What is the time constant in an LC circuit.

[OmniNewton]

Homework Statement

I wrote an exam today and a question worth quite a bit of points asked Determine the value of a capacitor in a series circuit that will give a time constant of 4.0 ms and is isn series with an inductance of 3mH

Homework Equations

The Attempt at a Solution

I do not believe there is a time constant since the circuit is in natural response so therefore this voids the problem

 

[robphy]

Could it be asking for the period of oscillation?

 

[OmniNewton]

Sorry let me update the description exactly, The question was: Determine the value of a capacitor in a series circuit that will give a time constant of 4.0 ms and is in series with an inductance of 3mH. I thought about the period of oscillation but it particularly said time constant. They are very strict and won't let you ask anything.
My thoughts were it was a trick question

 

[NascentOxygen]

I think it would be true to say that any expression comprising circuit constants and which has units of time (e.g., seconds) can be termed a time constant.

The reciprocal of seconds is written s^–1 and can be pronounced "Hertz".

 

[rude man]

The question is misleading and confusing. A time constant is associated with an exponential rise or decay, which is not the case here. A period is not a time constant in electrical parlance.

 

[LvW]

rude man - yes, in general I do support your view .
However, in this case, I am inclined to say that the ENVELOPE of the decaying sinusoidal step response (exponential envelope function) could be the answer (most probably)

 

[NascentOxygen]

I am inclined to say that the ENVELOPE of the decaying sinusoidal step response (exponential envelope function) could be the answer (most probably)

Doesn't that sound too easy, given that the circuit presented is lossless? Especially when ...

a question worth quite a bit of points asked Determine the value of a capacitor in a series circuit that will give a time constant of 4.0 ms and is isn series with an inductance of 3mH

In exams, be guided by the number of marks awarded.

No examiner gives marks for a candidate stamping his feet and complaining that a question is not well-phrased. The examiner has a particular answer in mind, and marks are awarded according to how well candidates manage to divine his answer from the clues he casts about.

 

[OmniNewton]

So the exam was returned and the answer was just the period of this circuit. However since the question can perhaps create confusion everyone was given full credit on the problem.