Heat and Specific Heat A person puts a pan on a stove heating ring and returns a few seconds later to find that the pan is hot. The same person puts a pan of water on the stove ring and returns minutes later to find that the pan is warm but far from hot. What does this tell you? Some substances will absorb a lot of heat for only a small change in temperature. Iron with a little heat will shoot up in temperature while water takes a lot of heat energy to make the temperature higher. Why is this?
Heat and Specific Heat Temperature has a lot to do with the translational back and forth motion of atoms or molecules Iron s electrons move rapidly back and forth and the temperature goes up quickly But water doesn t just shake back and forth, the molecules pucker in and out storing the energy in internal rotational states (potential energy) and the hydrogen bonding sticks them together so they don t shake as much, so the temperature is not driven up as much. We say that water stores an enormous capacity of heat for small temperature changes This is called specific heat
Heat and Specific Heat We can define the heat capacity, C, of an object as the energy required to raise its temperature by 1 K. This is different for different substances. C = Q / T units: J / K or J / 0 C Specific heat capacity, c, is the energy required to raise a unit mass of a substance by 1 K. c = Q / (m T) units: J / kg K or J / kg 0 C We say that water stores an enormous capacity of heat for small temperature changes This is called specific heat
Heat and Temperature Change: Specific Heat Capacity
Heat and Specific Heat It turns out that water will absorb a whole calorie of heat energy per gram and only change temperature by 1 degree Celcius (centigrade) We can think of specific heat as thermal inertia because it signifies the resistance of a substance to a change in temperature. Q = m c t m = mass of object c = specific heat capacity (cal/g 0 C) t = change in temperature c water = 1 cal/g 0 C or 4.184 J/g 0 C or 4184 J/kg 0 C
Heat and Specific Heat How much energy does it take to raise the temperature of 1.5 liters of water by 20 0 C? m water = 1.5 L ( 1kg / 1L) = 1.5 kg T = 20 o C c water = 1000 cal/kg 0 C or 4184 J/kg 0 C Q = m c t Q = (1.5 kg) (1000 cal/kg 0 C) (20 0 C) Q = (1.5 kg) (4184 J/kg0C) (20 0 C) = 30 000 cal = 125 520 J = 130 000 J
Heat and Specific Heat Some high temperature foods, you can eat comfortably when you take them out of the oven as they have a low specific heat capacity and therefore don t hold much thermal energy but water filled foods like pie filling you can burn your mouth on as the high temperature food will hold a lot of energy. A hot water bottle contains boiling water that cools gradually during the night releasing a large amount of thermal energy. Countries surrounded by water (which has a high specific heat) are heated by the warm winds that have absorbed thermal energy from the ocean (cons of energy). The ocean cools gradually during the winter so maintains a constant source of heat energy. The water acts as a temperature moderator, absorbing energy from the air above in the summer and releasing it in the winter.
Heat and Temperature Change: Specific Heat Capacity A Hot Jogger In a half-hour, a 65-kg jogger can generate 8.0x10 5 J of heat. This heat is removed from the body by a variety of means, including the body s own temperature-regulating mechanisms. If the heat were not removed, how much would the body temperature increase? Q mc T 5 8.0エ10 J Q D T = = = 3.5 C ( 65 kg) 鴿 3500 J ( mc kg ラC )
Heat and Temperature Change: Specific Heat Capacity CALORIMETRY If there is no heat loss to the surroundings, the heat lost by the hotter object equals the heat gained by the cooler ones.
Heat and Temperature Change: Specific Heat Capacity Measuring the Specific Heat Capacity The calorimeter is made of 0.15 kg of aluminum and contains 0.20 kg of water. Initially, the water and cup have the same temperature of 18.0 o C. A 0.040 kg mass of unknown material is heated to a temperature of 97.0 o C and then added to the water. After thermal equilibrium is reached, the temperature of the water, the cup, and the material is 22.0 o C. Ignoring the small amount of heat gained by the thermometer, find the specific heat capacity of the unknown material.
