# Electric Force on a Charge in a Solenoid

• Helly123
In summary, the conversation discusses the direction of the induced EMF in relation to a changing magnetic flux and the force experienced by a positive charge. The key word "increasing" refers to the Faraday EMF and the changing magnetic flux. The EMF is the integral of an induced electric field and the positive charge experiences a force based on this electric field and its velocity in a magnetic field.

## Homework Statement

http://[url=https://ibb.co/dgUy6T]https://preview.ibb.co/iyqS0o/20180525_213806.jpg
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Since i only know the field direction, increasing go into page. Why the answer is C?

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The R and r on the pic is respected to what?

[h2]Homework Equations[/h2]

[h2]The Attempt at a Solution[/h2]
Right hand rule : finger tip = current
Thumbs = force
Palm = B

Or
Finger tip = velocity
Thumbs = current
Palm = B

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The key word here in the statement of the problem is "increasing" and they should say "increasing with time". The question involves the Faraday EMF that occurs. The changing magnetic flux creates an EMF. They basically are asking the direction of the induced ## E ## of the EMF.

Helly123
Could it be like this?

Force direction the same as Emf's ?

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That is correct. The EMF is the integral of an induced electric field along the path. The electric field ## \vec{E}=\vec{E}_{induced} ## is what the positive charge responds to, and experiences a force ## \vec{F}=q(\vec{E}+\vec{v} \times \vec{B}) ##. ## \\ ## (And of course, when they say "placed at point b", etc. that means ## \vec{v}=0 ## so that ## \vec{v} \times \vec{B}=0 ## ).

Helly123