Heat
You should be handing in your completed lab reports this week. We are then doing a fairly straightforward lab demonstrating temperature measurements and heat transfer.
Heat is an amount of thermal energy that moves from one place to another during a thermal process. You must exclude all the energy transferred by work. Heat is measured in energy units (eg Joules). When something is hotter that its environment it has more average energy per molecule that the surrounding molecules. The hot object will transfer this excess energy to its surroundings. The amount of energy transferred in this way is called heat.
One of the best ways to develop some intuition about temperature and heat is to consider the billiard ball model of a gas. In this model we describe a bottle of gas as a container filled with billiard balls ricocheting all over the place. If you were to try to reduce the volume of the container you would clearly need to push on many of the balls during the process. If you were pushing then you were doing work. It is not so obvious but nevertheless true that the balls that were pushed during the process would have more energy. The extra energy would be quickly shared through collisions. The average energy of the billiard balls would increase. The temperature of the gas would increase. We have just done work and that work resulted in an increase of temperature. There was no energy in the form of heat. Now imagine a more interesting container, a container with a rubber diaphragm that separates two regions of the bottle. Imagine that you increase the energy on one side of the diaphragm. We won't concern ourselves with this process. You can imagine that in that part of the container there is a blender blade. You pulse the "blender" and the billiard balls go whizzing around in the first section. Now you have one section with billiard balls moving extremely fast and in the other section the balls are moving slowly. What will happen? The high-energy billiard balls will tend to pass energy to the low energy side through collisions with the diaphragm. This exchange of energy is heat. Energy flows from the fast to the slow. Eventually both sections will have their billiard balls moving with the same average energy. Thermal equilibrium has been reached.
Probably the most difficult aspect of describing heat and temperature is that thermal processes involve large numbers of particles, which are treated using statistics. Thermodynamics can account for how systems with large numbers of particles change under the laws of physics by incorporating statistics. Averages, for example, are used to describe properties of a system.
Review this material in your text book:
D U = Q - W
The increase in internal energy of an object is equal to the heat added minus the work performed. Changes in the internal energy of an object are reflected either in a change of temperature or a change of state (solid to liquid).
When no work is done and there are no phase changes.
D U = Q = m1 C1 D T.
C is the heat capacity per gram per degree Celsius. The heat transferred just changes the internal energy and the amount of that change is related to temperature by the equation above. (m is mass.)
If there are no temperature changes and no work done.
D U = Q = m1 L1.
Where L is the heat of fusion (solid to liquid) or vaporization (liquid to gas). These are called latent heats. The heat energy is used to change the phase of the object. The process where you heat water so that I boils (liquid to gas) is an example.
In this lab we will isolate our system so that no heat (ideally) will enter or leave. We begin with two objects that are not in thermal equilibrium. We put them together. They transfer heat until they reach thermal equilibrium. By measuring the temperature changes, phase changes, and masses involved we can measure C and L.
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This weeks quiz.
Read the lab in the manual and be sure you glance at any materials that are
available for this week's lab on the web.