Wikijunior:Particles/Summary

Introduction

• Properties of matter
  • Matter must have mass
  • Matter must take up space

Matter

• Different kinds of matter
  • Solids' shapes cannot be altered, and nor can its volume
  • Liquids' shapes can be altered, but not its volume
  • Gases' shapes can be altered, and so can its volume
• Change of states
  • Solid → Liquid: Melting
    • Happens at a fixed temperature
  • Liquid → Solid: Freezing
    • Happens at a fixed temperature
  • Gas → Liquid: Condensation
    • Does not happen at a fixed temperature
  • Liquid → Gas: Boiling
    • Happens at a fixed temperature
    • Evaporation does not happen at a fixed temperature

Particle theory

• Matter is made of particles;
• Particles are infinitesimal, and current scientific instruments cannot see them;
• Different kinds of particles have different sizes;
• There must be gaps between particles
• Particles are always in motion.

Diffusion

• A common phenomenon observed in particle movement
• Particles always go to a place of lower density

Brownian motion

• When particles collide, they move in random directions
• Observed in smoke cells
• Discovered by Robert Brown
  • In 1827
  • While observing pollen grains

Particle model

• A model that mimics the movement of particles
• Can explain:
  • The three states of matter
  • Gas pressure
  • Density
  • Thermal expansion and contraction
• Heat makes particles move more vigorously

Three states of matter

• Solid
  • Particles cannot move freely
  • Particles only vibrate in fixed positions
  • Particles take on a fixed shape
• Liquid
  • Particles do not take on a fixed shape; their shape depends on that of their container
  • Particles have a fixed volume
  • Particles can slide over one another
  • Particles do not move as freely as those in gas
• Gas
  • Particles move freely in all directions
  • The shape and volume depend on that of the container

Gas pressure

• Refers to the pressure exerted on a surface by gas
• Measured
  • in Pascals, or 'Pa'
  • With a Bourdon gauge or pressure sensor
• Appears in air, in which case it's called air or atmospheric pressure
• Vacuum
  • Absence of air
  • Magdeburg hemispheres
    • Two hollow hemispheres form an empty sphere
    • The hemispheres cannot be separated
    • The air pressure on the outside greatly exceeds that of the inside

Density

• $\text{Density} = \frac{\text{Mass}}{\text{Volume}}$
• If an object is submerged in a liquid or gas, it:
  • Floats if its density is less than that of the liquid or gas
  • Sinks if its density is more than that of the liquid or gas
• Density of water = 1
• Hot-air balloons
  • There is an engine in the balloon
  • As the engine is heated, the particles inside the balloon move more and are therefore further apart
  • As density of the air inside the balloon deceases, it floats.
• Ships
  • The overall density of ships are under that of the water because there is a lot of air inside
• Submarines
  • Whether it is submerged or surfaced depends on the ballast tank
  • When the ballast tank is filled with seawater, it sinks
  • When the water is pumped out, it surfaces

Thermal expansion and contraction

• Particles move more vigorously when heated, and are further apart as a result
• This makes the object larger, or expands
• When they are cooled, the particles move less vigorously and are closer together
• The object is made smaller, or contracts
• Bimetallic strips
  • Made of two metal strips composed of different metals
  • The metals do not expand to the same extent
  • The strip bends when heated as a result
  • Used in
    • Thermostats
      • The connection is cut off when an appliance is too hot
      • The bimetallic strip in the appliance expands and therefore no longer completes the circuit
    • Fire alarm
      • The circuit is closed when the fire alarm is too hot
      • This makes the alarm ring because the built-in bimetallic strip completes the circuit