# Voltage across inductor in AC

http://physics.bu.edu/~duffy/PY106/ACcircuits.html

The voltage across inductor is V=IXL but from the graph this doesn't seem to be true so what does it show ? Same for Capacitors

• Why do you think that these graphs are false? – G36 Mar 1 '18 at 14:50
• @G36 when voltage is maximum then current is zero which kinda violates this formulae in my opinion – Scáthach Mar 1 '18 at 15:38
• In a steady state for a sinusoidal extortion, the current in the inductor is lagging the voltage by 90 degrees. So if we ignore the phase shift we can find the "magnitude" of a current using this equation IL = VL/XL. And remember that the current is lagging the voltage by 90 degrees. electronics.stackexchange.com/questions/288380/… – G36 Mar 1 '18 at 15:55
• @G36 so we can't use this formulae for instantaneous values of current and voltage,right?Only for RMS values values – Scáthach Mar 1 '18 at 21:22
• No, you can use this equation for instantaneous values also. v(t) = Vpeak/XL * sin (ωt) and for current i(t) = Vpeak/XL * sin (ωt - 90 degrees) – G36 Mar 2 '18 at 15:08

The voltage across an inductor is V = $L\dfrac{di}{dt}$ so if the steady state voltage is a sinewave then the current, when differentiated is a sine wave. This means the current is an inverted cosine wave as shown in the picture.
• $V = L\dfrac{di}{dt}$ allows you to calculate instantaneous values. It's the base formula for the relationship between V and I in an inductor and from this is derived (via simplification) the RMS formula you have used. – Andy aka Mar 1 '18 at 15:51