A2L Item 217

• Description: Indicate the changes in stored charge, potential difference and stored energy in an isolated capacitor when a dielectric is inserted between the plates.
• Goal: Reason regarding capacitors and dielectrics.
• Source: 283-535 inserting a dielectric changes a capacitor
• Keywords: Capacitors, Dielectric, Electromagnetism, Energy, Voltage

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The question for students:

A capacitor with capacitance C is connected to a battery until charged, then disconnected from the battery. A dielectric having constant κ is inserted in the capacitor. What changes occur in the charge, potential and stored energy of the capacitor after the dielectric is inserted?

1. V stays same, Q increases, U increases
2. V stays same, Q decreases, U stays same
3. V increases, Q decreases, U increases
4. V decreases, Q stays same, U decreases
5. None of the above
6. Cannot be determined

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Commentary for teachers:

Answer

(4) It should be clear to students that the charge cannot change. Most students recognize that capacitance increases when a dielectric is inserted into a capacitor. The issue then becomes whether they appreciate the relationships between C, Q, V and U.

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