Car Temperature in Morning; Equilibrium? ```Name: Dave Status: Student Grade: Other Location: N/A Country: United States Date: Winter 2009-2010 ``` Question: Hi, I have a very simple question. If you leave your car outside all night, with the windows and doors shut, and the temperature drops to zero degrees, when you get in the car in the morning will the temperature inside the car be the same as the temperature outside the car? I am assuming that there is no additional heat source for the car (it has not been running) and that you are getting in the car right before the sun comes up. Replies: Hi Dave, The answer is "yes.... if the night is long enough"! With no additional sources of heat, the inside of the car will eventually reach thermal equilibrium with the outside, and both will have the same temperature. If the night is too short, and other sources of heat under the hood will not have fully cooled off, they may still cause a small amount of warming in the passenger compartment. This is only true if night time temperature is stable and does not change quickly. It is even possible for the interior of the car to be colder than outside. Why?... There is a time delay where the interior temperature lags slightly behind the changing outside temperature. What can happen is that the outside temperature may have remained at a steady low temperature for most of the night, and the interior will have cooled to this same temperature. But if the outside temperature started to rise before dawn, the interior temperature will also rise, but lag behind the rising exterior temperature rise. As a result, the interior temperature will remains slightly cooler than the rising exterior temperature, as both are rising. Regards, Bob Wilson It depends on how well insulated the car is, and how the temperature varies with time during the might. Generally, the temperature inside the car will lag behind the outside temperature, because it takes time for heat to be conducted -- the rate is proportional to the difference in temperatures, and to the thermal conductivity of all the paths by which heat can be conducted. As the temperature inside the car approaches the outside temperature, the rate at which heat is conducted decreases. Tim Mooney You did not specify whether you meant zero degrees F. or zero degrees C. That makes a big difference. The temperature in the car's interior will probably be similar to the exterior. That is different than what is going on under the hood of the car where you have that huge bunch of metal called the engine. It warms up more slowly just because of its large mass. On the long scale, the interior and exterior temperatures will be pretty much the same. Vince Calder A car is not perfectly insulated, so there is heat exchange between the car and its surroundings. A big factor is radiative heating due to sunlight. Conduction due to air temp is also a factor. Look up "conduction" and "radiative heat transfer" if you need background on these terms. Heat exchange does not happen instantaneously, so the internal temperature of the car will lag behind air temperature a bit as the heat moves in or out. In your example you are talking about morning, so I am assuming the air temperature is rising at the time you are talking about. In the case of a cold morning (but a rising air temperature) and a cold car, the car can actually be colder than the air because the air is heating up faster than the more-massive car interior. If the sun is shining, the car interior could be warmer than the air as conduction and radiative heating both warm it. If the air temperature is not changing, and if there is no radiative heating, then your car could exactly equal air temp (this is called thermal equilibrium), but in reality, things are never steady enough to truly reach equilibrium. Hope this helps, Burr Zimmerman Hi Dave. When you get in your car in the morning, the temperature inside the car should be the same as the temperature of the air outside the car because the heat of your engine, and any that may be left in the passenger area will dissipate into the surrounding air. So in an isolated system, If the car is parked in a garage, the heat of the engine will dissipate into the air of the garage. The garage air will get warmer, and the car will get cooler. This process was summarized by Isaac Newton in his statement of the second Law of Thermodynamics. One way to state it is that heat travels from hot to cold bodies and not the other way around until they reach a common equilibrium temperature. More technically: heat is a measure of disorder, or entropy, and entropy always increases for an isolated system (in this case the air in the garage). Sincere regards, Mike Stewart Click here to return to the Engineering Archives

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