Lecture 5. > Specific Heat. > Calorimetry. (Source: Cutnell, Giancoli, Tippens) Villacorta--DLSUM-SCIENVP-L Term01

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1 Lecture 5 > Specific Heat > Calorimetry (Source: Cutnell, Giancoli, Tippens) 1

2 Heat > An increase in a body's temperature is related to the increase in the kinetic energy of the molecules composing the body: + Molecules move more, making larger vibrations, covering more space. + The body expands as a result of the larger motions of its molecules. > Observation shows that heat flows from a body with lower temperature to one that has a higher temperature. + The caloric was a particle inferred to carry heat from one body to another. + Calorie (cal) became the unit describing the amount of heat needed to raise the temperature of one gram of water by 1 Cº. + Dietary products use kilocalorie (kcal) to describe the calorie content in foods. + The English system uses the British thermal unit (Btu) to describe the amount of heat: raising the temperature of one pound of water by 1 Fº. 1 Btu = 252 cal 2

3 Heat contd. > The caloric particle was developed in the 1800's to describe the flow of heat; however, this particle was never observed in experiments done at the time. + The caloric theory was abandoned in favor of the explanations associated with the experiments conducted by James Prescott Joule. + The link was between work done and heat. > Joule's apparatus + Weights go down + Turning the paddle + Disturbing the water + Water temperature measured > When the weights do work, the water temperature rises. > Equivalence: 1 cal = J 3

4 Heat contd. > Heat is the energy transferred between to bodies with different temperatures. + Heat: energy transfer + NOT contained in substance > The internal energy in the molecules increase due to the energy transferred to it. > Internal energy: sum total of the energies in the molecules + Kinetic energy + Potential energy wikipedia.org 4

5 Specific Heat > The heat absorbed by a body depends on + the temperature change: large changes leads to greater heat + body mass: large mass requires more heat to raise its temperature > The heat absorbed Q by a body of mass m is given by Q = m c T where T is the resulting temperature change and c is the specific heat capacity of the body. 5

6 Specific Heat contd. > Different substances have different heat capacities. It is a property of the substance itself, independent of its amount or shape in a problem. imgarcade.com 6

7 Calorimetry > Calorimetry refers measuring the heat flow from one body to another. > A system of bodies isolated from others have no heat flowing in or out of the system. + Total heat flow is zero. + Heat gains by a body inside the system equal heat losses by other bodies. Q tot = 0 Q gain + Q loss = 0 Q gain = Q loss 7

8 Calorimetry contd. > Ex. bomb calorimeter + System: water and mass + Walls prevent heat flow into and out of the system. > If the mass is initially at a higher temperature than the water before it is placed inside the calorimeter, + then the temperatures will change: + water becomes warmer; + mass becomes cooler. + Both will have the same final temperature. Q object = Q water m o c o T o = m w c w T w 8

9 Summary > The heat absorbed by a body depends on its mass and change in temperature. Q = m c T Q tot = 0 Q gain + Q loss = 0 Q gain = Q loss 9

10 Sample Problems 1. Blood can carry an excess energy from the interior to the surface of the body, where the energy is dispersed in a number of ways. While a person is exercising, 0.6 kg of blood flows to the surface of the body and releases 2000 J of energy. The blood arriving at the surface has the temperature of the body interior, 37.0 ºC. Assuming that blood has the same specific heat capacity as water ( c = 4186 J/kg-K ), determine the temperature of the blood that leaves the surface and returns to the interior. (Cutnell) 10

11 Sample Problems 2. A piece of glass has a temperature of 83.0 ºC. Liquid that has a temperature of 43 ºC is poured over the glass, completely covering it, and the temperature at equilibrium is 53.0 ºC. The mass of the glass and the liquid is the same. Ignoring the container that holds the glass and liquid and assuming that the heat is lost to or gained from the surroundings is negligible, determine the specific heat capacity of the liquid. (Cutnell) ( heat capacity for glass = 840 J/kg-K ) 11