Finding Electric Field intensity

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Homework Help Overview

The discussion revolves around a problem in electromagnetism, specifically focusing on the electric field intensity affecting a proton's motion along the x-axis. The original poster presents a scenario where a proton is decelerated by a uniform electric field, providing initial and final speeds at specific positions.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to calculate the electric field intensity using work-energy principles and seeks verification of their answer. Participants engage in discussions about the direction of the electric field and the force acting on the proton, questioning the relationship between the proton's velocity and the electric field direction.

Discussion Status

Participants have provided feedback on the correctness of the calculated electric field magnitude. There is an ongoing exploration of the directionality of forces and fields, with some guidance offered regarding the relationship between the proton's motion and the electric field direction. The discussion reflects a productive exchange of ideas without reaching a definitive conclusion.

Contextual Notes

Participants are navigating the implications of coordinate system definitions and the relationship between force and electric field direction, indicating a need for clarity on these concepts without resolving the underlying assumptions.

pat666
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Homework Statement


A proton traveling along the x-axis is slowed by a uniform electric field E. At x = 20 cm, the proton has a speed of 3.5 x 106 ms-1 and at x = 80 cm, its speed is zero. Find the magnitude and direction of electric field intensity.

Can someone please! check my procedure and if your really nice/helpful my answer-----thanks.


Homework Equations


W=Fx
W=Eqx
W=ΔK_E
ΔK_E=1/2 mv^2-1/2 mu^2



The Attempt at a Solution


1st i found the change in energy (work) to slow the proton to a stop. then that's equal to Eqx. I found E to be 1.06*10^5 N/C (positive direction) please check my answer if you have time - its worth marks.
 
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The magnitude of the electric field is correct. In what direction should the force on the proton be and why?
 
thanks kuruman - I thought positive (is that what's meant by direction??) because they need to be "like" to repel??
 
I am talking about the force on the proton. If it moves left to right (x is increasing), in what direction should the force be to stop it?
 
towards the origin - - but left or right would be dependent on the way the cc were defined? +x could be towards the left or right couldn't it?
 
pat666 said:
towards the origin - - but left or right would be dependent on the way the cc were defined? +x could be towards the left or right couldn't it?
Yes it could, so let me rephrase my question. Relative to the direction of the proton's velocity, in what direction should the force on it point? Same or opposite direction? The answer to this question has nothing to do with which way is positive and which way is negative.
 
opposite to slow it down??
 
Yes. Now how is the direction of the force on a positively charged particle related to the direction of the electric field? Same or opposite?
 
same- the direction of an electric field is the way a positively charged test particle would go
 
  • #10
So put it together. Is the direction of the electric field the same as or opposite to the direction of the proton's velocity?
 
  • #11
so the full answer is 1.06*10^5 N/C in the opposite direction of the protons velocity.
 
  • #12
That is correct.
 
  • #13
thanks kuruman!
 

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