Thermal properties

Teaching challenges Latent heat: The transfer of energy which accompanies a change of state is ‘hidden’ and so can be difficult to grasp.

Thermal transfers Energy transfer from one store to another because of a temperature difference. Thermal store: internal energy (particles in motion)

Cooling or heating curves previously schools used naphthalene (now hexadecanol) Rate of heat loss is proportional to temperature difference between sample and its surroundings.

Heat capacity: thermal store with no change of state Start simple: Why is a bite of hot potato more likely to burn the tongue than a bite of cabbage at the same temperature? Which food stays hot longer on your plate? Thermal (heat) capacity of a material: energy stored or released by an object per degree of temperature change. J oC-1 ‘Specific’ thermal capacity: energy stored or released by a kg of material per degree of temperature change. J kg-1 oC-1 Materials used as coolants have a high specific thermal capacity. Table On global and regional scales, water can store and transfer huge amounts of energy.

Experiments to determine c Electrical method Method of mixtures e.g. solid placed in water energy lost by hotter object = energy gained by cooler object NOTE: Both the above assume no heat loss. Insulate calorimeter & include it in calculations. Using a cooling curve ref: Nelkon & Parker Advanced level Physics Practicalphysics.org Energy collection ‘Thermal physics’

Latent heat: thermal store associated with a change of state No temperature change Term ‘latent’ introduced ~1750 by Joseph Black [derived from the Latin latere, to lie hidden]. Fusion: reversible change solid to liquid Vaporisation: reversible change liquid to vapour Table Again water has relatively large values. Latent heat of vaporisation is the main energy source for hurricanes.

Latent heat Experiments Fusion: Melting ice in a calorimeter Vapourisation: passing steam through a calorimeter; electrical method. Ref: Nelkon & Parker Advanced level Physics Practicalphysics.org Energy collection ‘Thermal physics’ Applications releasing energy as liquid changes to solid hot pad hand-warmer thermal energy storage in buildings

Real world thermal processes Jearl Walker (2nd ed 2010) The flying circus of physics with downloadable pdf file for all YouTube links and journal citations in the book. e.g. Chapter 4: Striking At The Heat In The Night