Register to reply 
Apply Kirchoff's rules with a nonohmic resistor? 
Share this thread: 
#1
Jan3009, 05:15 AM

P: 22

I'm trying to work out the correct algorithm for using Kirchoff's rules when using a nonohmic resistor like a light bulb. The problem is that the resistance depends on the voltage, but the voltage may depend on the resistance (a simple example: a bulb and a resistor in series: how should the voltage be divided?)
I'm sure there must be an algorithm for this, but at the moment all I can think of is to use some kind of relaxation method like simulated annealing to home in on the correct answer. Thanks Hob 


#2
Jan3009, 08:11 AM

P: 22

Just following up on this, two further questions (slightly edited, because rereading it I realised I wasn't clear):
1) Does anyone have a link to some actual data on the resistance variation in an incandescent bulb? (and indeed in a diode, which I'm also going to have to do) I can't find it anywhere  just vague lines and handwaving. 2) Given that resistance of a bulb, and therefore power, is variable, is there a reasonably simple relationship between that stated power and voltage of a bulb and the resistance variation? My guess is that a lower wattage bulb gets less hot and therefore its resistance is less variable, but I'm working a bit blind here. None of this has to be perfect, but it does need to match up to the picture that kids are taught in school. Thanks 


#3
Feb109, 04:23 AM

P: 4

If you can measure the current through the series connection of the resistor and bulb, you can determine the voltage across the bulb with ohm's law and Kirchoff's current law. Can't help you with the voltage/current relationship for the bulb though :/



#4
Feb109, 01:38 PM

Mentor
P: 17,271

Apply Kirchoff's rules with a nonohmic resistor?



#5
Feb309, 03:33 AM

P: 22

A root *is* an optimum, isn't it? (it just depends on your utility function). But yes, any kind of numerical solver will do the job. The reason I'm thinking of using some kind of iterative method is that I suspect it's pretty close to reality: the voltage is applied, it creates a current, the current makes the filament hot and increases the resistance, this decreases the current, and so on until equilibrium is reached. But I'm nervous because of course in an arbitrary circuit this could get a bit nasty. And it's all academic until I can get a decent power/current/resistance relationship for these damn bulbs



#6
Feb409, 04:14 PM

Mentor
P: 17,271




#7
Feb409, 05:45 PM

P: 22




Register to reply 
Related Discussions  
Kirchoff's Rules  Introductory Physics Homework  5  
Kirchoff's rules  Introductory Physics Homework  1  
Kirchoff's Rules  Introductory Physics Homework  1  
Kirchoff's Rules  Introductory Physics Homework  1  
Kirchoff's Rules  Introductory Physics Homework  5 