Coulomb's Law describes the force between two point charges. In a vacuum, the force is given by:
```
F = k * q1 * q2 / r^2
```
where:
* F is the electrostatic force
* k is Coulomb's constant (approximately 8.98755 × 10^9 N⋅m^2/C^2)
* q1 and q2 are the magnitudes of the charges
* r is the distance between the charges
When a medium (like air, water, or a dielectric material) is present between the charges, the force between them is modified. The reason is that the medium's constituent particles (atoms or molecules) can be polarized by the electric field of the charges. This polarization creates an opposing electric field within the medium, which effectively reduces the net electric field experienced by the charges.
The effect of the medium is captured by a quantity called the permittivity (ε). It's a measure of how easily a medium can be polarized. The permittivity of vacuum is denoted as ε₀ (approximately 8.854187817... × 10^-12 F/m).
The modified Coulomb's Law in a medium is:
```
F = k * q1 * q2 / (ε * r^2)
```
Key Points:
* Higher permittivity means a weaker force: The force between charges decreases as the permittivity of the medium increases.
* Dielectric constant: The relative permittivity (ε_r) is the ratio of the permittivity of the medium to the permittivity of vacuum (ε_r = ε / ε₀). This dimensionless quantity tells us how much stronger or weaker the force is compared to the force in a vacuum.
* Common examples:
* Air has a relative permittivity of about 1.00059, meaning the force is almost the same as in a vacuum.
* Water has a relative permittivity of about 80, so the force between charges in water is significantly reduced.
In summary, the presence of a medium between charges affects the force between them by reducing it. This effect is quantified by the permittivity of the medium, which reflects its ability to be polarized.
