P.3.7.4.2 Parallel plate capacitor

An air-filled parallel-plate capacitor is charged from a source of emf. The electric field has a strength E between the plates. The capacitor is disconnected from the source of emf and the separation between the isolated plates is doubled.

What is the final electric field between the plates?

• 2E

• E

• E/2

• E/4

A parallel-plate capacitor has square plates of length l separated by distance d and is filled with a dielectric. A second capacitor has square plates of length 2l separated by distance 2d and has air as its dielectric. Both capacitors have the same capacitance.

What is the relative permittivity of the dielectric in the first capacitor?

• 1/2

• 1

• 2

• 6

A parallel-plate capacitor is fully charged and then disconnected from the power supply. A dielectric is then inserted between the plates. Which row correctly identifies the charge on the plates and the electric field strength between the plates?

• Charge: stays the same

  Electric field strength: increases

• Charge: increases

  Electric field strength: decreases

• Charge: increases

  Electric field strength: increases

• Charge: stays the same

  Electric field strength: decreases

A capacitor consists of two parallel square plates of side l separated by distance d.The capacitance of the arrangement is C.

What is the capacitance of a capacitor with square plates of side 2l separated by a distance d/2 ?

• C

• 2C

• 4C

• 8C

The separation of the plates of an isolated charged parallel−plate capacitor is increased.

What also increases?

• the capacitance of the capacitor

• the charge on the plates

• the strength of the electric field between the plates

• the pd between the plates

Which of the following statements about a parallel plate capacitor is incorrect?

• The capacitance of the capacitor is the amount of charge stored by the capacitor when the pd across the plates is 1 V.

• A uniform electric field exists between the plates of the capacitor.

• The charge stored on the capacitor is inversely proportional to the pd across the plates.

• The energy stored when the capacitor is fully charged is proportional to the square of the pd across the plates.