particle model

• electrons are negatively charged
• polythene rods repel eachother
• acetate rods repel eachother
• polythene and acetate rods attract
• losing electrons = positive charge
• having electrons = negative charge
• polythene and acetate have equal but opposite charges
• friction causes electrons to move to/from a material
• rod and cloth experiment

  
• what is density?


  the density of a substance is defined as the mass per unit volume. an object made from a low density substance will be light for its size

  density = mass/volume
• to measure the density of a solid object or a liquid, measure its mass and volume then use the equation density = mass/volume
• objects that have a lower density than water (1000kg/m^2) can float
• the particles in a solid are held next to each other in fixed positions and vibrate about their fixed positions so each solid keeps its own shape. they are the least energetic states of matter.
• the particles of a liquid are in contact with eachother, moving randomly. this is why liquid doesn't have its own shape and is free flowing
• the particles of a gas move about at random much faster than in a liquid. they are much further apart from eachother than the particles of a liquid so the density of a gas is less than a solid or liquid. almost no forces of attraction
• when a substance changes state, its mass stays the same because the number of particles stays the same.
• for a given amount of a substance, its particles have more energy in the gas state than liquid state but more energy in the liquid state than the solid state
• a dense material has its particles packed together. the particles in a less dense material are more spread out - if you compressed the material the particles would move closer together and it would become more dense.
• internal energy


  1. the internal energy is the total kinetic energy and potential energy of all the particles that make up a system.
  2. heating changes the energy stored within the system by increasing the energy of the particles that make up the system.
  3. this can either raise the temp of the system of induce a change of state
• temperature change in a system

  if the temperature of the system increases, the increase in temperature depends on the mass of the substance heated, the type of material and the energy input to the system.
• specific latent heat


  1. is the energy needed to change state of one kg of a substance without changing temperature
  2. for cooling, specific latent heat is the energy released by a change in state
  3. different for different materials and states
  4. energy = mass x specific latent heat
• particle motion in gases

  1. the molecules of gas are constant randomly moving
  2. the temperature of the gas is related to the average kinetic energy of the molecules.
  3. changing the temperature of the gas (at a constant volume) changes the pressure exerted by the gas.
• how does particle motion in gas create pressure?

  1. as gas particles move with high speed and collide, a force is exerted. these occur with a net force at right angles to the surface of the gas container.
  2. increasing temperature increases the speed and pressure of particles if kept at constant volume
  3. if temperature is constant and volume increases the pressure decreases
  4. pressure and volume are inversely proportional
  5. pressure x volume = constant
• Doing work on a gas increases the internal energy of the gas and can cause an increase in the temperature of the gas