Electric field lines

  • #1
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Homework Statement:
The top plate is negatively charged and the bottom earthed.

which of the diagrams is correct?
Relevant Equations:
none
Shouldn't the plate that is earthed be with no charge? making B correct.
Untitled6.png


but the accepted answer was A.
 

Answers and Replies

  • #2
Orodruin
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Shouldn't the plate that is earthed be with no charge?
No, grounded means zero potential, not zero charge.
 
  • #3
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No, grounded means zero potential, not zero charge.
so, even if it's zero potential, it's possible to have +ve charges?
shouldn't the plate with +ve charges be at a higher potential
 
  • #4
Orodruin
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so, even if it's zero potential, it's possible to have +ve charges?
Yes, that is perfectly possible. You can also have negative charges. It depends on what charges exist in the surroundings. What characterises ground is zero potential.

shouldn't the plate with +ve charges be at a higher potential
Higher relative to what? Let me give you a different example where there is no ground:
Consider two spherical concentric shells with the same magnitude charge. The outer shell is positively charged and the inner is negatively charged. Since the overall charge of the system is zero, there is no electric field outside of the outer charge and therefore its potential is zero (assuming potential at infinity is set to zero) although it is positively charged.

Yes, the grounded plate will have a higher potential than the negatively charged one, but this tells you nothing about the actual charges on the plate.
 
  • #5
The plate which is grounded should have zero charge on its left face so charge on outer face of 2 plate also have zero charge, so negative charge of 2 plate will appear on inner face and equal and opposite charge induce on inner surface of grounded plate then electric field will move from positively charged face to negatively charged face
 
  • #6
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then can we say that the field lines are from high potential to low potential instead of from +ve to -ve?
 
  • #7
Orodruin
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Field lines point in the direction of the electric field, which is given by the negative of the gradient of the potential. So, by definition, field lines point from high potential to low potential.
 
  • #8
robphy
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B and D can't be correct since the field starts or stops at the bottom plate without a surface charge density [technical point: "boundary condition of the perpendicular component of E"].
Field lines start at positive charges and terminate at negative charges.
Without an equal magnitude positive surface charge density, the field should continue below the lower plane.
 

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