Notes on Heat by pptfiles


									Notes on Heat Rev1 Heat is similar to mechanical energy but on the molecular scale. It is the kinetic and potential energy of individual molecules and atoms. For pure heat interactions, [i.e. no macroscopic work done and no significant changes in the position of the body with respect to a force field or its speed] With no phase change Q in = mass c T, where c is the specific heat. where there is a phase change we must include latent heats of fusion, Q = mL f (remove for freezing –add for melting), or vaporization Q = mL v (remove for condensing –add for evaporating). Also note that the specific heat can change with the state of the substance (or even with its temperature) Water has different specific heats as ice, liquid and steam. For a closed system with no outside heat or work added total energy is conserved. If no work is added then the total heat changes add to zero. Example: 200 g of metal with a c of 350J/kg Kelvin is dropped into 500g of liquid with a c of 500 J/kg Kelvin. If the metal gains 30 Kelvin in temperature, how much did the liquid temperature drop? Q liquid = -Q metal so .500x500Xliquid .200x350x30 or liquid= -2100/250 = 8.4 Kelvin or 8.4 deg C. During evaporation or condensing or melting and freezing a substance can gain or lose heat with no change in temperature. These appear as flat spot in a temperature vs heat in curve and the longer the flat spot the greater the L. In the places where temperature is rising, the steeper the slope the smaller the specific heat is (greater change in T per unit of Q added). Heat is transferred by 1) conduction where heat passes through an object or fluid. The rate of heat transfer through an object or body of fluid is given by

Q kA  T t L where k is the thermal conductivity, A is the cross sectional area, and L is the length of the object in the direction of the heat flow.

2) convection where the transfer of heat occurs by motion of a fluid. For example: a fluid receives heat or provides heat to a body or region moves away; a fluid passes over a surface and absorbs or releases heat to a surface which is why windy days feel cooler. In many cases the heat itself generates convection flows because hotter fluids are less dense and rise, This a reason for strong winds in thunderstorms and tornadoes.. 3) Radiation: The heat emitted by an object due to radiation from the electrons in its atoms/molecules is roughly given by

Q  AT 4 where t
A is area(m2) ;


 is the Stefan Boltzmann constant = 5.67 x 10-8 W/m2 K 4  is the emissivity coefficient and T is the temperature in K

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