Is it hard to change the current across a solenoid, or coil of wire?

Question

Here's my thinking:

Suppose we establish a current across a solenoid by placing a voltage between the two ends of the solenoid. We know that a magnetic field is generated inside the solenoid, and the magnetic field within the solenoid is roughly uniform and normal to the cross-sectional area within the solenoid. Suppose we increase the voltage across the solenoid, which, will increase the current. However, this increasing current increases the magnetic field through the solenoid, which generates a changing magnetic flux through the solenoid. From induction, we know that the increasing magnetic flux through the solenoid induces a current through the solenoid that is opposite the original current (I suppose, if I decreased the voltage, then the induced current is parallel to the original current.) Notice that a voltage is also induced across the solenoid that opposes the increase in voltage. Thus, in the end, the current across the solenoid changes very little, if at all, due to the induced voltage and current, even though we physically attempted to change it by changing an external voltage.

Is this correct? Why does this seem so confusing? What happens after the current is induced in the solenoid. What is the eventual current through the solenoid? How does this apply to inductors?

Thanks!

Answer 1

your correct there, in a Straight wire circuit a voltage change has a virtually instantaneous effect on the current in the circuit, but with an inductor, it produces a back-emf (emf being the same thing as voltage) in the opposite direction of the change in current( and in this case change in voltage) so instead of the current quickly droping from I1 to I2, it porduces two voltages in opposite directions that oppose each other, thus slowing the change in current. same thing happens when current is increased.



but if one were to simply remove the battery from the circuit, the back emf would eventully run out of power, so the current would drop to zero eventuly even with the back-emf. there is an exponentialy decay equation that governs this effect. it is on the inductor wikipedia page

Answer 2

This is a tricky question because a solenoid doesn't have to be connected to a power source but it remains a solenoid even though it's not carrying a current. Generally, I wouldn't call it a solenoid unless the coil is intended to pull in a metal object. But the basic description is correct.

Answer 3

There are 2 factors, voltage and time!

If you just apply a voltage, that means DC, the current will be that of the wire resistance.

If you increase the voltage it will behave like a resistor, the current is proportional to the voltage.

While you change-it then it is not proportional

So when you apply a constantly varying voltage like AC, then it is not linear





Hope this properly answers your question





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