26.12
Condutividade Elétrica
Em condutores perfeitos, o campo elétrico no interior é sempre zero, por conta da abundância de elétrons livres, que anulam qualquer campo com o seu d…
Dentro de condutores perfeitos, o campo elétrico é zero. No entanto, para condutores práticos, os campos elétricos aplicados levam ao fluxo de elétrons, causando corrente.
Na teoria da condução metálica, a densidade de corrente resultante é proporcional ao campo elétrico aplicado. A constante de proporcionalidade é chamada de condutividade elétrica. É uma propriedade intrínseca do material.
A corrente através da área da seção transversal está relacionada ao campo elétrico aplicado. A relação entre o campo elétrico e a diferença de potencial ao longo do comprimento do condutor é então usada para relacionar a corrente e a diferença de potencial.
A constante de proporcionalidade, chamada de resistência, é inversamente proporcional à área e diretamente proporcional ao comprimento, que são fatores geométricos. É diretamente proporcional à condutividade inversa, chamada resistividade, que é outra propriedade intrínseca do material.
Essa relação é chamada de lei de Ohm. É equivalente à relação entre a densidade de corrente e o campo elétrico aplicado.
View the full transcript and gain access to JoVE Core videos
Q1: What is electrical conductivity and how does it relate to current flow?
Electrical conductivity is an intrinsic material property that describes how readily a material allows current to flow when an electric field is applied. In metallic conduction theory, current density is proportional to the applied electric field, with electrical conductivity as the proportionality constant. Materials with high conductivity permit greater electron flow for a given field strength.
Q2: Why is the electric field zero inside a perfect conductor?
In perfect conductors, the abundance of free electrons responds immediately to any applied field by flowing to neutralize it, resulting in zero electric field inside. Any residual charge accumulates on the surface. This contrasts with practical conductors, where an applied electric field can be sustained, causing controlled electron flow and current production.
Q3: How do geometric factors affect a conductor's resistance?
Resistance is directly proportional to conductor length and inversely proportional to cross-sectional area. A longer conductor has more bound positive ions for electrons to drift past, increasing resistance. A larger cross-sectional area provides more space for electrons to pass through, reducing resistance. These geometric relationships follow from current density principles.
Q4: What is the difference between resistivity and resistance?
Resistivity is an intrinsic material property independent of shape or size, while resistance depends on both material properties and geometry. Resistance is directly proportional to resistivity and inversely proportional to conductivity. The relationship R = ρL/A shows how resistivity combines with length and area to determine total resistance.
Q5: How does Ohm's law connect current density to potential difference?
Ohm's law relates current and potential difference through resistance, serving as both an integral and differential form. The differential form connects current density to the applied electric field, while the integral form relates total current to potential difference across the conductor. Both forms are experimentally verified models of current flow in conductors.
Q6: Why do electrons move slowly through conductors despite applied electric fields?
Electrons drift slowly through conductors because they continuously collide with bound positive ions in the lattice structure. Although the electric field accelerates electrons, these frequent collisions limit their net velocity. The resulting drift velocity is much slower than the field strength might suggest, making the magnetic component of the Lorentz force negligible in conductors.
Q7: Is Ohm's law a fundamental physical law?
Ohm's law is not a fundamental law but rather an experimentally verified model of current flow in conductors. It emerges from the relationship between current density and electric field in metallic conduction theory. This model accurately describes behavior in ohmic materials but does not apply universally to all conducting systems, such as non-ohmic devices.