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: \(\vec{F}=q\vec{E}\)
  • group Small Group Activity schedule 30 min. build Yellow plastic surface (1 per group), Contour map for parallel plate capacitor with plastic sleeve (1 per group), Big whiteboard (1 per group), Dry-erase markers and erasers (1 each per student), Student handout (1 per student) description Student handout (PDF)
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