GoForSmoke:
You don't want to put much current through the Arduino so the Arduino controls the FET gate only. The FET is a switch that allows lots more current (not all FETs, check the datasheet, 2N7777 TTL-level FET is 250mA max, always check before buy) to flow through and THAT should not come through the Arduino board but straight from a power supply.The Arduino regulator will get very hot if you draw too much current through it. It takes current flow to make heat, you can spot weld sheet metal with amps of 5V.
Will the heating pad take 6V or 4.5V? That's 4 or 3 AAA's. Could you use mains power to demonstrate operation? You can get 5V supplies in 60W or higher, that's 5V x 12A = 60W but you don't want to push maximums --- if you need 12A continuous, get 20A max and the switching PS will last for years. Enough batteries should work but will run down.
Sorry for the late reply, I have been doing research. So i can use 4 triple A batteries and then just a resistor at the end to reduce it to 5V. Furthermore, is there no way I can use the mosfet I have right now?