P1.3 PRESSURE

• • Molecules in a gas are in constant random motion at high speeds
  • Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
  • With the walls of its container
  • With other molecules
  • The random motion of tiny particles in a fluid is known as Brownian motion
• as the temperature of a gas increases, the average speed of the molecules also increases.
• work is the transfer of energy by a force.
• work can be done on a gas by ____?

  compression
• what happens when the volume of a gas decreases?

  • there is less space available for the same number of molecules. The molecules have to travel further between collisions with the walls. As a result, the frequency of collisions decreases but the force of impact remains the same. Therefore, the pressure increases.
• doing work on a gas involves a transfer of energy. what 2 things does this increase?

  internal energy and temperature
• what happens if we increase the temperature of a gas?

  • the kinetic energy of the molecules will increase so they will move faster. This causes them to hit the walls more frequently and with greater force, which increases pressure.
• how is work done on a gas by compression?

  a force is used to push a piston by a certain distance.

  this decreases the volume of the gas.

  the molecules move faster + have a higher kinetic energy.

  this increases the temperature.
• what pressure does the earths atmosphere exert at sea level?

  about 101 kPa
• what 2 things can a fluid be?

  a liquid or a gas
• what happens when an object is immersed in a liquid?

  the liquid exerts a pressure on the object, which is is addition to the atmospheric pressure.
• the atmospheric pressure becomes less dense with increasing height above sea level.

  this also means that pressure becomes less too.
• TRUE OR FALSE: atmospheric pressure does not vary day-to-day, nor does it depend on the weather.

  false
• what is upthrust?

  Upthrust is the upward force exerted on an object submerged in a fluid, such as water.
• is upthrust in the opposite or same direction to the object's weight?

  opposite
• what 2 things does the size of the upthrust depend on?

  density of the fluid and the volume of the fluid that is displaced (which is equal to the volume of the object)
• the denser the liquid, the ____ the upthrust it will exert on an object.

  greater
• $pressure (pascals) =$$force (n) / area (m^2)$
• • If the upthrust on an object is equal to (or greaterthan) the object’s weight, then the object will float
• • If the upthrust is smaller than the weight then the object will sink
• • If it has a density less than the density of the fluid it is immersed in, the object will float
• • If it has a density more than the density of the fluid it is immersed in, the object will sink
• • When an object is immersed in a liquid, the liquid will exert a pressure, squeezing the object
  • This pressure is exerted evenly across the whole surface of the liquid and in all directions
• the greater the ____ and the _____ of the liquid, the greater the pressure.

  depth and density
• in a column of water, where would the highest pressure be?

  Bottom
• $pressure (Pa)=$$height (m) *$$density(kg/m^3) *$$gravitational field strength$