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 Aug 3rd 2011, 12:49 AM #1 Member   Join Date: Jul 2010 Posts: 70 Generator question I'm having trouble understanding one aspect of q6 https://sites.google.com/site/xtheunknown0/physics-1 If the light blows, why does the current in the primary decrease? Can you explain it without inductance and back EMF? xtheunknown0
Aug 5th 2011, 02:13 PM   #2
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Join Date: Dec 2009
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It has to

 Originally Posted by xtheunknown0 I'm having trouble understanding one aspect of q6 https://sites.google.com/site/xtheunknown0/physics-1 If the light blows, why does the current in the primary decrease? Can you explain it without inductance and back EMF? xtheunknown0
You can think of the bulb in the secondary as something that is allowing the conversion of potential energy to kinetic energy. The potential energy comes from the primary and is delivered to the secondary. With the bulb not blown some of that PE is converted into KE moving the little electrons around through the bulb doing work.

Work done is energy taken. So the primary trying to keep the PE in the secondary the same has to source a little more current into that side to deliver the additional energy that is being used by the bulb.

When the bulb blows that side acts as an open and no current can flow. So no energy will be converted into KE. Therefore the primary side does not need to deliver as much energy to the secondary side to keep that side at some PE value. Therefore a little less current needs to flow within the primary side.

Isolation transformers like this deliver electrical energy from one place to another without using any wires. They do so completely through the energy that is contained within the magnetic fields that flow within the core.

I have a very good book written by some brilliant EE who uses a full form of Maxwell's equations and describes magnetism like this as a magnetic current. He can show mathematically that the magnetic current when the bulb is not blown is inducing an electrical current that is doing work within the bulb. Therefore power is being used by the secondary which comes from the primary. When the secondary no longer draws any power from the primary, the primary need not "work" as hard.

Does that help?
Craig