Chapter 5 - Pressure

Question 1

Define “pressure”

Answer 1

The force acting normally per unit area

Question 2

Formula to calculate pressure

Answer 2

P = F/A

[ pressure = force / area ]

Question 3

What is the SI unit for pressure?

Answer 3

Pascal (Pa)

Question 4

1 Pa = 1 ____

Answer 4

N m^-2

Question 5

Factors affecting pressure

Answer 5

1. Area on which force acts
2. Magnitude of the force acting on the surface
3. Direction of the force acting on the surface

Question 6

Formula to calculate pressure due to a liquid column

Answer 6

P = hρg

[ pressure due to a liquid column = height of column x density of liquid x gravitational acceleration ]

Question 7

The pressure in a liquid is ________ along the same horizontal level of the liquid.

Answer 7

the same

Question 8

The pressure in a liquid is proportional to the ________ and ________ of the liquid.

Answer 8

depth

density

Question 9

The pressure in a liquid is ________ in all directions

Answer 9

equal

Question 10

What is the density of water?

Answer 10

1.0g cm^-3 = 1000kg m^-3

Question 11

What is the density of mercury?

Answer 11

13.6g cm^-3 = 13600kg m^-3

Question 12

What makes hydraulic systems work?

Answer 12

Liquids are almost incompressible and they transmit pressure equally in all directions (Pascal’s Law)

Question 13

What is the advantage of using a hydraulic system?

Answer 13

It can multiply the force applied

Question 14

Formula relating F1, A1, F2, A2

How is this derived?

Answer 14

F1/A1 = F2/A2

• Effort=F1 acts on an Area=A1 to supply an input pressure=P1
• Input pressure is transmitted onto Area=A2
• Since P1 = P2 (output pressure=P2), F1/A1 = F2/A2

Question 15

Since A2 > A1, it follows that F2 ____ F1

Answer 15

>

Question 16

Formula relating A1, d1, A2, d2

How is this derived?

Answer 16

d2/d1 = A1/A2

• When F1 is applied, small piston needs to be pushed down a longer distance=d1 for big piston the move up a shorter distance=d2
• Volume of liquid pushed by small piston=V1 is equal to volume of liquid that goes below large piston=V2
• Since V2 = V1, A2d2 = A1d1

Question 17

Since A2 > A1, it follows that d2 ____ d1

Question 18

Atmospheric pressure is due to…

Answer 18

the force per unit area exerted against a surface by the weight of air above that surface

Question 19

In which direction does atmospheric pressure act?

Answer 19

In all directions

Question 20

What surfaces experience atmospheric pressure?

Answer 20

All surfaces exposed to the atmosphere

Question 21

What are common non-SI units for atmospheric pressure?

Answer 21

Atmosphere (atm)

Centimetres of mercury (cm Hg)

Question 22

What is the atmospheric pressure at sea level?

Answer 22

1 atm

76 cm Hg

Question 23

What happens to the mercury column when more mercury is poured into the reservoir? Why?

Answer 23

As reservoir level rises, the mercury column level will rise by the same amount to maintain the column height h since pressure, density and gravitational acceleration remains constant.

Question 24

What happens to the mercury column when the glass tube is tilted? Why?

Answer 24

Perpendicular height of mercury column to the reservoir is unchanged as pressure is dependent on the vertical height and not on the length of the column.

Question 25

What happens to the mercury column when the barometer is taken to the top of a mountain? Why?

Answer 25

Mercury column height decreases. Lower atmospheric pressure at higher altitudes.

Question 26

What happens to the mercury column when water is introduced into the space above the mercury column? Why?

Answer 26

Mercury column height decreases. Water vaporises and sets up vapour pressure in the space above the mercury column and presses downwards on the mercury column. Column indicates the pressure difference between atmosphere and water vapour pressure.

Question 27

What happens to the mercury column when there is a crack in the glass tube along the mercury column above the mercury reservoir? Why?

Answer 27

Height of mercury decreases to the same level as the reservoir. Air outside the tube at the crack is at atmospheric pressure while the pressure inside the tube is lower than atmospheric pressure. The air will move from a higher pressure region to a lower pressure and so enters the column. The air only stops when pressure difference inside and outside the column is zero, hence height difference will be zero.

Question 28

Concept of pressure in a drinking straw

Answer 28

- When you suck, your lungs expand and pressure in the mouth decreases - The greater atmospheric pressure pushes down on the surface of the liquid and so forces the liquid up to your mouth

Question 29

Concept of pressure in a rubber sucker

Answer 29

- When the rubber sucker is pressed into the smooth surface, air is removed and a near vacuum is created behind the sucker - Sucker stays in place because atmospheric pressure > pressure of vacuum

Question 30

What is excess pressure?

Answer 30

The difference in the gas pressure and the atmospheric pressure

Question 31

Calculating gas pressure from manometer (if gas pressure > atmospheric pressure)

Answer 31

PG = PA + h | [ gas pressure = atmospheric pressure + height difference ]

Question 32

Calculating gas pressure from manometer (if gas pressure

Answer 32

PG = PA - h | [ gas pressure = atmospheric pressure - height difference ]

Question 33

State Boyle's Law

Answer 33

For a fixed mass of gas at a constant temperature, the volume V of the gas is inversely proportional to its pressure P.

Question 34

Formula (Boyle's Law)

Answer 34

PiVi = PfVf | where the subscripts i and f indicate the initial and final conditions respectively

Question 35

When a capillary tube with a column of air trapped by a column of mercury is placed horizontally, what is the pressure acting on the column of air?

Answer 35

Pressure = PA

Question 36

When a capillary tube with a column of air trapped by a column of mercury is placed vertically, what is the pressure acting on the column of air?

Answer 36

Pressure = PA + h

Question 37

When a capillary tube with a column of air trapped by a column of mercury is placed vertically upside down, what is the pressure acting on the column of air?

Answer 37

Pressure = PA - h