P1.3 - Pressure

Question 1

why do balloons expand when air is blown into it? (3)

Answer 1

• gases move quickly in all directions
• so, they collide with the surface of the ballon
• each collision produces a force
• so, when more air is blown, more force is applied to the balloon causing it to expand

Question 2

how does temp affect gas pressure? (3)

Answer 2

• particles gain more kinetic energy (1)
• causes more frequent collisions (1)
• makes more force on surface (1)
• more pressure

Question 3

what is the unit of pressure?

Answer 3

Pa (Pascals)

Question 4

what is the unit of force?

Answer 4

N (newtons)

Question 5

what is the unit of area/surface area?

Answer 5

m^2 (meters squared)

Question 6

what is the equation to work out pressure

Answer 6

force / area

(force over area)

Question 7

what is extrapolating?

Answer 7

• basically ‘estimating’ what the result would be on a graph
  (shown as a dotted line)
• an extended dotted line downwards

Question 8

what can you use to measure the pressure of a gas?

Answer 8

a manometer

Question 9

what can you use to measure the pressure of air that is trapped in a bottle?

Answer 9

a pressure gauge

Question 10

explain why it is dangerous to throw a canister full of pressurised gas in a fire? (3)

Answer 10

• the gas particles heat up (1)
• increasing the pressure within the can
• start moving quickly in all directions, exert a high force on the surface area of the canister (1)
• if canister is unable to width stand the force, it would explode (1)

Question 11

explain the extent to which pressure is, or is not proportional to the temperature of a gas

Answer 11

• temp and pressure have a directly proportional relationship as long as the volume is constant
• the pressure of gas also depends on its volume as well as its temperature

Question 12

State the direction of the force of a gas on the surface of a container

Answer 12

90 degrees (right angle)

Question 13

If pressure is increased (within a closed system) how does the volume change?
- and explain the relationship (name)

Answer 13

The volume decreases

INVERSELY PROPORTIONAL

Question 14

What is the equation for pressure and volume?

+ explain why

Answer 14

Pressure(Pa) x volume (m^3) = constant

+ since pressure and volume are inversely proportional, if you half one the other doubles, and so their product would always equal to the same amount

Question 15

In terms of the particles why are pressure and volume inversely proportional?

Answer 15

• if you half the volume, the number of collisions per second between the gas particles and container doubles

Vice versa. If you double volume, the number of collisions per second halves

Question 16

Why is there a higher atmospheric pressure at the surface compared to higher up?

Answer 16

Air is more dense at Earth’s surface, meaning the particles are more tightly packed together, so the particles collide more frequently.

Question 17

If the force is so great (100 000N per square metre on earth’s surface), why do objects not collapse?

Answer 17

As there is the same pressure pushing outwards at the same time (forces are equal in all directions = equilibrium)

Question 18

Why does a bag of crisps expand as it rises?

Answer 18

• since within the bag the air pressure is still the same as the ‘surface pressure’ - high
• but the outside has a lower pressure LESS PARTICLES PUSHING DOWN
• so force pushing outwards > force pushing inwards, so not in equilibrium

Question 19

Explain why people who climb mt Everest need oxygen cylinders

Answer 19

• as there are less gas particles pushing down upon the other gas particles at a higher level
  -so, there is a smaller pressure
• but, as pressure and volume are inversely proportional, the gas particles have more space to move around in
• therefore, oxygen levels decrease per volume (so not enough for us)

OR

• gravity is weaker higher up (away from core), so pull is less = can move around easier = less density

Question 20

What is the equation to calculate pressure in a liquid at a particular depth?
1) think ‘what factors would affect pressure in a liquid?’

Answer 20

Pressure = height x density x gravitational field strength
(Pa). (M). (Kg/m^3). (N/kg)

Question 21

Why does the pressure of a liquid vary with depth?

Answer 21

• higher depth = more water molecules are compressed by the weight of all the water molecules above that layer
• pushes water molecules, and so has higher pressure

Question 22

What is liquid pressure?

Answer 22

Water molecules colliding with each other, or the container

Question 23

How is pressure affected by a more dense/less dense liquid?

Answer 23

• more dense = more mass per volume = more more weight pushing down = a higher pressure

Question 24

In reality, what other factor affects a submerged objects pressure?

Answer 24

• the atmospheric pressure, as it also pushes down on the water, which then pushes down on more water
• and pushes down on the object

Question 25

State which law reinforces the idea of an upwards force on a floating object?

Answer 25

Newton’s third law

• anything that is floating has an upwards force to balance its weight (remember weight is a force)

Question 26

In terms of pressure, explain why objects float

Answer 26

• top of object = smaller force pushing downwards (lower depth = lower pressure)
• bottom of object = larger force pushing upwards (higher depth = higher pressure)
• so the pressure difference causes an upwards resultant force (so it floats)

Question 27

What is the equation to calculate pressure?

Answer 27

Pressure = force/ area
(Pa). (N). (m^2)

Question 28

What is the equation to see if an object would float?

Answer 28

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

Question 29

Simply put, what factor decides whether an object would float or sink?

Answer 29

Whether the pressure difference with area is enough to balance the weight

Question 30

If you want a heavy object (like a submarine) to float, what should you do?

Answer 30

• decrease the mass (lower weight acting downwards)
• increase the surface area (so it can balance the weight easier with the upthrust)