P1.3 - Pressure

• Gas pressure is caused by the collisions gas particles have on a surface which exert an outward force over a certain area.
• If gas particles are constantly added to a container that cannot expand, its gas pressure increases.
• If the temperature of a gas increases, the particles have a higher average speed. They collide more frequently with the sides of a container, causing a bigger force and subsequently bigger pressure onto it.
• A pressure gauge is used to measure the pressure of a gas.
• The pressure of gas is measured in Pascals (Pa).
• 1 Pa = 1 N/m$^2$
• Gas pressure reaches 0 Pa at -273.14 $^{\circ}$C (0 K), also known as absolute 0.
• It is not possible to reach absolute 0 for pressure, but by extrapolating graphs it is.
• As a gas' volume doubles, its pressure halves, and vice versa. In other words, gas pressure and volume are inversely proportional.
• Pressure (Pa) * Volume (m$^3$) = constant
• Work on gas affects temperature, to increase the internal energy of a gas you:

  • Heat it up.
  • Do work on it.
• When you apply a force to a bicycle pump and move it, you do work on the gas and it gets hotter. The average speed of the particles increase because their kinetic energy increases when they collide with the moving piston.
• The atmosphere is a single layer of gas that covers the Earth of a height of about 700 km. In the simplest model of the atmosphere, scientists assume that its density is the same in all parts of it.
• The gases of an atmosphere exerts atmospheric pressure.
• To prove that air exerts a pressure, pump air out of a can and see that the shape of it collapses.
• A pressure gauge reads the atmospheric pressure before a reading is given, similar to how a thermometer reads the room temperature.
• A force exerted by a liquid in all directions is known as liquid pressure.
• The molecules in any layer in a liquid are being compressed by the weight of the liquid above it. This pushes the liquid molecules together so the pressure is bigger.
• When comparing the density of two liquids, at any particular depth the pressure would always be bigger in the denser liquid. This is because there is a greater weight pushing down in the denser liquid.
• Pressure (at the bottom of a column of liquid) = height of column (m) * density of liquid (kg/m$^2$)* gravitational field strength (N/kg)
• Anything that floats must have an upwards force (upthrust) to balance its weight.
• In a pool, the air exerts a lower downwards force onto you than the upwards force exerted by the water.
• Formula for a floating object's weight:

  • (pressure at bottom * area at bottom) - (pressure at top * area at top) = weight.
• When a submarine is at the ocean surface, tanks are full of air to keep it floating. To let it sink, water is filled into the tanks instead to increase the submarines weight. There is now not enough upthrust to balance it so it sinks. To rise back up, the tanks carry compressed air to fill the tanks.
• Fish have a swim bladder that has an amount of gas that varies, working similar to a submarine. This allows them to adjust their depth in water.