# Could Someone Please Clarify A Brief Textbook Technical Paragraph For Me?

#### positive8

Joined Sep 27, 2016
61
Given this statement:
"I can write the equation for the power absorbed by the inductor as the product of the voltage across it and the current flowing through it. Then, since the current i is changing as the magnetic field builds up, I simply integrate all these different values of i from switch-on to the steady state current I = V/R to get the total power. The energy needed to supply this power has been borrowed from the circuit and is now stored in the inductor's magnetic field for as long as the field persists."
and the equation:
P=(L I^2)/2

By "L" do they mean the static inductance value you can calculate for an unconnected coil based on number of turns, diameter, etc.?
And by "I=V/R" do they mean the the Vrms of the peak voltage generated by the inductor "discharging" dividing by the purely resistive value that would be measured by an ohmmeter across the coil?

This is P as is watts, I'm assuming, due to the di/dt differential, but if the magnetic field of the inductor collapses in, say, 20 ms, then is total power generated the P value x 0.02s, rather than the P value derived from the above equation?

I just want to be absolutely sure on all this.

#### ErnieM

Joined Apr 24, 2011
8,199
Given this statement:
"I can write the equation for the power absorbed by the inductor as the product of the voltage across it and the current flowing through it. Then, since the current i is changing as the magnetic field builds up, I simply integrate all these different values of i from switch-on to the steady state current I = V/R to get the total power. The energy needed to supply this power has been borrowed from the circuit and is now stored in the inductor's magnetic field for as long as the field persists."
and the equation:
P=(L I^2)/2
OK, though this is the very first time I have seen energy denoted by the symbol P.

P is usually reserved for power. E is more typical for energy.

By "L" do they mean the static inductance value you can calculate for an unconnected coil based on number of turns, diameter, etc.?
Yes.

And by "I=V/R" do they mean the the Vrms of the peak voltage generated by the inductor "discharging" dividing by the purely resistive value that would be measured by an ohmmeter across the coil?
Nope, where did you get that from? That is discussing a resistor.

This is P as is watts, I'm assuming, due to the di/dt differential, but if the magnetic field of the inductor collapses in, say, 20 ms, then is total power generated the P value x 0.02s, rather than the P value derived from the above equation?

Nope, rubbish. Energy. ENERGY. In Joules.

Power is in watts, and a watt is one joule per second.

• positive8