Density

Density

Remember that density is the mass of one cubic centimetre (or cubic metre) of a substance.

So for example, gold has a density of 19.3 g/cm3. This means one cubic centimetre (cm3) of gold has a mass of 19.3 grams (or one cubic metre of gold has a mass of 19300 kg)

Density (g/cm3) = mass(g)

volume(cm3)

Or using the formula triangle;

m

D x V

kg/m3

kg

m3

Can you copy this please!

Density of regular shapes

volume = length x width x height

density = mass/volume

mass using a scale

lengthwidth

height

http://www.youtube.com/watch?gl=IE&feature=related&hl=en-GB&v=14nahP_FVnM

Precision and Accuracy

• Precise – High number of significent figures. Repeated measurements are similar

• Accurate – Near to the “real” value

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Density of liquids

Mass of liquid and cylinder

Mass of cylinder

Mass of liquid

Density = mass/volume

Volume

Density of irregular shapes (1)

Difference in level gives the volume of the shape

mass

Density = mass/volume

Density of irregular shapes (2)

Displacement can

volume of object

mass

Density = mass/volume

Pressure

Pressure = Force

AreaN/m2 or Pa

N

m2

F

AP x

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An example

A woman of weight 600N has a total shoe area of 150 cm2 and a man of weight 750 N has a total shoe area of 360 cm2. What is the pressure beneath their feet?

Angelina pressure = force/area = 600/150 = 4 N/cm2

Brad pressure = force/area = 750/360 = 2.1 N/cm2

The pressure acts in all directions

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Pressure and depth

Pressure increases with depth (P = ρgh)

The pressure acts in all directions

• Pressure difference between top and bottom = ρgh

= 1000 kg/m3x9.8x0.2

= 1960 N/m2

0.2m

Draw these sentences!1. The particles in a solid are close packed.

2. The particles in a solid are in regular positions vibrating around a fixed point.

3. The particles in a liquid are also close packed.

4. The particles in a liquid also vibrate and move around randomly.

5. The particles in a gas are far apart.

6. The particles in a gas are moving very quickly.

7. For the same substance (e.g. water), the particles are the same size in the solid, liquid or gaseous forms.

8. Mr Porter is the world’s best science teacher.

Solids

• Fixed shape

• Cannot flow

• Difficult to compress

• Generally dense

Liquids

• Shape can change

• Can flow

• Not easy to compress

• Generally dense

Gases

• Shape can change

• Can flow

• Easy to compress

• Low density

Changes of state

Brownian motion – Fat droplets in milk

Einstein's Explanation of Brownian Motion

http://www.practicalphysics.org/fileLibrary/wmv/brownian_motion.wmv

Brownian Motion

• Brownian motion is the seemingly random movement of particles suspended in a fluid (i.e. a liquid or gas). It is due to the instantaneous imbalance in the combined forces exerted by collisions of the particle with the much smaller liquid molecules surrounding it.

Pressure in a gas

Collisions of the gas particles with the side of a container give rise to a force, which averaged of billions of collisions per second macroscopically is measured as the pressure of the gas

PHET!

• http://phet.colorado.edu/en/simulation/gas-properties

Pressure versus temperature (at constant volume)

P/T = constant

• P1/T1 = P2/T2

The temperature MUST

be in kelvin

This is only true for a constant mass of gas at constant volume.

At -273°C, P = 0!!

Absolute/Kelvin temperature and Celsius

T (in Kelvin) = T (in degrees Celcius) + 273

Kelvin Temperature

The kelvin Temperature is proportional to the average kinetic energy of the particles in a substance.

Note that they are not all travelling at the same speed.

Temperature

The hotter the temperature, the faster the average speed of the particles

Note that they are not all travelling at the same speed.

pV = constant

• p1V1 = p2V2 (at constant temp)

This is only true for a constant mass of gas at constant temperature.