B-Fields: Deriving Faraday's Law & Why No Work?

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In summary, the conversation discusses the confusion surrounding the concept of work in relation to B-fields. Work is mathematically defined as the path integral of a force, but it is also restricted to conservative fields. B-fields do not do work on point charges because the Lorentz force is always perpendicular to the charge's velocity. Additionally, the closed line integral of B is zero, as B is a conservative field with zero divergence. This is tied to Faraday's law and the concept of emf.
  • #1
ChanceLiterature
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This statement has always confused me. Now my confusion is coming home to roast while we cover EMF.

If we consider work mathematically as ∫f.dl and require integral to be path independent then of course the B-field does no work.

However, it seems like there is a deeper meaning to B-fields do no work. Is there?

Tied into this is faradays law. Faraday's Law can be "derived" from emf (it's in quotes because I understand faradays is first principles). In this derivation emf force is stated to be closed integral ∫E.dl. I do not understand why this why emf or this induced force around the loop is necessarily due to an E-field as opposed to to B-field.
 
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For some clarification I think you are saying that B-field do no work to charges. B-field do work to magnetic dipoles as we see two magnets attract and collide.
ChanceLiterature said:
In this derivation emf force is stated to be closed integral ∫E.dl.
it equals to
[tex]-\int_A\frac{\partial \mathbf{B}}{\partial t} \cdot d\mathbf{A}[/tex]
using B.
 
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anuttarasammyak said:
For some clarification I think you are saying that B-field do no work to charges. B-field do work to magnetic dipoles as we see two magnets attract and collide.

it equals to
[tex]-\int_A\frac{\partial \mathbf{B}}{\partial t} \cdot d\mathbf{A}[/tex]
using B.
Yes, I'm trying to highlight two points of confusion. You're right I was not clear enough. I hope this helps.

1) I am asking why B-fields do no work on a point charge when the path integral of a charge on a B-field is non-zero? Did we define work as path integral only over conservative fields? Is this why we say B-fields do no work?

2) I am not sure why the closed line integral of B is necessarily zero (if I still had my copy of Boas' Mathematics in the physical sciences, I might know why/if this holds). Additionally, if it is not necessarily zero, I am not sure why the resulting the emf is defined as closed E.dl.

My confusion with Faraday's law in the original question derives from not understanding emf I suspect. Thus, I think ironing out point of confusion 2 will help
 
  • #4
A familiar reply to 1) is to see the formula of Lorentz force the part originated from B of which is
[tex]q \mathbf{v} \times \mathbf{B}[/tex]
and is always perpendicular to velocity of charge q point, ##\mathbf{v}##, thus no work done on the charge.
 
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Thanks. I realized 1 was very dumb after remembering what conservative and close integral meant.

Additionally, even I'm not sure what I'm asking or saying for 2
 
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Thanks for the help
 
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ChanceLiterature said:
2) I am not sure why the closed line integral of B is necessarily zero
Zero we see as for B is
[tex]\nabla \cdot \mathbf{B} =0[/tex]
and thus
[tex]\int_S \mathbf{B} \cdot d\mathbf{S}=0[/tex]
for a closed surface S. Not line integral.
 

FAQ: B-Fields: Deriving Faraday's Law & Why No Work?

1. What is a B-field?

A B-field, also known as a magnetic field, is a region in space where a magnetic force can be detected. It is created by moving electric charges and can exert a force on other moving charges.

2. How is Faraday's Law derived from B-fields?

Faraday's Law states that a changing magnetic field can induce an electric field. This can be derived from B-fields using the principle of electromagnetic induction, which states that a changing magnetic field can create an electric field.

3. Why is no work done by a B-field?

A B-field does not do work because it is a conservative force. This means that its energy is conserved and does not change as an object moves through it. Therefore, no work is done by the B-field on the object.

4. How are B-fields measured?

B-fields can be measured using a device called a magnetometer. This device can detect the strength and direction of a magnetic field and can be used to map out the shape and strength of a B-field in a given region.

5. What are some real-world applications of B-fields?

B-fields have many practical applications, such as in electric motors, generators, and MRI machines. They are also used in compasses for navigation and in particle accelerators for scientific research. B-fields are also important in understanding the behavior of the Earth's magnetic field and its impact on our planet.

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