About Heat

The transfer of heat through a substance is often explained using the particle model. When heated the particles in a substance vibrate faster and push each other further apart. Thus, the substance expands. On cooling the particles vibrate less and the substance contracts. For this reason bridges and railway lines are designed with gaps between sections. Liquids expand more than solids. Thermometers use this property. Alcohol boils at 79oC so it cannot be used as the liquid when measuring the temperature near the boiling point of water. When fluids are heated, they expand, become lighter, and then rise, while cooler parts sink. This particle movement in liquids and gases is called convection. When one end of a metal strip is heated, the vibrating particles collide with others which then pass on the vibration. Thus the heat spreads throughout the material. This process is called conduction. Not all solids do this well - insulators do not pass on the vibrations very well. They are poor conductors. Aluminium is a good conductor, whereas wood is a good insulator. Heat may travel from a hot object through space by radiation. The hot object emits heat to its surrounds. When an object receives radiated heat it can transmit some of it if it is transparent, otherwise the heat can be reflected back or absorbed, warming it up.