Comparing DC to AC: Is A Always Correct?

[hidemi]

Homework Statement

An inductor produces a back emf in a DC series RL circuit when a switch connecting the battery to the circuit is closed. we can explain this by,

A) Lenz's law
B) increasing magnetic flux within the coils of the inductor
C) increasing current in the coils of the inductor
D) all of the above
E) only (A) and (C) above.

The answer is D.

Relevant Equations

Maxwell equations:
(See better interpretations at "Attempt at a Solution" section)

https://www.physicsforums.com/attachments/282071
https://www.physicsforums.com/attachments/282072

If the question changes DC to AC, the correct answer would be just A right?
The reason why (B) and (C) are wrong is that the current keeps changing direction and doesn't remain constant throughout for the coil to store energy to eventually hit the maximum. Any other better explanations?

 

[onatirec]

It's true that the magnetic flux and current aren't monotonically increasing in an AC inductor, but they lag in phase and so continue to "oppose" the circuit

 

[hidemi]

It's true that the magnetic flux and current aren't monotonically increasing in an AC inductor, but they lag in phase and so continue to "oppose" the circuit

I thought the current lags the voltage by π/4 phase? I do agree that it oppose the circuit clockwise/counterclockwise interchangeably

 

[berkeman]

Homework Statement:: An inductor produces a back emf in a DC series RL circuit when a switch connecting the battery to the circuit is closed.

Closed? Did you mean to say "opened"?

 

[hidemi]

Closed? Did you mean to say "opened"?

The book says 'closed'

 

[onatirec]

The book says 'closed'

When the DC circuit is initially closed and current starts flowing - this makes sense.

I meant they (as in the inductors) lag in phase. You are right; it is the current that lags.

 

[hutchphd]

Well technically it produces a "back" emf to any change in current (meaning the emf tries to oppose the change). Notice that in an LR configuration the change in the current is proportional to the existing current...this makes "off" a more exiting event

 

[onatirec]

My point was that the problem statement is physically consistent and there's no reason to think the switch should be 'opened' instead of 'closed'

Notice that in an LR configuration the change in the current is proportional to the existing current

Maybe I'm misunderstanding, but I don't agree with this as written. For one, when the circuit goes from open -> closed, the existing current is 0A. How could the change be proportional to 0?

 

[hutchphd]

You are correct and I was being sloppy.