Electronic DC Load #3 - What is the function of the dummy load resistor?

Описание: An explanation of how the circuit works for the dummy load and some tweaks to increase the constant current...and is that load resistor required?
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Link to Kibi's "Dave Jones' dummy load with added features" :
http://mjlorton.com/forum/index.php?t...

Dave Jones / EEVblog video:
   • EEVblog #102 - DIY Constant Current Dummy ...  

WHAT IS AN OP-AMP?:
http://talkingelectronics.com/project...

My components:

Infineon MOSFET N-channel 200V 13.5A TO-220 - BUZ31L H
Vishay 534 Series Pot with 6.34mm shaft, 50K
2 x 10 k Ohm resistors
Arcol HS50 Al house wirewound high power resistor,1R (1 ohm) 50W - HS50 1R J
National Semiconductor Quad op amp,LM324N 1MHz DIP14

How does an N Channel logic level MOSFET work - gate voltage, variable resistance, heat dissipation.
How does a LM324N Op Amp work - voltage follower, comparator, rail to rail, head room, feedback loop.
What is the heat / power dissipation from a variable resistor?