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Please help: A coil of insulated wire is wound many times around an iron bar. The wire is connected to a battery and a contact switch. The contact switch is alternately flipped closed and left closed, so the wire carries a current through it, and then flipped open and left open, so that the current stops. Describe what magnetic fields are created, if those magnetic fields in turn induce an emf, and whether the induced emf is in the direction of the original current or opposite to it. Justify your answer using Lenz's law and the conditions which can cause a magnetic field to induce an emf. Y

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Help on homework is what OpenStudy is for, isn't it? @awesomegal22053 Anyways, sorry, I don't know how to help but I'll keep thinking about it.
As the current in the coil (inductor) grows, it will produce a changing magnetic field in the core (and outside the core too). This is a statement of Ampere's Law. While there is a changing magnetic field, there will be eddy currents produced in the core, which because of Lenz's Law will be in the opposite direction to the original current. Lenz's Law is basically the negative sign in front of Faraday's Law. Faraday's Law is what makes the nonconservative electric field in the core in the presence of the changing magnetic field, thereby inducing the current. You can picture this by remembering the changing magnetic flux through the core will produce a current whose own magnetic flux will act to oppose the original changing flux. Its like inertia when driving your car. When you are at a constant speed you feel no force but when your speed changes, your body tries to stay at the speed it was going before. When you try to change the flux nature tries to oppose that change with an opposite flux. Sorry for the long answer but that concept is at the heart of all of electromagnetics and makes our modern day world possible.

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I did not explicitly state it but the condition for a magnetic field to induce an EMF is that it is changing in time. A static field will not work. Thus Faraday's and Amperes Laws are time derivatives. Changing magnetic fields make changing electric fields. Changing electric fields make changing magnetic fields. That is how an electromagnetic wave propagates.
I just found out an answer myself, can you guys tell me if it could work for answering this question? "At each turn of the wire around the iron bar, a magnetic field is created when and if a current is passed through it. Since this system is connected to a battery, this system will induce an emf. Since the induced emf's direction tends to oppose the change producing it, the direction of the induced emf will be opposite the original current." @egenriether
why don't draw a diagram to explain the situation in a better way..?
I was going to, why though? is my explanation not good enough?
|dw:1337635729420:dw|... yes it is but how would you determine the direction of induced emf otherwise..
Induced emf's direction would be opposite the current flow like i stated in my answer.
the basic thing is you will have induced emf only while opening & closing the switch...
so induced emf only happens when the switch is left open?
and it stops when the switch is closed?
when we are closing a switch current is increasing instantaneously.. & when we are opening the switch current is decreasing instantaneously.. so a momentary production of induced emf will happen..
How do you know induced emf is only created between the time the switch is being open and closed?
yes it is.. once we have closed the switch current becomes constant in wire..
oh so induced emf is produced when there is a change in current, and that change in the current happens when the switch is opened at closed? so thats why induced emf only happens between the time the switch is opened then closed.
yes we can apply lenz's law to determine the direction of induced emf produced..
when the current is increasing the induced emf will create a magnetic polarity to reduce this effect... & also when the current is decreasing..
oh because it doesnt like change?
once we have a diagram facts will become clear.. say like if current is flowing in such a way that it would produce a north pole in right hand side so the induced emf will produce a south pole in left hand side if the current to minimize the effect if the current is increasing..
yes.. exactly it doesn't like change.. when there is no change like when current is constant no induced emf no lenz's law..
okay thanks so much I totally understand now.
Do you think you could help me with two more problems? It might take time so you don't have to
& if the current is decreasing.. (when we are opening switch).. the north pole produced due to flowing current is still in right hand side because the direction of current is constant.. but now the current is dying and hence the north pole so induced emf will produce a north pole in rhs to support it..
can i help u later .. bcoz it is 4AM morning where i am i need to sleep.. you message me a link and i will try it tomorrow,.
okay ill wait. thank you
Child-of-the-sea, you have the right idea. Realize that as we said the magnetic field must be changing. It is not a requirement that the switch be opening or closing, only that the current continues to change. If the inductance of the coil is large enough, it will take more time to cause saturation. This means the current will build and build until all of the magnetic domains in the core are aligned. At that point the field cannot continue to change and the induced emf stops. Since most coils have a small inductance, it appears that with DC current it quickly saturates and the emf occurs only at the closing and opening of the switch, but no physical law says it has to be then only. Given any sized inductor the emf could last as long as you'd like.

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