A dielectric slab, battery on or off
"A dielectric slab, battery on or off" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Effect of dielectric on capacitance" in Electrostatic Potential and Capacitance. A charged parallel-plate capacitor with the battery kept on or removed while a slab of chosen K and thickness slides in, tracking Q, V, E, C and U. NEET has asked about this idea 3 times in 10 years of papers. JEE Main asked 48 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Effect of dielectric on capacitance in Electrostatic Potential and Capacitance. Change the values and watch what happens.
What it shows
NCERT §2.13
A charged parallel-plate capacitor with the battery kept on or removed while a slab of chosen K and thickness slides in, tracking Q, V, E, C and U.
Controls
4 to try
- Togglebattery connected / disconnected
- Sliderslab insertion 0–100%
- Sliderslab thickness 0–d
- SliderK 1–10
Try this
Disconnect the battery, insert a K = 4 slab fully and show U falls to one quarter while Q is fixed; then reconnect and show U rises four-fold instead.
Common mistake
Inserting a dielectric with the battery removed and saying the charge rises by K.
In NEET
10 years of papers
Asked 3 times in 10 years of NEET.
See the past questions: Capacitance, Electrostatic Potential and Capacitance (chapter)
In JEE Main
42 shifts analysed
48 questions from Electrostatic Potential and Capacitance across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §2.13
- With vacuum between the plates, E₀ = σ/ε₀, V₀ = E₀d and C₀ = ε₀A/d.
- A dielectric filling the gap is polarised; its bound surface charges ±σₚ cut the effective charge density to σ − σₚ, so E = (σ − σₚ)/ε₀.
- For a linear dielectric σ − σₚ = σ/K, where K > 1 is the dielectric constant. The field and V both fall by the factor K for the same charge.
- Hence C = Kε₀A/d = KC₀. The dielectric constant is the factor by which capacitance grows when the dielectric fills the space fully.
- Permittivity of the medium ε = ε₀K; K = ε/ε₀ is dimensionless. For vacuum K = 1.
- C = KC₀ holds for a capacitor of any shape, so it can serve as the definition of K.
- Slab partly filling the gap: a slab of constant K and thickness 3d/4 (full plate area) makes V = E₀d(K + 3)/(4K) for the same charge, so C = 4KC₀/(K + 3).
Questions
5 answers
- What does the "A dielectric slab, battery on or off" simulation show?
- A charged parallel-plate capacitor with the battery kept on or removed while a slab of chosen K and thickness slides in, tracking Q, V, E, C and U.
- What mistake does the "A dielectric slab, battery on or off" simulation fix?
- A common mistake it clears up: inserting a dielectric with the battery removed and saying the charge rises by K.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 3 times in 10 years of papers.
- Does JEE Main ask questions from Electrostatic Potential and Capacitance?
- Yes. JEE Main asked 48 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Effect of dielectric on capacitance" in Electrostatic Potential and Capacitance (Class 12 Physics).
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