# Why is current induced into a solenoid

#### bongas

Hi all Physics (masters)

I was reading a paper on the faraday's induction experiments; i didn't quite understood the reason why is current induced into a solenoid.

There is this solonoid and a Permanant magent is pushed inside the solonoid and we observe that a current flows (by a galvanometer or a compass). Can any one please explain the reason of induced magnetism?

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#### bongas

This is my first thread i don't think that it leaves a very good impression of the PF as no one seems to answer my query.

#### cabraham

Induction is an observed phenomenon. Nobody knows why it happens, but the behavior is consistent enough so that Faraday & Ampere published their equations describing it. Maxwell added a needed term to Ampere's law, then Lorentz published his force law.

Searching under the following key words will produce good info.
Ampere's law.
Lorentz force law.
Maxwell's equations.

Does this help?

Claude

#### stevenb

Hi all Physics (masters)

I was reading a paper on the faraday's induction experiments; i didn't quite understood the reason why is current induced into a solenoid.

There is this solonoid and a Permanant magent is pushed inside the solonoid and we observe that a current flows (by a galvanometer or a compass). Can any one please explain the reason of induced magnetism?
Keep in mind that you addressed physics "masters". I was thinking of trying to answer the question, but I don't consider myself a physics master. This question can quickly spiral toward something that does require a real master.

In line with that, it's not clear what point of view you want this question answered from. Are you trying to just get a physical understanding and a statement of the law (Faraday's Law), or are you trying to get a deep meaning answer, such as something relating to fundamental symmetries, or maybe you are asking one of those unanswerable questions like "why are the laws of physics what they are and not something different?".

Can you clarify the question and also give a rough idea of what level you are at in studies of math, physics and electromagnetism?

#### bongas

i asked my teacher about his ...."why does elecrtomagnetic induction take place"
His answer: "Whenever magnetic field lines are cut a current is induced into the wire placed in a magnetic field."
His Answer:"According to the explanation by Faraday, this happens when there is a disturbance in the magnetic field"

Now, based on his answers i am more and more curious with the fact that why does this happen?
Come on , the above answers are in now way an explanation but rather a little information on Induction.
What i need is the real reason why is current "Induced" in the wire ... how does a disturbance in the wire causes a current to flow?

#### cragar

I don't know if this is what your looking for , The B field is pushing on the electrons so the electrons start to move in the wire . each electron has its own magnetic dipole.

#### cabraham

This is like asking why do 2 charges exert a mutual force, or 2 masses due to gravity. Forces due to e/m & gravity are observed to happen, but the underlying reason is not known. Time changing magnetic fields & electric fields cannot exist independently. THe H field acts normal to a charge carrier's velocity, while the E field acts tangential. Lorentz' force equation details this, I suggest you refer to it. Faraday & Ampere's laws detail the mathematical relation between magnetic flux, current, voltage, & freqency.

Claude

#### stevenb

Come on , the above answers are in now way an explanation but rather a little information on Induction.
What i need is the real reason why is current "Induced" in the wire ... how does a disturbance in the wire causes a current to flow?
Questions of "why" are not that easy to answer in physics. The scientific process is one of doing experiments and uncovering simple laws, so that complex things can be understood. We often are successful in explaining why complex processes happen in terms of the fundamental laws. So, answering why a complex process happens is sometimes possible if we are intelligent enough to figure out how to use fundamental laws to calculate and predict the behavior of complex systems. Still, we end up scratching our heads if we try to explain why the fundamental laws are true. Some laws do seem obvious from symmetry principles if you study deeply, but a full explanation has never been possible. In a very real sense, science is more often a "description" of nature, rather than an "expanation" of nature. However, even that is not perfectly accurate since most people feel they have deeper understandings of nature after years of studying physics, which is the one field of science that tries to dig into the most fundamental laws, more than any other field.

In this case, you are asking a question of "why is current induced in a wire?" and "how does a disturbance in the wire cause current to flow?". These questions are basically asking why a fundamental law (called Faraday's Law) is true, which is very hard to answer. It is possible to delve into a deeper understanding of Faraday's Law by looking at symmetries, special relativity and the known fact that electric and magnetic fields are not separate entities, but intimately related as components of one electromagnetic field tensor. In a sense, it comes down to thinking about what a simple electric field from a charge looks like to a moving observer (looks like a magnetic field from a current) and following this through its implications for electric/magnetic fields and inclusion of the principles of relativity (speed of light is constant and physical laws have the same form, as determined in all inertial frames).

In a nutshell, you are asking why a fundamental law called Faraday's Law is true. From one point of view, you can just accept that as an unexplainable fact of nature. From another point of view, you can develop a deeper understanding if you study (which takes considerable time) the various ideas above (and much more).

You can also go a little deeper if you are willing to accept as facts the definition of Lorentz force (and the implied definition of electric field and magnetic field), the tenents of special relativity (particularly Lorentz invariance), the conservation of charge and the conservation of magnetic flux. These fundamental principles can be used to show that Faraday's Law must be true. However, it is equally valid to accept Faraday's Law as a fundamental law in itself, and then say that some of the the other principles are a consequence of that. This kind of axiomatic approach can get very tricky, and is not all that helpful for intuitive understandings. Also, as soon as you think you've got the axioms and structure right, a theoretician will tell you it's the wrong way.

Anyway, I doubt any of the above helps greatly, but maybe it will inspire you to study over the long term. You didn't answer my queston as to what level you are at in terms of math and science, but I get the impression you were talking to your high school teacher. Hence, I've attached a simple description of Faraday's Law (it's not perfect or complete, but sufficient for a general introduction) appropriate for that level.

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#### gau_physics

"EMF is induced when magnetic flux(B.A) through a loop changes"

Now why does this occur? Initially Faraday observed it and could not explain it.
There are 2 types of EMFs induced - 1) motional EMF 2)EMF due to time varying magnetic field.

for the first case "HALL EFFECT" related to "Lorentz force" is said to be somewhat an explanation when magnetic field is constant but conductor moves.

In the second case since electrons aren't initially in motion therefore no Lorentz force then what makes an electron move ? I think therefore scientists brought the concept on INDUCED ELECTRIC FIELD which is non conservative. Thus time varying magnetic field induce a non conventional electric field which is quite different from electric field due to static charges.

Hope this will help a bit :)

#### gau_physics

now you may ask "Why Lorentz force occurs?" or "Why this particular equation of Lorentz force ?" . . . This was an observed phenomenon and that no magnetic force when charged particle moves parallel to magnetic field.

#### bongas

I think i get it...there is no explanation for this topic.

Although i think that gau_physics gave somewhat convincing response but again (for the 1st type of EMF) he could not explain the why lorentz force occurs?

Can any one please tell me in summary, what Lorentz force is ? OR a link to a nice article of my level...(my level is high school)

#### jtbell

Mentor
Lorentz force is simply another name for the elecromagnetic force on a charged particle, the sum of the electric and magnetic forces:

$$\vec F = q ( \vec E + \vec v \times \vec B )$$

#### A Dhingra

hi there....
so it seems there is no clear reason as to why is current induced ?...
but thanks anyway...

"Why is current induced into a solenoid"

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