Electric Circuits – What Happens in an AC Circuit When the Power Delivered to the Inductor is Negative?

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The instantaneous power delivered to a resistor by an alternating voltage source $v(t)=V_0\sin\omega t$, is always nonnegative. But the instantaneous power delivered to an inductor by $v(t)=V_0\sin\omega t$, is positive for $0<\omega t<\pi/2$ and negative for $\pi/2<\omega t<\pi$.

During the first quarter of the cycle ($0<\omega t<\pi/2$), both $v_L(t)$ and $i(t)$ have the same signs, and the power delivered to the inductor is positive. This energy is stored in the inductor as magnetic energy. In the next quarter of the cycle ($\pi/2<\omega t<\pi$), $v_L(t)$ and $i(t)$ have the opposite signs, the power delivered to the inductor is negative! I do not understand this part of the cycle in terms of what the circuit is doing.

During this interval, is the magnetic energy stored in the inductor negative? What would that mean?

During this interval, the inductor delivers power to the source? How does an inductor deliver power to the source?

Best Answer

During this interval, the inductor delivers power to the source? How does an inductor deliver power to the source?

Yes, by doing work against the electrostatic field of the voltage source.

An inductor can accept work of the voltage source and store it as magnetic energy at one time interval (while the current magnitude increases and goes in the direction of potential drop), and at other time interval, it can release this magnetic energy by doing work on mobile charges running through it (current magnitude decreases and goes in opposite direction to the potential drop).

When an inductor releases energy via work back to the source, this work is done by induced electric forces ( due to the induced electric field of the inductor) acting on the mobile charge carriers in direction of their motion. Since these charge carriers' kinetic energy remains negligible all the time, all this work is spent to increase electric potential energy of all the charges. In other words, the induced electric field forces deliver positive work against the forces of conservative electric field, and this work is stored (usually) as increased electrostatic energy of those charges in the source and on surfaces of all circuit elements, including the inductor. In other words, magnetic energy turns into electric energy.