Student handout: Electric Field of Two Charged Plates

  • Students need to understand that the surface represents the electric potential in the center of a parallel plate capacitor. Try doing the activity Electric Potential of Two Charged Plates before this activity.
  • Students should know that
    1. objects with like charge repel and opposite charge attract,
    2. object tend to move toward lower energy configurations
    3. The potential energy of a charged particle is related to its charge: \(U=qV\)
    4. The force on a charged particle is related to its charge: \(\vec{F}=q\vec{E}\)
What students learn
  • Potential and potential energy are different. The value of potential is independent of the sign of charge of the test particle.
  • Force and energy are both ways to understand how charged objects interact.
  • Review that electric field and electric potential are related to force and potential energy.
  • Electric field vectors are perpendicular to equipotential surfaces and are short if the curves are closely spaced.

Before you is a plastic surface and a contour map each representing the electric potential. A \(1\)-\(cm\) height difference corresponds to an electric potential difference of \(1~V\).

Consider the Motion of a Positive Charge: If you were to place a positively charged particle at rest at the blue square, which way do you expect the particle to move?

  • What direction is the force on the charged particle?
  • Does the charged particle move toward higher or lower electric potential?

  • Does the electric potential energy increase, decrease, or stay the same?

Consider the Motion of a Negative Charge: If you were to place a negatively charged particle at rest at the blue square, which way do you expect the negative charged particle to move?

  • What direction is the force on the charged particle?
  • Does the charged particle move toward higher or lower electric potential?
  • Does the electric potential energy of the system increase, decrease, or stay the same?

Consider the Electric Field at the Blue Square: Draw a vector on the contour map to indicate \(\vec{E}\) at the blue square.

  • Explain your reasoning.

  • Does your answer depend on the sign of the charge?

  • How is the vector oriented with respect to the contour lines?

Consider the Electric Field at Several Points: Draw vectors at several additional points to represent \(\vec{E}\), making sure the lengths of the vectors are qualitatively accurate. Choose points near the middle and edges of the map.

  • How do the electric field vectors near the middle compare with the vectors near the edge of the map?

  • How are the electric field vectors related to the equipotential lines?


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Learning Outcomes