# Heating a 7 cm long copper wire of cross sectional diameter .75mm

1. Feb 1, 2012

### dr.ankit

hi guys,
m a lil stuck here
what voltage would be rquired to heat a 7cm long copper wire of .75mm diameter in about 2-3 seconds.the target temp is 400-450c.

dr ankit

2. Feb 2, 2012

### Bobbywhy

Can you please describe the entire project that uses this 7 cm wire? What is the resistance of that copper wire? What kind of conductors do you plan to connect to that length of copper wire to carry the current? What power supply do you use? How will you measure the target temperature?

3. Feb 2, 2012

### dr.ankit

power supply can be ac/dc whatever...can we generate that high temp using dc current?i did some research.the resistance will be 11.76/10^8ohms.still trying to find out how to measure the temperature

Last edited: Feb 2, 2012
4. Feb 2, 2012

### DarioC

If you are trying to make a heating element I would suggest you use something other than copper. Nichrome is pretty common for that purpose.

If it has to be that short, therefore low resistance, you will need to use something like a step down transformer for high current, low voltage. Is there any reason you cannot use a transformer hooked to the AC mains? That would be the easiest.

If you connected it via a variac you could adjust the temperature to exactly whatever you need. Probably easier to do cut and try than to calculate the temperature.

DC

Additional thought: a bunch of "D" cells hooked in parallel might work. Car battery would be way to much voltage.

If this is just a paper problem on a test or whatever...never mind all of the above.

Last edited: Feb 2, 2012
5. Feb 2, 2012

### sophiecentaur

If you really do have to use such a low resistance then you may be better to use a current source rather than a voltage source. Contact resistances would not then be the same problem.

6. Feb 3, 2012

### sophiecentaur

If you use a highish supply voltage and a series resistor that is much higher than the load then the current can be defined much better because the % variation of total resistance due to contact is less.
This is at the expense of wasting power but will give the current you want.
The same thing can be done using switch mode solid state.

7. Feb 3, 2012

### gsal

While you are pushing current through the copper wire, for as long as you know the current going though it and for as long as you can measure the voltage drop across the wire, you can determine its temperature, you just need to know the resistance of the wire before you heat it up, like at room temperature...read up on "temperature coefficient of resistance"...actually, I think I may have posted about it some time ago...search.

8. Feb 3, 2012

### sophiecentaur

Actually, this is a hard problem, if you want an accurate solution. The power dissipation needs to be perhaps a Watt or so and, from experience of burned fingers, I'd say that would need around 5A but the temperature reached would depend a lot on the surroundings and heat loss.
I think the best solution would be to use AC, from a very few turns of thick wire on a transformer with only a few volts on its primary - i.e. you may need two step down transformers if the turns per volt on your mains transformer are too high - making the voltage steps too coarse. You would need a really thick conductor, bonded very well to your wire or the contact resistance will dominate.

Hang on , Is this just a foam cutter that you're planning to make??? The nichrome wire from a broken toaster in a skip would cost nothing.

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