P1.3 - Pressure

Question 1

How do gas particles move?

Answer 1

• Particles of a substance in the gas state, move
  about at high speed, in random directions
• Random, Equally in all directions

Question 2

What happens when gas particles are in a container?

Answer 2

The particles collide with each other, and with the walls of the container and exert a pressure.

Question 3

Why do gases exert a pressure?

Answer 3

A gas exerts a pressure due to the force with which particles collide with the container walls, over a given surface area.

Question 4

Define temperature.

Answer 4

Temperature is a measure of the average kinetic energy of particles in a substance.

Question 5

Explain the relationship between gas pressure and temperature.

Answer 5

There is a directly proportional relationship between the temperature of a gas and its pressure, at a constant volume.

Question 6

Why does an increase in temperature increase gas pressure and what assumptions are made?

Answer 6

1. Assuming that the volume is constant, an
   increase of temperature means the particles
   have a higher average speed.
2. They will collide more frequently with the sides
   of the container which results in a bigger force
   over a certain area per unit time.
3. This results in a bigger pressure.

Question 7

What does extrapolation show?

Answer 7

• Extrapolation shows the estimate of what would
  happen if you continued to change the
  independent variable.
• It is a dotted line.

Question 8

What happens to the gas pressure when the volume of a container halves? Explain.

Answer 8

• As the volume halves, the pressure doubles.
• If you halve the volume of a container, the number
  of collisions of particles with the container walls
  doubles and force exerted by the gas particles per
  unit time doubles.
• Therefore the pressure of the
  gas doubles.

Question 9

Describe the relationship between pressure and volume.

Answer 9

If temperature is constant, pressure and volume are inversely proportional.

Question 10

What are the two equations linking pressure and volume?

Answer 10

• Pressure (Pa) x Volume (m3) = constant

- p1V1 = p2V2 (the product of pressure and volume of a gas is the same, before and after a change)

Question 11

What happens when a gas is compressed?

Answer 11

• When a gas is compressed, a force is required
  to move the gas particles together.
• Work is done on the gas particles.
• Gas particles gain kinetic energy. The gas
  particles move faster.
• And hence, the gas temperature increases

Question 12

What happens when a gas expands?

Answer 12

• When a gas expands, a force is applied by gas
  particles together.
• Work is done by the gas.
• The gas particles lose kinetic energy and the
  particles move slower.
• Gas temperature decreases

Question 13

What is the simple model of the Earth’s atmosphere?

Answer 13

• A single layer of gas that surrounds the Earth with
  a height of around 700km.
• In this simple model, the density of the layer is the
  same throughout, top to bottom

Question 14

Why can the Earth maintain an atmosphere?

Answer 14

The Earth can maintain an atmosphere, due to its gravitational field strength.

Question 15

What is atmospheric pressure and what is the atmospheric pressure at the surface of the Earth?

Answer 15

• Atmospheric pressure is pressure exerted by
  gases of the atmosphere.
• Atmospheric pressure (at sea level) is large at
  around 100,000 Pa = 100kPa

Question 16

What happens to atmospheric pressure as you go up and why?

Answer 16

As you go away from the surface of the Earth the atmospheric pressure decreases, owing to fewer gas particles exerting a force, due to a weaker gravitational field.

Question 17

What does the liquid pressure in a container depend on?

Answer 17

The pressure in a liquid container only depends upon the depth below the surface.

Question 18

What is liquid pressure independent to?

Answer 18

The pressure is independent of the container shape and surface area.

Question 19

What does liquid pressure at a certain depth depend on?

Answer 19

The pressure at any given depth in a liquid in a container, is determined by the weight of the water above that must be supported.

Question 20

What happens to liquid pressure in relation to depth?

Answer 20

The greater the depth, the more mass (per unit area) and weight of water, the greater the pressure.

Question 21

What happens to liquid pressure in relation to density?

Answer 21

The greater the density, the more mass (per unit area) and weight of water, the greater the pressure.

Question 22

What is the liquid pressure equation?

Answer 22

Pressure (Pa) = Height of column (m) x Density of liquid (kg/m3) x gravitational field strength (N/kg)

Question 23

What is the liquid pressure equation (difference in height)?

Answer 23

Pressure (Pa) = Density x Gravitational field strength x Difference in height

Question 24

When does the force of upthrust occur?

Answer 24

The force of upthrust occurs whenever a body is immersed (fully or partially) within a fluid.

Question 25

What does upthrust result from?

Answer 25

Upthrust results from the pressure difference that exists between the top and bottom of the object.

Question 26

What is upthrust equal to?

Answer 26

The upthrust experienced by a body is equal to the weight of the fluid displaced.

Question 27

How can you tell is an object is floating (equation)?

Answer 27

An object floats if:

(pressure at bottom x area at bottom) - (pressure at top x area at top) = weight

Question 28

What is the equation for upthrust?

Answer 28

• Upthrust = Weight of fluid displaced
• Upthrust = (mass of fluid) x g
• Upthrust = (density of
  fluid) x (volume) x g
• Upthrust = (Density of fluid) x (Surface area) x
  (Depth submerged) x g
• Upthrust = pghA

Question 29

When will a body only float?

Answer 29

A body made of a given substance will float only if it is able to acquire enough upthrust before becoming fully submerged.