EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 5-1�Density and Pressure

All content applies for Triple & Double Science

Edexcel Specification

Section 5: Solids, liquids and gases

b) Density and pressure

know and use the relationship:

density = mass / volume ρ = m / V

describe experiments to determine density using direct measurements of mass and volume

know and use the relationship:

pressure = force / area p = F / A

understand that the pressure at a point in a gas or liquid which is at rest acts equally in all directions

know and use the relationship:

pressure difference = height × density × g p = h × ρ × g

Measuring the volume of a regular solid

V = w x l x h

V = π x r² x h

Measuring the volume of an irregular solid

Smaller solid

Measure the change in level of the water in a measuring cylinder

Larger solid

Measure the volume of water displaced. The string is assumed to have no volume.

Volume units

1 cubic metre (1 m³)

= 1m x 1m x 1m

= 100cm x 100cm x 100cm

= 1000 000cm³

​

1 m³ = 1000 000 cm³

​

NOTE: 1 cubic centimetre (cm³ OR ‘cc’) is also the same as 1 millilitre (ml)

​

Density (ρ)

density = mass

volume

ρ = m / V

​

mass, m is measured in kilograms (kg)

volume, V is measured in cubic metres (m³)

density, ρ is measured

in kilograms per cubic metres (kg/m³)

also:

mass = density x volume

​

and:

volume = density

volume

ρ

V

m

Conversion between kg/m³ and g/cm³

A 1g mass of water has a volume of 1cm³

but 1g = 0.001kg

and 1cm³ = 0.000 001 m³

Therefore: 1m³ of water will have a mass of 1000 000 x 1g = 1000kg

​

1000 kg/m³ is the same as 1 g/cm³

​

Density examples

0.0989

0.179

1.29

0.534

19 100

850 to 1400

10^-25 to 10^-15

13 500

150 000

700

1000

2 700

7 900

11 300

22 610

19 300

10¹⁷

> 4 x 10¹⁷

osmium

Question 1

Calculate the density of a metal block of volume 0.20 m³ and mass 600 kg.

​

density = mass

volume

= 600 kg / 0.20 m³

density of the metal = 3000 kg / m³

Question 2

Calculate the mass of a block of wood of volume 0.050 m³ and density 600 kg/m³.

​

ρ = m / V

becomes:

m = ρ x V

= 600 kg/m³ x 0.050 m³

mass of wood = 30 kg

Question 3

Calculate the volume of a liquid of mass 45 kg and density 900 kg/m³.

​

ρ = m / V

becomes:

V = m / ρ

= 45 kg ÷ 900 kg/m³

volume of liquid = 0.05 m³

Question 4

When a small stone is immersed into the water inside a measuring cylinder the level increases from 20.0 to 27.5 ml. Calculate the density of the stone in g/cm³ if its mass is 60g.

​

Volume of stone = (27.5 – 20.0) ml

= 7.5 cm³

ρ = m / V

= 60g / 7.5cm³

density of the stone = 8.0 g/cm³

Question 5

Calculate the density in g/cm³ and kg/m³ of a metal cylinder of radius 2cm, height 3cm and mass 400g.

​

Volume of a cylinder = π x r² x h

= π x (2cm)² x 3cm

= 3.142 x 4 x 3

= 37.7 cm³

ρ = m / V

= 400 g / 37.7 cm³

metal density = 10.6 g/cm³

= 10 600 kg/m³

Question 6

Calculate the mass of a teaspoon full (1 cm³) of a neutron star. Density of a neutron star = 1.0 x 10¹⁷ kg/m³.

​

1.0 cm³ = 0.000 0001 m³

ρ = m / V

becomes:

m = ρ x V

= 1.0 x 10¹⁷ kg/m³ x 0.000 0001 m³

mass = 1.0 x 10¹¹ kg

​

Note: 1 tonne = 1000 kg = 1.0 x 10³ kg

Therefore mass = one hundred million tonnes!

Question 7

Calculate the weight of a gold ingot of dimensions (20 x 10 x 4) cm. The density of gold is 19 300 kg/m³.

​

volume of gold = 800 cm³

= 0.0008 m³

mass = volume x density

= 0.0008 x 19 300 = 15.4 kg

weight = mass x gravitational field strength

= 15.4 x 10

​

weight of gold ingot = 154 N

Answers

6 g/cm³

3 g/cm³

1.5 m³

64 g

Complete:

Choose appropriate words to fill in the gaps below:

Density is equal to ______ divided by _________ and can be measured in kilograms per ______ metres.

A density of _______kg/m³ is the same as a density of 1 g/cm³. This is the density of ________.

The ________ of a stone can be measured by immersing the stone into water. The volume of water ________ by the stone is equal to the volume of the stone. The volume of the water displaced is found using a _________ cylinder.

displaced

cubic

1000

mass

water

volume

WORD SELECTION:

density

measuring

displaced

cubic

1000

mass

water

volume

density

measuring

Pressure, p

pressure = force area

​

p = F

A

units:

force, F – newtons (N)

area, A – metres squared (m²)

pressure, p – pascals (Pa)

also:

force = pressure x area

​

and:

area = force

pressure

p

A

F

Note:

1 Pa is the same as 1 newton per square metre (N/m²)

​

Question 1

Calculate the pressure exerted by a force of 200N when applied over an area of 4m².

​

p = F / A

= 200N / 4m²

pressure = 50 Pa

Question 2

Calculate the force exerted by a gas of pressure 150 000 Pa on an object of surface area 3m².

​

p = F / A

becomes:

F = p x A

= 150 000 Pa x 3 m²

force = 450 000 N

Question 3

Calculate the area that will experience a force of 6000N from a liquid exerting a pressure of 300kPa.

