๋࣭ ⋆｡ °✩ chapter 10

• The SI unit of temperature is kelvin (K)
• When an object gains thermal energy, the atoms in the object starts to move further apart
• Atoms and molecules never expand in size, but it is the distance between the atoms and molecules that changes due to the effects of thermal energy
• Thermal expansion and contraction occurs for all 3 states of matter
• The forces of attraction in a gas are much weaker than the forces of attraction between particles in a liquid.
• The weak forces in a gas are easily overcome and the particles in a gas can move apart easily, increasing the volume greatly when heated.
• In a liquid, the string forces of attraction are not easily overcome, and the particles can only move slightly apart when heated.
• A solid has strong intermolecular forces of attraction which hold its shape even at high temperatures.
• Solids have very little change in volume with temperature because the particles cannot move far enough apart to increase their volume significantly.
• Liquid expands more than solids as they heat up because the particles in liquids can move around freely and so there is greater space available for them to occupy.
• Expansion & contraction in daily life

  In hot weather, a metal railway track gains heat & expands

  The track can buckle if it is made in one continuous metal piece & may cause a train to derail
• Expansion gaps between railway tracks provide space for expansion
• When a bimetallic heated is heated or cooled, the flat strip bends because one metal expands or contracts more than the other (In the common form of brass and another form of metal, brass always expands and contracts more when heated or cooled)
• Conduction is the transfer of thermal energy through a material without the movement of the medium itself
• Through conduction, heat is transferred from the warmer end to the cooler end of the medium
• Conduction can occur in all 3 states of matter - solid, liquid and gas.
• All conduction processes involves molecular vibrations. The particles closest to the heat source gain thermal energy and vibrates faster, hit neighbouring particles and cause them to vibrate faster too. This process is repeated from particle to particle along the rod.
• Good conductors of heat conduct heat faster than poor conductors.
• Poor conductors of heat is also called an insulator
• Vacuum is the perfect insulator as it has no particles. As matter is needed to conduct heat, no heat transfer occurs through a vacuum by conduction at all.
• Using the particulate model of matter, suggest why liquids and gases are not as good conductors as solids.

  (liquid) Particles in a liquid are closely packed with weaker forces of attraction between the particles compared to solids, hence it is harder to pass on the molecular vibrations

  (gas) Particles in a gas are far apart and have very weak forces of attraction between the particles compared to solids, hence it is harder to pass on the molecular vibrations
• Convection is the transfer of thermal energy from one place to another by the movement of the medium itself due to a difference in density
• Liquids and gases are poor conductors of heat. Heat is transferred through liquids and gases mainly by convection
• Convection can only occur in liquids or gases
• when the water at the bottom of the (flask) is heated, it expands and decreases in density, causing it to rise. Cooler water at the top is denser and sinks to the bottom of the flask. This sets up a convection current which heats up the entire water in the flask
• Convection in gases:

  The difference in density of the hot and cold air at the different (chimneys) causes the air to move, forming convection currents.
• Why does the ice not melt?

  Water is a poor conductor of heat. Convection currents are not set up when the water is heated at the top.

  
• Radiation is the transfer of thermal energy without the need for a medium. Objects warmer than their surroundings give out heat as radiation. Radiation can take place in a vacuum
• Objects that gain thermal energy via radiation from a heat source are known as better absorbers of thermal radiation.
• Objects that lose thermal energy via radiation from a heat source are known as better emitters of thermal radiation.
• We need to identify if an object is acting as an absorber or emitter of radiation, but not both at the same time