# What explains the current flow in a LC circuit?

• Est120
Est120
TL;DR Summary
Explain why current flow keeps changing directions in a LC circuit. Use potential difference arguments.
I'm just not able to grasp the concept of a LC circuit intuitively, and yet I have found zero answers to my doubts.

I can't understand why does the current keeps flowing counter-clockwise between the 3rd and the 4th circuit (see image attached)
I know that when the capacitor has 0 charge, in that instant, "i" has a maximum value (3rd image).
The key to understand my doubt is just exactly between 3rd and 4th image. In the PRECISE moment the capacitor has 0 charge and i is maximum there is NO voltage across them, BUT there should be some sort of voltage across the inductor (because the magnetic flux changed across time, then there should be a non conservative electric field that would create a magnetic flux opposing the change) the emf across the inductor is always opposing the current flow, so How can i (current flow) keep flowing in the same direction, even though there is a emf acting in the opposite direction? Please, just use voltage arguments, i don't care about the spring-mass system analogy.

Last edited by a moderator:
The charge on the capacitor drives the current through the inductor until the charge has been transferred to the other side of the capacitor.
Est120 said:
How can i (current flow) keep flowing in the same direction, even though there is a emf acting in the opposite direction?

The inductor's back emf resists the initial surge from the capacitor but the charge on the capacitor continues to flow building up to a maximum while the resisting back emf is reduced. At maximum current which is now not changing the capacitor, the capacitor begins to discharge reducing the present current until the current goes to zero and the capacitor is again charged as it was initially.
That is, at max current, the current is no longer being resisted by the back emf so the capacitor will now discharge in the opposite direction. The inductor resists this change initially strongly only causing a small decrease in the maximum current but as before the resisting back emf now in the opposite direction slows the current to zero.

Is that clear?

A finite energy is circulating between the L and the C.
EL= ½⋅L⋅i2
EC= ½⋅C⋅v2

Have you studied calculus yet?

a) the capacitor discharges thru the inductor
b) the inductor builds up a magnetic field due to the current
c) as the capacitor discharges to Zero volts, the magnetic field collapses
d) as the field collapses it induces a voltage in the inductor
e) the polarity of the induced voltage is the reverse of the original capacitor voltage
f) this reverse voltage then charges the capacitor in the opposite polarity

The above 'reversing-polarity' process continues until the circuit losses dissipate all the stored energy as heat.

The overall process is referred to as 'Ringing' in a circuit and occurs at the circuits resonant frequency. The amplitude decay is referred to as "Damping".

Hope this helps!

Cheers,
Tom

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