A2L Item 205

• Description: Identify the field inside a charge-free cavity lying inside a conductor.
• Goal: Hone the concept of a conductor
• Source: 283-230 E in the cavity of a neutral conductor
• Keywords: Electromagnetism, Electric Field

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The question for students:

A positive charge +Q is placed outside a neutral conductor. Inside the conductor is a cavity containing no charge. What is the electric field at the “center” of the cavity, a distance D from the charge +Q?

1. kQ/D², direction away from the charge Q
2. kQ/D², direction toward the charge Q
3. kQ/D², direction away from the center of the conductor
4. kQ/D², direction toward the center of the conductor
5. E = 0
6. Cannot determine the electric field

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Commentary for teachers:

Answer

(5) Induced charges and resulting fields pose problems for most students. Having students compare this case to one where the charge is inside the cavity helps. They tend to get hung up on the problem of envisioning the exact distribution of induced charge that will cancel the field everywhere inside the conductor.

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