What is Density? Density is defined as mass per unit volume. Density = Mass Volume.
Density
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Transcript of Density
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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)
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Density (g/cm3) = mass(g)volume(cm3)
Or using the formula triangle;
m D x V
kg/m3
kg
m3
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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
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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
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Density of irregular shapes (1)
Difference in level gives the volume of the shape mass
Density = mass/volume
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Density of irregular shapes (2)
Displacement can
volume of object
mass
Density = mass/volume
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Pressure
Pressure = Force Area
N/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
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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
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• Pressure difference between top and bottom = ρgh
= 1000 kg/m3x9.8x0.2= 1960 N/m2
0.2m
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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.
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Solids
• Fixed shape• Cannot flow• Difficult to compress• Generally dense
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Liquids
• Shape can change• Can flow• Not easy to compress• Generally dense
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Gases
• Shape can change• Can flow• Easy to compress• Low density
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Changes of state
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Brownian motion – Fat droplets in milk
Einstein's Explanation of Brownian Motion
http://www.practicalphysics.org/fileLibrary/wmv/brownian_motion.wmv
Brownian Motion
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• 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.
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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
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PHET!
• http://phet.colorado.edu/en/simulation/gas-properties
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Pressure versus temperature (at constant volume)
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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.
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At -273°C, P = 0!!
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Absolute/Kelvin temperature and Celsius
T (in Kelvin) = T (in degrees Celcius) + 273
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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.
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Temperature
The hotter the temperature, the faster the average speed of the particles
Note that they are not all travelling at the same speed.
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pV = constant
• p1V1 = p2V2 (at constant temp)
This is only true for a constant mass of gas at constant temperature.