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Why is initial current zero in an RL ciruit with an emf, but it is not in a charging RC circuit?
Thanks for your answer.It starts from zero in an RC circuit as well, but in a very small time ##\Delta t## (this time depends on the self inductance of the circuit which is small, and on the dimensions of the circuit/speed of light-practically this time ##\Delta t## is zero, theoretically is not zero) it climbs up to a value ##\frac{E}{R}## and then it starts exponential decay according to what is well known for an RC circuit. Most books omit this phase of the time ##\Delta t## because it is very small, practically zero as I said.
What you saying is almost correct, It is just that even if we attach no mass to a spring, the mass of the spring itself will prevent it of changing length instantly. However if we also assume that the spring is massless, then there is no way to prevent the mini apparent paradox that the spring will change length instantly.You might like to think about a mechanical analogy. For instance, a spring (with zero mass) is like a capacitor; if we apply a force, it changes length instantly and stores the supplied energy as elastic PE. On the other hand, a mass is like an inductor; when we apply a force, it accelerate slowly (F=MA) and stores the applied energy as kinetic energy.
The mechanical laws are an analogue of the electrical ones. For instance, for an inductor Energy =LI^2/2 whereas for a mass Energy=Mv^2/2.