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How it Works »New Instructable »Cookies n' Ice Cream

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Consider the following koan, to help prepare your mind:

Well, the answer is that the amount of power dissipated by a resistor is I

^{2}*R, and if you have a bunch of resistors all in series, the largest R dissipates the most power.Ah... but what if you make one of the Rs infinite (same as an "open" circuit)? What happens then? Then the amount of power dissipated by all the resistors becomes zero, because the current is zero.

So you have to make the R of your heating element large, larger than the other resistances in the circuit, but not too large... Not too small either, or your R then it might just burn itself up.

The resistance of a length of wire...is proportional to its length, inversely proportional to the cross-section area, and proportional to this other number called resitivity. The equation looks like this:

Nichrome has larger resistivity ρ than copper.

Divide both sides by unit l, and you get resistance per unit length. Just like you'd expect, thin wire (small A) has large resistance per unit length.

So that's, roughly, how you calculate the resistance of a given length of wire. Often there is also a correction for temperature too. The resistance of metals usually increases somewhat with increasing temperature.

These equations, along with Ohm's Law and Kirchhoff's rules, will indeed be helpful, if you want to take a wire and make it hot,

andkeep everything else cool at the same time.Without these equations, you'll need a lot of trial and error.

Re-design's suggestion is the closest thing to recipe among these answers, since he gives you some numbers that sound believable,

(12V)

^{2}/(20 Ω )= 7.2 W,Plus, following this recipe, you don't have to go to the trouble of designing a resistor, You can just buy one. In fact if you're totally new to the subject of Ohm's law and circuits, and math, etc, then RD's answer might be a good place to start.

If the wire isn't thin, you'll need a heavy power supply to get it warm...

L

Then start very low and see what temperature you get. Don't go over 12 volts. Both the resistor and wire will get hot so be careful.