Thursday, November 21, 2013

Layered Learning

One common problem with teaching and learning HVACR is that there is just so much material to master. It is easy to drown the students in details if you are not careful. When I was using a text which presents enthalpy diagrams in its early chapters on basic refrigeration, students would come up to me and ask for drop slips after reading the chapter. They looked at the PH diagrams and concluded that they just were not smart enough for HVACR. It was just way too much information for them to comprehend while they were still wrestling with refrigerant boiling while it is cold! The real skill in presenting HVACR subject matter is to take inherently complex material and break it down into a series of comprehensible lessons, without leaving out material. Presenting information in stages, at a rate which most students can more easily digest is preferable to feeding them the whole enchilada at once and causing cognitive indigestion. You can still reach the peak, but you have to get to base camp first.

Electricity is a perfect example. We start with the basic concepts of voltage, current, resistance, and simple circuits. If you think of a simple balance beam with voltage in the middle – if resistance goes up, current goes down. Students can in fact understand how current, voltage and resistance are related without using any formulas. Similarly, you don’t have to bring in Ohm’s Law to understand the concepts of source, path, and load. In fact, I believe circuits are best approached at first without discussing Ohm’s Law. Students can build circuits and operate them to get a mental concept of how switches  and loads behave. They can even build series and parallel circuits to see how they differ: all without formulas. Then AFTER the students understand the basic concept of an electrical circuit, you introduce Ohm’s Law as a mathematical description of a circuit. Now they have something real to relate to the parts of the formula and it is less abstract. You can even build circuits with heaters and take measurements to demonstrate how Ohms Law works.

After students have learned to solve Ohm’s Law problems, then we tell them that Ohm’s Law does not work in most AC circuits. At first, we don’t discuss the effects of alternating current, inductive, and capacitive reactance. It is just too much to take in at once. However, since these concepts have far more to do with most air conditioning circuits than Ohm’s Law, we can’t really afford to ignore them. The effect of inductive reactance can be demonstrated by operating a small motor in a circuit and comparing its resistance, current, and voltage. You can have students build circuits that demonstrate the effect of inductive and capacitive reactance and how they relate to each other. Depending on how much AC electricity you teach, you can put an oscilloscope on the circuits to show what happens in capacitive and inductive circuits (the teacher would be doing this.) Finally, you can bring in reactance and impedance calculations, although I must admit we do not have our Air Conditioning classes at Athens Tech doing LCR calculations. It’s a little like boiling a frog – you do it a little at a time. Before the students know it, they have learned some complex subject matter that would have sent them running for the exits if it had been presented all at once.

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