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Given the setting,

What is the potential difference between point $Q, ( {q ~\rm cm} , {0~\rm cm} )$ and point $P, ( {p~\rm cm} , {0~\rm cm})$? Let $q = 17.2~\rm cm$ and $p = 2~\rm cm$.

When solving this problem, I know we have to find $E=\frac{\sigma}{2\epsilon_0}$ for each infinite plate. However, I am not sure about the direction of each infinite plate. The solution indicates that the $σ_1$ points right and $σ_2$ point left. How do we know that?

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The solution indicates that the σ1 points right and σ2 point left. How do we know that?

The charge on each plate is positive. The electric field due to a positive charge points away from that charge.

Therefore the field due to each plate points to the left on the left side of the plate, and to the right on the right side of the plate.

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  • $\begingroup$ I see. If they were negative, σ1 would have pointed left and σ2 would have pointed right? $\endgroup$
    – Dawn17
    Commented May 3, 2018 at 3:30
  • $\begingroup$ Yes, a negative charge attracts a positive charge. And we assume a positive test charge when defining the electric field. $\endgroup$
    – The Photon
    Commented May 3, 2018 at 3:35

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