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AP · AP C:E&M · multiple choice

AP Physics C: Electricity and Magnetism · Question 2

AP Physics C: Electricity and Magnetism · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
demanding
Marks
1
Topics
1
Answer
Complete
A charged parallel-plate capacitor filled with a dielectric, battery disconnectedparallel-plate capacitordielectric κ+Q−Qbatteryswitch open — battery disconnected
Fig. 2.1Two long horizontal plates, one directly above the other, form a parallel-plate capacitor; the upper plate is marked +Q at its right-hand end and the lower plate is marked -Q. A lightly shaded slab labelled "dielectric" with the constant k fills the whole space between the plates. A wire leaves the left-hand end of the upper plate, runs left and then down, passes through a battery and then an open switch along the bottom of the figure, and rises again to meet the underside of the lower plate. The switch is drawn open and labelled to show that the battery has been disconnected.
multiple choice1 mark

A parallel-plate capacitor is charged by a battery and the battery is then disconnected. A dielectric slab of dielectric constant κ > 1 is inserted, filling the space between the plates. What happens to the charge on the plates and to the potential difference across them?

  1. A

    charge unchanged, potential difference decreases

  2. B

    charge unchanged, potential difference increases

  3. C

    charge increases, potential difference unchanged

  4. D

    charge decreases, potential difference unchanged

Ready to self-mark?Reveal the detailed answer guide
Answer overviewCorrect answer: A — charge unchanged, potential difference decreases
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is A: charge unchanged, potential difference decreases

  2. 2

    C is the correct answer to the same question with the battery still connected, and telling the two cases apart is the entire item. With the battery removed the charge has nowhere to go, so Q is fixed; C rises by κ, and V = Q/C therefore falls.

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