​

p = F / A

becomes:

A = F / p

= 6000 N ÷ 300 kPa

= 6000 N ÷ 300 000 Pa

area = 0.02 m²

Complete:

5

20

400

100

2

Pressure exerted by a block question

The metal block, shown opposite, has a weight of 900 000N. Calculate the maximum and minimum pressures it can exert when placed on one of its surfaces.

​

Maximum pressure occurs when the block is placed on its smallest area surface (2m x 3m)

p = F / A

= 900 000N / 6m²

Maximum pressure = 150 000 Pa

​

Minimum pressure occurs when the block is placed on its largest area surface (3m x 5m)

p = F / A

= 900 000N / 15m²

Minimum pressure = 60 000 Pa

Pressure examples

Pressure exerted by a person on a floor

1. Weigh the person in newtons. This gives the downward force, F exerted on the floor.

2. Draw, on graph paper, the outline of the person’s feet or shoes.

3. Use the graph paper outlines to calculate the area of contact, A with the floor in metres squared.

(Note: 1m² = 10 000 cm²)

4. Calculate the pressure in pascals using: p = F / A

Typical results

1. Weight of person: _____ N

2. Outline area of both feet in cm² ____

3. Outline area of both feet in m² _____

4. Pressure = ________

​

= _______ Pa

500

60

0.06

500 N

0.06 m²

8300

Why off-road vehicles have �large tyres or tracks

In both cases the area of contact with the ground is maximised.

This causes the pressure to be minimised as:

pressure = vehicle weight ÷ area

Lower pressure means that the vehicle does not sink into the ground.

How a gas exerts pressure

• A gas consists of molecules in constant random motion.
• When a molecule collides with a surface it reverses direction due to the force exerted on it by the surface.
• The molecule in turn exerts a force back on the surface.
• The pressure exerted by the gas is equal to the total force exerted by the molecules on a particular area of the surface divided by the area.
• pressure = force / area

Other pressure units�Note: You do not need to learn any of these for the IGCSE exam

Atmospheres (atm)

Often used to measure the pressure of a gas.

An atmosphere is the average pressure of the Earth’s atmosphere at sea-level at a temperature of 0°C.

Standard atmospheric pressure = 101 325 Pa (about 101 kPa)

​

Bars and millibars (bar; mbar)

Also used to measure gas pressure. One bar is about the same as one atmosphere.

​

Millibars are often found on weather charts.

1000 millibars = 1 bar = 100 kPa

Pounds per square inch (psi)

Often used to measure car tyre pressures.

1 psi = 6895 Pa

1 atm = 101 kPa = 14.7 psi

​

Inches of mercury (inHg)

Often found on domestic barometers.

1 inHg = 3386 Pa

1 atm = 101 kPa = 29.9 inHg

Examples:

Fair weather – high pressure: 30.5 inHg

Rain – low pressure: 29.0 inHg

Pressure in liquids and gases

The pressure in a liquid or a gas at a particular point acts equally in all directions.

At the same depth in the liquid the pressure is the same in all directions

The pressure in a liquid or a gas increases with depth

The pressure of the liquid increases with depth

Pressure, height or depth equation

pressure difference = height × density × g

​

p = h × ρ × g

units:

height or depth, h – metres (m)

density, ρ – kilograms per metres cubed (kg/m³)

gravitational field strength, g

– newtons per kilogram (N/kg)

pressure difference, p – pascals (Pa)

Question 1

Calculate the pressure increase at the bottom of a swimming pool of depth 2m.

Density of water = 1000 kg/m³

g = 10 N/kg

​

pressure difference = h × ρ × g

= 2m x 1000 kg/m³ x 10 N/kg

pressure increase = 20 000 Pa

Question 2

At sea level the atmosphere has a density of 1.3 kg/m³.

(a) Calculate the thickness (height) of atmosphere required to produce the average sea level pressure of 100kPa.

(b) Why is the actual height much greater?

​

g = 10 N/kg

​

(a) p = h × ρ × g

becomes:

h = p / (ρ × g)

= 100 kPa / (1.3 kg/m³ x 10 N/kg)

= 100 000 / (1.3 x 10)

= 100 000 / 13

height = 7 692 m (7.7 km)

​

(b) The real atmosphere’s density decreases with height.

The atmosphere extends to at least a height of 100 km.

Choose appropriate words to fill in the gaps below:

Pressure is equal to _______ divided by ______.

Pressure is measured in _______ (Pa) where one pascal is the same as one newton per ________ metre.

The pressure of the Earth’s ___________ at sea-level is approximately 100 000 Pa.

Pressure increases with ______ below the surface of liquid. Under _______ the pressure increases by about one atmosphere for every ______ metres of depth.

water

pascal

square

force

atmosphere

area

WORD SELECTION:

depth

ten

water

pascal

square

force

atmosphere

area

depth

ten

Online Simulations

Density - PhET - Why do objects like wood float in water? Does it depend on size? Create a custom object to explore the effects of mass and volume on density. Can you discover the relationship? Use the scale to measure the mass of an object, then hold the object under water to measure its volume. Can you identify all the mystery objects?

Bouyancy - PhET - When will objects float and when will they sink? Learn how buoyancy works with blocks. Arrows show the applied forces, and you can modify the properties of the blocks and the fluid.

Balloons & Bouyancy - PhET - Experiment with a helium balloon, a hot air balloon, or a rigid sphere filled with different gases. Discover what makes some balloons float and others sink.

Density Lab - Explore Science

Floating Log - Explore Science

Hidden Word Exercise on Tractor Tyres - by KT - Microsoft WORD

Water ejected from a hole in a tank - NTNU

Hydrostatic Pressure in Liquids - Fendt

Buoyant Forces in Liquids - Fendt

BBC KS3 Bitesize Revision:

Pressure - includes formula triangle applet

Pressure in gases