Dielectrics play an important role in electronic devices. We can use what we learnt about screening effect to understand how various materials with different dielectric constants respond to external electric fields.
By applying an electric field across an atom we can push negatively charged electrons and positively charged atom nuclei in opposite directions making it polarized. In larger atoms electrons are more loosely held because of which polarizability generally increases with atom size. Molecules are also polarizable, and like atoms, their polarizability varies significantly. Use the slider below to see how atoms with different polarizability respond to electric field.
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We never deal with individual atoms or molecules. Let's see what happens if we have an array of atoms. Can you use what you learnt about screening effect in Chapters 16 and 17 to explain the electric field profiles you see on the right based on the net charge inside the array?
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In practice, even a thin layer of any material is formed by countless number of atoms or molecules. While all these atoms or molecules get polarized by an applied electric field, the net charge is going to appear only very near the surfaces of the material. Check how positive and negative charges get shifted by an applied electric field and how net charges appear on both sides of the thin film.
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Can you say which of the two materials have a higher dielectric constant? Remember that you can use Q=CV and V=πΈβπ to obtain the electric field inside the material as
πΈβ=(π/A)/(ΙrΙ0)
where π is charge on the electrodes, A is the area of the capacitor, and Ι0 is the permittivity of free space.
Let's look at the values of charge and electric field for the cases of SiO2 (ππ=3.9) and Si (ππ=11.9) with a film thickness of d=10nm.
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Charge Density (Β΅C/cm
2
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Can you calculate the charge densities at the surfaces of the materials? (Hint: You can find what charge densities will give you the same electric field values when there is no material between the two electrodes and then use what you learned about the screening effect to find the charges on the surfaces).
Letβs see what happens if we have two materials sandwiched between the two electrodes. See how net charges appear at the interface of the two dielectric materials. Can you again explain the changes in the electric field based on the screening effect?
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Add charge on the electrodes to create electric field. Hover your mouse over the graph to see changes in the field strength.
Charge Density (Β΅C/cm
2
):
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The net charges that appear at the surfaces of the dielectrics are distributed over very thin layers and we often assume their thicknesses are infinitesimal. This way, the changes in electric field at the interfaces would be abrupt as you see in this scene. One can also show that the electric displacement D=rE remains continuous wherever there is a change in dielectric constant.
Important Note:
In our calculations, we almost never need to account for the net charges that appear at the surfaces of dielectrics because the impact of those charges on electric field is accounted for by the relative permittivity of dielectrics. Hence, we need to only account for the free charges on the electrodes in relationships like Q=CV or when using Gauss's law.
Less Polarizable Material
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Positive Charge
Negative Charge
Add charge on the electrodes to create electric field. Hover your mouse over the graph to see changes in the field strength.
Charge Density (Β΅C/cm
2
):
Toggle visible charge: