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8.1Fluids

Internal Structure and Density — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 14 1 quiz

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Lesson summary

8.1 Internal Structure and Density — AP PHYSICS 1 notes: Describe the properties of a fluid, its internal structure and its density.Fluid = a substance with no fixed shape that can flow; liquids and gases are fluids.Liquids — particles weakly bonded, close together, able to move …

  • Solids versus fluids
  • Incompressible
  • No viscosity
  • Steady flow
  • Real fluids
  • Bulk properties

Section 1 of 9What a fluid is

Fluid
Fluid a substance with no fixed shape that can flow; liquids and gases are fluids.
Liquids
  • Liquids particles weakly bonded, close together, able to move past each other, nearly incompressible;
    • they fill the base of a container.
Gases
  • Gases particles not bound, far apart and moving freely;
    • they are easily compressed and fill a container completely.
Solids versus fluids
  • Solids versus fluids differences arise from the interactions between atoms and molecules.

Section 2 of 9Ideal fluid

1Incompressible
  • Incompressible constant density, so a force on one region is exerted on other regions.
2No viscosity
  • No viscosity no internal friction, so resistive forces are ignored in flow.
3Steady flow
  • Steady flow without turbulence.
4Real fluids
  • Real fluids have viscosity and may be compressible;
    • AP Physics 1 models fluids as ideal unless told otherwise.
5Bulk properties
  • Bulk properties a fluid can be modelled using density and pressure.

Section 3 of 9Density

Density
ρ = m/V
What the symbols mean: ρ = density (kg m−3 or g cm−3) · m = mass (kg) · V = volume (m3)
  • mass per unit volume, in kg/m3 (1 g/cm3 = 1000 kg/m3).
Typical values
  • Typical values water about 1000 kg/m3, air at sea level about 1.2 kg/m3.
Property of material
  • Property of material at a given temperature and pressure, not of the sample size.
Closer particles
  • Closer particles higher density; gases have very low density.
Average density
  • Average density total mass divided by total volume; it decides whether an object floats.
    • (a steel ship floats because steel plus air is less dense than water)

Section 4 of 9Densities of materials

MaterialDensity (g/cm3)In water
Cork0.24Floats
Ice0.92Floats
Oil0.9Floats
Water1.00Reference
Aluminium2.7Sinks
Iron7.9Sinks
Gold19.3Sinks

Section 5 of 9Measuring density of a regular object

1
Place the object on a digital balance and note its mass.
2
Measure the dimensions with a ruler, callipers or micrometer.
3
Repeat and average the readings.
4
Calculate the volume with a formula for the shape.
5
Substitute mass and volume into ρ = m/V.

Section 6 of 9Measuring density of an irregular object

1
Measure the mass of the object with a balance.
2
Fill a displacement (eureka) can with water just below the spout and place an empty measuring cylinder under it.
3
Lower the object carefully into the can.
4
Measure the volume of displaced water, equal to the object's volume.
5
Repeat and average, then calculate ρ = m/V in kg/m3.

Section 7 of 9More from the specification

More from the specification
Density measurement at variable depth
  • Density measurement at variable depth evaluating liquid hydrostatic column pressure at known vertical depth provides a direct physical means to determine density.
Gauge pressure
  • Pressure above atmospheric, ρgh.
Absolute pressure
  • Total pressure, P0 + ρgh.
Perpendicular
  • At right angles.
Component
  • Part of a vector along one axis.

Section 8 of 9Key terms

TermMeaning
FluidSubstance with no fixed shape that can flow.
DensityMass per unit volume.
Ideal fluidIncompressible fluid with no viscosity.
Displacement methodFinding volume of an irregular object by the water it displaces.
Eureka canOverflow can used for displacement.

Section 9 of 9Exam technique

1
Thinking density depends on the size of the sample.
2
Forgetting to convert g/cm3 to kg/m3.
3
Treating real fluids as ideal without being told.
4
Saying heavy objects always sink — it is the average density that decides.

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