Current leading voltage or vice versa concept

[member 392791]

Hello,

I was wondering if there is a conceptual explanation for when current leads voltage or vice versa for capacitors or inductors with AC voltages, or is it just the way the math pans out?

 

[tiny-tim]

Hi Woopydalan!

In a resistor, voltage is proportional to current, so there's no lead or follow.

In an inductor voltage is proportional to rate of change of current, so when the current is at phase zero, the voltage must be maximum, at phase 90°, ie leading.

In a capacitor, voltage is proportional to charge, so rate of change of voltage is proportional to current, so it's the other way round, ie current leading.

 

[technician]

This is a difficult idea to show in the early stages of learning AC theory. When I teach this I use a battery with an ammeter and connect this to an inductor. When the switch is closed it is clear that it takes some time for the current to rise to its steady value after the voltage is connected. In some way it seems reasonable to state that 'the current is behind the voltage'.
Do the same with a capacitor and a battery and when the switch is closed the current is at a maximum and you have to wait for the voltage to rise to its steady value. So in some way the voltage is 'behind' the current.
Not rigorous...but I find it a useful, non mathematical aid. ( some of my physics students do not do maths)

 

[mikeph]

I'm not sure what specific example you're referring to, but a voltage is a potential difference, and voltage per unit length of wire is a measure of the electric field strength, which is what causes the electrons to move and overcome resistance in the wires. For that reason I always think of voltage as the cause and current as the effect.

 

[sardar]

Thank you for your question. The concept of current leading voltage or vice versa in capacitors and inductors with AC voltages is a result of the underlying physics and is not simply a mathematical coincidence. It is rooted in the concept of reactance, which is the opposition to current flow in a circuit caused by the presence of capacitance or inductance.

In capacitors, the voltage leads the current because as the voltage across the capacitor increases, the electric field is established between the plates, which causes a displacement current to flow. This displacement current leads the actual current flowing through the circuit. Similarly, in inductors, the current leads the voltage because as the current through the inductor changes, it creates a changing magnetic field which induces a voltage to oppose the change in current. This induced voltage leads the current flowing through the circuit.

So, it is not just a mathematical relationship, but rather a fundamental concept in electromagnetism that explains the phenomenon of current leading voltage or vice versa in capacitors and inductors with AC voltages. I hope this helps clarify the concept for you.