Question

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

Answer 1

If this is homework do it yourself!

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 6

why don't draw a diagram to explain the situation in a better way..?

Answer 7

I was going to, why though? is my explanation not good enough?

Answer 8

...

yes it is but how would you determine the direction of induced emf otherwise..