2ª Lei de Ohm - Brasil Escola

Summary

In this video, Rafael, a physics teacher, explains Ohm's second law and its application to electrical resistance. He discusses how resistance depends on the material, length, and cross-sectional area of a conductor. Through examples, such as the calculation of the potential difference between a bird's feet on a transmission line and a comparison of two wires with different dimensions, he demonstrates the practical implications of resistance. Rafael also emphasizes the role of resistivity in determining whether a material is a good conductor or an insulator, providing viewers with a solid understanding of the topic.

Takeaways

• 😀 The second law of Ohm explains that electrical resistance is both a geometric and material property.
• 😀 Resistance depends on the shape and size of the conductor, particularly its cross-sectional area.
• 😀 A conductor with a larger cross-sectional area has lower resistance, as there is more space for electrons to flow.
• 😀 Resistance increases with the length of the conductor because the electrons have a longer path to travel.
• 😀 The resistance of a material is also affected by its resistivity, a property that indicates how well it can conduct electricity.
• 😀 Materials with low resistivity, like silver and copper, are good conductors, while materials with high resistivity are insulators.
• 😀 The resistivity of common materials, such as silver (1.5 x 10^-8 Ω·m), copper, and aluminum, varies greatly.
• 😀 When a bird lands on a power line, the potential difference between its feet is very small, preventing an electric shock.
• 😀 In a given electrical circuit, the potential difference (voltage) between two points can be calculated using Ohm's Law.
• 😀 Doubling both the length and the radius of a wire affects its resistance, resulting in a lower overall resistance due to the larger cross-sectional area.
• 😀 The relationship between length, area, and resistivity determines the total resistance in a conductor, following the formula R = ρ(L / A).

Q & A

What is Ohm's second law?

Ohm's second law explains that electrical resistance is a property of both the geometry and the material of a conductor. It is affected by the length, cross-sectional area, and resistivity of the material.

How does the geometry of a conductor affect its resistance?

The resistance of a conductor depends on its geometry. A conductor with a larger cross-sectional area will have lower resistance, while a longer conductor will have higher resistance, as electrons face more opposition over longer distances.

What is the relationship between resistance and length of a conductor?

Resistance is directly proportional to the length of a conductor. The longer the conductor, the higher the resistance, as electrons must travel a greater distance.

How does the cross-sectional area of a conductor affect its resistance?

Resistance is inversely proportional to the cross-sectional area of the conductor. A larger cross-sectional area allows more room for electrons to flow, reducing resistance.

What is resistivity and how does it affect resistance?

Resistivity is a material property that quantifies how strongly a material opposes the flow of electric current. Materials with low resistivity, like silver, are good conductors, while those with high resistivity, like rubber, are insulators.

Why is silver considered a better conductor than copper?

Silver has a lower resistivity compared to copper, meaning it allows electrical current to flow more easily. Its resistivity is 1.5 x 10^-8 ohm meters, while copper's is slightly higher.

What is the potential difference between the feet of a bird sitting on an electric wire?

In the given example, the potential difference between the bird's feet is very small, around 3 millivolts, which is not enough to cause an electric shock, as there is insufficient energy for a current to flow between the feet.

How is the potential difference across the bird’s feet calculated?

The potential difference is calculated using Ohm's law: Voltage = Current × Resistance. Here, resistance is derived from the resistance per unit length of the wire, the length between the bird's feet, and the current flowing through the wire.

What happens to the resistance when the length of the wire is doubled and its radius is also doubled?

When the length and radius of the wire are doubled, the resistance is affected in two ways. The doubling of length doubles the resistance, but the doubling of the radius reduces the resistance by a factor of four, resulting in a total resistance that is half of the original resistance.

What formula represents the relationship between resistance, length, and cross-sectional area?

The formula for resistance is R = ρ × (L/A), where R is resistance, ρ is resistivity, L is the length, and A is the cross-sectional area of the conductor.