A 5microF capacitor is charged to 30V and is then connected in series with a 10microH inductor and............

Question

...a 50Ohm resistor. What is the current after a long time has passed?

The answer is 0, But i don't know why. Please help, thanks.

Answer 1

You are correct!  Check out this applet to see what is happening.

http://www.coilgun.info/mark2/rlcsim.htm

You will have to click on "show capacitor voltage".  Then hit the reset to see the voltage waveform.

You have a "damped oscillator".  A simple explanation follows.

The capacitor acts as a source.  As the electric field of the capacitor is discharged a current flows.  This current flow creates a magnetic field in the inductor.  Energy is being transferred from the electric field to the magnetic field.

When the capacitor is completely discharged, there is nothing to maintain the magnetic field of the inductor, so the magnetic field collapses, which creates a voltage, which creates a current.  The current recharges the capacitor.  Energy is being transferred from the magnetic field to the electric field.

Now, if there was no resistor, this transfer of energy would go on forever.  But there is a resistor in the circuit.  

In fact, all inductors are made of wire and wire has resistance, so the oscillations would die out in any real circuit with a charged capacitor and an inductor.

Now, each cycle the resistor wastes some of the energy until the current is 0.  So you have a damped oscillator.

Now, the actual values you have may be problematic.  50Ω seems large.  If you plug your values into the applet, you may have to decrease the resistance to see oscillations.

Hope this helps...