Concepts Of Physics MCQ Edition [Volume 2]
- Exam
- Concepts Of Physics MCQ Edition [Volume 2]
- Subject
- Physics
- Chapter
- Capacitors
Concepts Of Physics MCQ Edition [Volume 2] · Physics · Capacitors
A parallel-plate capacitor with a plate area of 400 cm^2 and a...
A parallel-plate capacitor with a plate area of and a separation of is connected to a power supply. A thick dielectric slab with a dielectric constant of is inserted into the gap, and later, after disconnecting the power supply, the slab is removed. In both steps, the electrostatic energy increases. Which of the following best explains why the energy increases during both the insertion and the removal of the slab?
Options
- ADuring insertion, the battery does work to supply additional charge; during removal, an external agent does work against the attractive electrostatic forces.
- BDuring insertion, the dielectric does work on the battery; during removal, the battery does work on the external agent.
- CDuring insertion, the capacitance decreases; during removal, the capacitance increases.
- DDuring insertion, charge is lost to the dielectric; during removal, charge is regained from the dielectric.
Correct answer
A. During insertion, the battery does work to supply additional charge; during removal, an external agent does work against the attractive electrostatic forces.
Step-by-step solution
When the dielectric slab is inserted with the battery connected, the capacitance increases. To maintain the constant voltage, the battery supplies additional charge, performing work. The work done by the battery () is twice the increase in the stored electrostatic energy, allowing the energy to increase while the slab is naturally pulled in.
When the slab is removed with the battery disconnected, the charge on the plates remains constant. The induced bound charges on the dielectric surface are attracted to the charges on the capacitor plates. An external agent must exert a force to overcome this electrostatic attraction and pull the slab out. The work done by the external agent adds to the system's stored electrostatic energy, causing it to increase.
When the slab is removed with the battery disconnected, the charge on the plates remains constant. The induced bound charges on the dielectric surface are attracted to the charges on the capacitor plates. An external agent must exert a force to overcome this electrostatic attraction and pull the slab out. The work done by the external agent adds to the system's stored electrostatic energy, causing it to increase.