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Conservation of Charge
 Charges of an Atom
 Polarization of Atoms
 Force of a Point Charge on an Atom
 Source and Reference

Conservation of Charge

The net charge of a system and its surroundings cannot change. But charge can move from the systtem to surroundings or from surroundings to the system. However, charges can be created or destroyed in (+,−) pairs. For examples, electron-positron annihilation.

Charges of an Atom

image Charge of electron cloud equals that of nucleus implies neutral atom. If the electron cloud is centered on the nucleus implies electric field produced by electrons exactly cancels the field produced by nucleus.

Polarization of Atoms

image By placing a point charge close to an atom, point charge exerts electric field on the atom. Atom becomes polarized by electric field. An induced dipole can be represented as follow image A more easily polarized atom has a larger induced dipole moment, which produces a larger electric field. And a more easily polarized atom experiences a greater attraction to a point charge. Polarization is defined as 𝑝=𝛼𝐸 where 𝛼 is polarizability of a material
In an induced dipole, the distance between the charges is proportional to the strength of the applied field.

Force of a Point Charge on an Atom

image A point charge first creates electric field 𝐸0 at atom, 𝐸0=14𝜋𝜀0𝑞0𝑟2𝑟 Then the electric field 𝐸0 polarizes atom image and the polarization is 𝑝=𝛼𝐸0=14𝜋𝜀0𝛼𝑞0𝑟2𝑟 The induced dipole in turn creates field 𝐸𝑎 at point image The electric field is 𝐸𝑎=14𝜋𝜀02𝑝𝑟3=14𝜋𝜀02𝛼𝐸0𝑟3=14𝜋𝜀02𝛼𝑟314𝜋𝜀0𝑞0𝑟2𝑟=14𝜋𝜀022𝛼𝑞0𝑟5𝑟 ~1/𝑟5 The electric field 𝐸𝑎 due to the induced dipole exerts force on point charge image The force is 𝐹0=𝑞0𝐸𝑎=14𝜋𝜀022𝛼𝑞20𝑟5𝑟 By reciprocity, the force 𝐹𝑎 that the point charge exerts on atom will be equal and opposite to the force 𝐹0 image The force is 𝐹𝑎=−𝐹0=−𝑞0𝐸𝑎=14𝜋𝜀022𝛼𝑞20𝑟5𝑟 Neutral atoms are attracted by charges with interaction strength ~1/𝑟5

Source and Reference


ID: 191101002 Last Updated: 11/10/2019 Revision: 0

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