Why am I asked to find the frequancy at which an LR circuit is resonant? Every formula that I saw always included a negative reactance for a capacitor in series.

 

There is no resonance without C. There shoul be something missing in the question. LR can be used as a filter at the most.

 

Thanks, I would assume it's a mistake in my assignment.. :/

 

Resonance can only occur in a second order system. The series RL circuit is a first order system.

 

As already mentioned no resonance, but the question may be asking,
"at what frequency, does (XL = R), such as used in filter branches.

 

Maybe the question refers to parasitic capacitance which resonates with the inductance.

 

Maybe the question refers to parasitic capacitance which resonates with the inductance.

if we were to consider stray capacitance then it will be an LC and still not an LR resonance.

 

A series LR circuit is an idealization, without capacitance, and thus incapable of resonance. Now it is true that in a real circuit as you increase the applied frequency you reach a point where there is a small capacitance between the individual windings of the inductor whose reactance becomes significant. Now it is no longer an RL circuit and we can get resonance.

 

Even then it is a LC resonace and not a RL resonance.

 

Even then it is a LC resonace and not a RL resonance.

Of course. That is what we have been trying to say. There is no such thing as either an RL resonance or an RC resonance. You need BOTH an L and a C for rsonance to occur.