Posted: Sun Jan 17, 2016 10:11 am
I have a few questions involving systems. What are the significance involving systems and can you please tell me each of the way these systems can be heated ?
Posted: Sun Jan 17, 2016 11:14 am
When looking at a problem, one should be able to identify the system, or the object of interest (such as a chemical reaction that is occurring), and the surroundings. This is important because the way that you answer a question can change depending on if you're considering the system or the surroundings. For example, the system DOING work is different from the surroundings doing work on the system (w=- if the system does work, w=+ if work is done on the system). Another important aspect of this is that the energy lost by the system is the energy gained by the surroundings. So for example, if an ice cube is melting (the system), you can think of it as the ice cube is absorbing this energy from the surroundings. Thus you can use the equation
q(sys) = -q(surroundings) or heat gained by ice = heat lost by surroundings
Therefore it is very important to be able to identify which is the system, which is the surroundings, and what type of interaction is occurring between the two.
An open system means that both matter and energy can be exchanged with the surroundings. For example, in an open beaker, matter can go in and out, along with energy. Dr. Lavelle used the example in class of water evaporating in a beaker - the matter, the evaporated gas molecules, can go into the surroundings, along with energy flow, because the beaker is not insulated. The energy of an open system can be changed by heating or cooling it (for example, putting a flame beneath an open beaker containing matter) or by adding/removing the amount of substance in a system.
In a closed system, only energy can be exchanged with the surroundings, and not matter. Think of a closed beaker. Another example is a cold pack used for treating athletic injuries -- heat can be transferred from the injury to the cold pack, but matter cannot be exchanged. A closed system's can be changed by heating or cooling the system or by (if the system is gas inside a piston or something similar) compression/expansion.
In an isolated system, neither energy nor matter can be exchanged with the surroundings. It has rigid, thermally isolating walls. This occurs in a calorimeter. The book also gives the example of a hot liquid inside a high quality thermos bottle. The energy of an isolated system remains constant because no matter or heat can be transferred.
I hope this clarifies things! Also, check out page 23 of the course reader lecture notes.