. Barometers measure this pressure.
