Lab 7 - Specific Heat of Solids.pdf - Specific Heat of...

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1 PHYS 40B: Lab 7 PHYS 40B: Lab 7 Specific Heat of Solids (Includes Pre-Lab Assignment) Objectives These lab activities will focus on concepts of thermodynamics in materials. You should read all the steps in each part before you start. Work in your assigned groups and maintain a collaborative and communicative team. We will measure the specific heat of different solid samples. The experimental setup allows us to develop an understanding for how temperature changes and phase changes may be expressed in terms of heat energy. Introduction The amount of thermal energy ( heat ) required to raise the temperature of an object is determined by the heat capacity of the object. The heat capacity is defined as the amount of energy required to raise the temperature of an object by one Kelvin: ! = #∆% where Q is the amount of heat energy, C is the heat capacity, and D T is the change in temperature. The heat capacity changes depending on the mass of the object we are considering. It is convenient to introduce a quantity that does not depend on the mass of the object, but only on the material of the object. This quantity is the specific heat , which is defined as the heat capacity per unit mass. Twice as much material (mass-wise) will have twice as much heat capacity, but the specific heat will remain the same for all objects made of the same material. The specific heat is related to the heat capacity as: & = #/( Specific Heat of Solids 2 PHYS 40B: Lab 7 When a substance changes state (solid to liquid, or liquid to gas, or solid to gas) there will be a transfer of heat associated with the change of internal energy in that substance. The heat energy absorbed by the material goes to breaking the bonds (potential energy) that hold the atoms/molecules together to form the solid or liquid. There is no change in temperature associated with the change of state, it can (mathematically) be treated separately from heat associated with changes in temperature (as described above) . The heat of transformation depends on how much material is present, so: ! = () where m is the mass of the material, and L is the latent heat or heat of transformation . For example, the solid (ice) to liquid transformation of water has a latent heat of L = 3.33x10 5 J/kg = 79.7 cal/g. Note that the temperature of the ice remains at 0 º C (273 K) while the heat is being absorbed by the ice. We will then use this value to determine the specific heat of a solid material. Measurement of Specific Heat by Vaporization of LN 2 : The relationship between the heat lost by the sample and that gained by the boiling liquid nitrogen is: ! = ( *+ ) *+ = ( ,-./0 & ,-./0 (% 233, − % *+ ) In this experiment, you will place a Styrofoam cup (to be used as the Dewar) containing liquid N 2 on a balance and measure its mass as a function of time as shown in the figure on the right. When the nitrogen vaporizes, it enters the atmosphere and the scale no longer records its weight. Even before the sample is immersed in the LN 2
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