13. Pressure

Question 1

Density

Definition and Formula

Answer 1

Density of a substance is its mass per unit volume.
ρ=m/V

Density of a pure substance is unchanged, irrespective of the amount of substance

Question 2

How does density affect sinking and floating

Answer 2

Object is denser: object sinks
Object is less dense: object floats
Object has equal density: Object remains wherever it is released/in the middle of the liquid

Question 3

Pressure

Definition and Formula

Answer 3

Pressure is the force acting per unit area

P=F/A
Pressure = Force/Area

Question 4

How can pressure be increased/ decreased

Answer 4

Increase pressure:
1. Increase the force
2. Decrease the contact area

Decrease pressure:
1. Decrease the force
2. Increase the contact area

Question 5

What 3 properties does a hydraulic press make use of?

Answer 5

1. Liquids are incompressible
2. If an external perssure is applied to an enclosed fluid, the pressure is trasmitted equally to all other parts of the fluid
3. Liquid has no fixed shape, and can fill up the space in a container

Question 6

How does a hydraulic press work?

Answer 6

Input pressure = output pressure

F1/A1=F2/A2
Since A2 > A1, F2> F1

Smaller piston travels a longer distance than larger piston.
Volume of liquid pushed by smaller piston is equal to that pushed by the bigger piston.

Question 7

What happens when there is a pressure difference?

Answer 7

There will be a net force which acts in the direction of a region of higher pressure to a region of lower pressure

Question 8

Applications of Atmospheric Pressure

Answer 8

1. Sucking through a straw
   Lowers air pressure in straw/mouth, atmospheric pressure is larger than air pressure in straw, pushes on liquid surface. Pressure difference results in net force which pushes the liquid up the straw.
2. Suction cup
   Most air underneath suction cap is squeezed out when suction cup is pressed against a wall. Creates a pressure difference between the atmosphere and the trapped space under the suction cup. Net force acts inwards on the suction cup to hold it against a surface.

Question 9

Liquid Pressure at a Depth

Formula

Answer 9

Pressure = height x density x gravitational field strength
P = hpg

Liquid pressure is greatest at largest depth

Question 10

Barometers to Measure Pressure

Function

Answer 10

Mercury/Water Barometer:
To measure atmospheric pressure

Atmospheric pressure is expressed in the height of the mercury/water column.

Question 11

Manometers to Measure Pressure

Function

Answer 11

Used to measure differences in pressure of gases or liquids

Height difference is equivalent to the pressure difference between gas pressure and atmospheric pressure.