Heat and Temperature Change: Specific Heat Capacity mc T Al mc T water mc T unknown c unknown = ( mcd T ) + ( mcdt ) Al ( mdt ) unknown water = 鴿 ( ) ( )( ) 鴿 + ( ) ( )( ) ( 0.040 kg)( 75.0 C ) 2 9.00エ 10 J kg ラC 0.15 kg 4.0 C 4186 J kg ラC 0.20 kg 4.0 C ( kg C ) = 1300J ラ
Phase Changes Why do you feel cold when you get out of a pool and a breeze is blowing? Water evaporates off your body and cools you down. How come? Why is evaporation a cooling process? Molecules in a liquid have a distribution of speeds and the average relates to what we call Temperature. When faster moving molecules leave a liquid s surface they leave behind the slower moving molecules, which lowers the average speed and therefore the temperature of the liquid. Normal distribution curves of molecular speeds for ideal gas at 100 K and 900 K respectively
Phase Changes In hot climates, sacking is put over clay pots and kept wet. Why? Evaporation of water from the sack cools the sack and thus draws thermal energy outwards from the water inside the pot. This keeps the drinking water nice and cool. How do we keep cool? We perspire (sweat), so evaporation takes place from our skin s surface. Why do dogs pant? (or other animals without sweat glands) They can t sweat so they create a large surface area (tongue and bronchial tract) from which liquid can evaporate and therefore cool them down. Similarly rubbing your hands under a hand dryer creates a larger surface for water to evaporate.
Phase Changes Evaporation depends upon the temperature of the liquid and air surrounding the liquid, the surface area of the liquid and the moisture content in the air (humidity). Evaporating molecules form a vapor above the liquid. Equilibrium is reached when the vapor pressure exerted on the fluids surface returns liquid molecules to the liquid at the same rate they leave. How do fans keep people cool? Lower vapor pressure so more evaporation so.more cooling Why are their warning signs on hot tubs about staying in too long? In a hot tub you sweat like mad but the water can t evaporate off the skin so your body heats up and can be dangerous if you stay in too long. Name at least two ways to cool a hot cup of coffee? Increase evaporation by blowing on it, pouring it into a saucer Cool by conduction by pouring in cool saucer, adding ice cubes, stirring with cool silverware
Phase Changes Is condensation a heating or a cooling process? It is a heating process. The slower moving water vapor molecules stick together (forming droplets) while the higher speed molecules remain in the gaseous state thus increasing the average KE of the air molecules and thus the temperature. Why do people dry off in a steamy bathroom? The cooling effect of water evaporating off their body is balanced by the heating effect of steam condensing on it. That is why my 4-year old daughter asks for the warm water to be left in the tub while she is drying off.
Phase Changes If evaporation is a cooling process is boiling a heating process? Heating the water in a pan is a heating process but the act of boiling is actually a cooling process. Like evaporation, when boiling, molecules leave the surface of the liquid taking thermal energy with them. What influences the ability of a liquid to boil other than temperature? The air pressure acting on the liquid Heat pan of water, bubbles form but 30km of air pushing down (atmospheric pressure) squashes these bubbles. With more heating the temp keeps increasing and the molecules become more energetic until bubbles are able to withstand the pressure. This temps for water is 100 0 C at sea level. What happens to this process if you are up a mountain? Closer to top of air therefore less pressure so bubbles stay formed at a lower temperature.
Phase Changes You normally cook eggs for 3 minutes at sea level. Do you cook them for a longer or shorter time when living at higher altitude? Longer as water boils at a lower temp If you heat water at atmospheric pressure it boils at 100 0 C but if you turn up the heat it doesn t get any hotter, how come? The more you heat, the more vigorous the boiling but you also have more liquid escaping therefore more cooling so they offset each other. (you can also think of it as the heat input going towards breaking intermolecular bonds and not towards changing the average kinetic energy of the molecules). If you put a cover on a pan of spaghetti will it cook faster? Yes, the lid increases the pressure so temp goes up more before bubbles stay formed.
Phase Changes Why are sensitive fruit crops in the South sprayed with water before a forecasted frost? Like condensation (gas to liquid), changing state from liquid to solid is also a heating process. The heat liberated in freezing the layer of water around the outside of the strawberry, actually protects the strawberry inside.