Question

Explain the following statement giving reason :
When a dielectric is placed in an external electric field, the electric field inside the dielectric is less than that outside it.

Hint:

The factor by which the external electric field is reduced inside the dielectric is exactly equal to the dielectric constant (relative permittivity) $K$ of the material, mathematically expressed as $E_{net} = E_{ext} / K$.

Answer 1

Concept:
• A dielectric is fundamentally an insulating material containing bound charges that cannot move freely, but can shift slightly from their equilibrium positions.
• When exposed to an external field, these bound charges undergo physical polarization.
• Polarization creates an opposing internal electric field that strictly alters the net field present inside the material.

Step 1: Analyze the effect of the external field
When a non-conducting dielectric slab is actively placed inside a uniform external electric field $\vec{E}_{ext}$, the external field strongly exerts forces on the microscopic molecules of the dielectric.
This forcefully stretches or actively rotates the molecules, causing their positive and negative charge centers to physically separate slightly.
This phenomenon is known scientifically as dielectric polarization.

Step 2: Identify the creation of the induced field
Due to this precise polarization, a net positive surface charge density securely accumulates on one face of the dielectric, while a net negative surface charge density builds up on the opposite face.
These newly induced surface charges naturally produce their own internal electric field, known strictly as the polarization field, denoted as $\vec{E}_p$.

Step 3: Determine the net internal electric field
By fundamental laws of electrostatics, this induced polarization field $\vec{E}_p$ points exactly in the opposite direction to the original applied external field $\vec{E}_{ext}$.
According to the principle of superposition, the net electric field $\vec{E}_{net}$ physically existing inside the dielectric material is the vector sum of both opposing fields:
\[ \vec{E}_{net} = \vec{E}_{ext} - \vec{E}_p \]

Step 4: Conclusion
Because the induced field $\vec{E}_p$ aggressively subtracts from the applied field, the overall magnitude of the net internal electric field is significantly reduced.
Therefore, the electric field inside a polarized dielectric is always strictly less than the external electric field existing outside it.