A2L Item 132

• Description: Identify the correct relationship for total energy when the distance between charges changes upon release.
• Goal: Link energy conservation and electromagnetism
• Source: 283-421 Change of total energy
• Keywords: Conservation, Electromagnetism, Energy

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The question for students:

A uniform volume distribution of charge has radius R and total charge Q. A point charge -q is released from rest at point b, which is a distance 3R from the center of the distribution. When the point charge reaches a, which of the following is true regarding the total energy, E?

1. E_a = -E_b
2. E_a = -2E_b/3
3. E_a = -3E_b/2
4. E_a = -9E_b/4
5. E_a = E_b
6. E_a = 2E_b/3
7. E_a = 3E_b/2
8. E_a = 9E_b/4
9. None of the above
10. Cannot be determined

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Commentary for teachers:

Answer

(5) Students often forget to include the kinetic energy, especially after a lot of discussion of potential energy. Many will simply misinterpret the energy to mean potential energy. Teasing apart these issues is important.

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