How Wrong Is VERITASIUM? A Lamp and Power Line Story

Summary

In this video, the speaker responds to a question posed by Derek from Veritasium regarding the behavior of electricity in a circuit with long wires. The question revolves around the time it takes for a lamp to turn on when a switch is flipped, and the speaker challenges Derek’s answer, arguing that the light remains on due to leakage current. The speaker also critiques Derek’s explanation of electricity, emphasizing the role of electromagnetic fields in energy transfer and the inaccuracies in his analogies. The video delves into complex circuit theory, offering insights on how engineers model and approach these problems.

Takeaways

• 😀 Derek's video on a physics question about a giant circuit with long wires posed a tricky question that sparked a debate in the electronics community.
• 😀 The question involved a circuit with a battery, switch, light bulb, and two wires 300,000 km long. The central issue was the time it takes for the light bulb to turn on after the switch is flipped.
• 😀 Derek's answer to the time it takes for the light bulb to turn on was incorrect, according to the narrator. The correct answer is 'none of the above' because the light is always on due to leakage current.
• 😀 The video touches on the concept of leakage current, which occurs even when the switch is off, causing a small but continuous current to flow, keeping the light on permanently.
• 😀 The narrator challenges the idea that electrons themselves carry energy in an AC circuit, which Derek suggests in his video. The narrator argues that fields, not electrons, are responsible for carrying energy.
• 😀 In AC circuits, magnetic fields are stronger close to the wires, and the energy flow is more concentrated near the wires than further out. This differs from what Derek suggests in his explanation of energy flow in electrical circuits.
• 😀 The narrator disagrees with Derek's analogy about energy transfer in AC circuits. They argue that the current creates a magnetic field around the wire, but the energy is carried by the fields, not the electrons themselves.
• 😀 The video explains the concept of the Poynting vector and how energy flows in an electric circuit, with fields being responsible for energy transfer, but the narrator disputes the oversimplified view Derek presents.
• 😀 The narrator performs simulations to analyze the time it takes for energy to propagate in a circuit with long wires, demonstrating that the voltage rise in the lamp takes time due to the properties of the transmission line.
• 😀 The narrator highlights the importance of accurately modeling transmission lines and their impedance in understanding the time it takes for the current and voltage to reach the lamp, emphasizing the complexity of such circuits.

Q & A

What is the central issue with Derek's question in the video?

The main issue with Derek's question is that it is framed as a trick question. It assumes a lamp can turn on immediately when current flows, but this is not realistic in a typical circuit due to the time required for the current to propagate through the wires.

Why does the author think Derek's answer to the circuit question is wrong?

The author believes Derek's answer is wrong because Derek overlooks the fact that the lamp in question would not actually turn on immediately due to the limitations of current propagation through long wires and the existence of leakage current. This results in the lamp being on continuously, regardless of the state of the switch.

What is the significance of the 300,000 km length of the wires in Derek's question?

The 300,000 km length of the wires is significant because it represents the distance light travels in one second. This is used to emphasize the delay in signal propagation and energy transfer through the circuit, which contradicts Derek's assumption that the lamp would turn on instantaneously.

What does the author mean by 'leakage current' and why is it important in this context?

Leakage current refers to the tiny, continuous flow of current that occurs even when the switch is off, due to electric fields across the switch contacts. This small current means the lamp could stay on even if the switch is off, which is important in understanding the behavior of the circuit and why Derek's assumption of the lamp turning on instantly is inaccurate.

How does the author challenge the idea of electrons carrying energy in a circuit?

The author challenges the idea that electrons themselves carry energy by arguing that it's the electromagnetic fields, not the electrons, that transport energy. The motion of electrons creates these fields, which in turn transfer energy through the circuit, especially in AC circuits.

What role do inductance and capacitance play in the behavior of long wires in a circuit?

Inductance and capacitance act as components that influence the flow of current through long wires. Inductance resists changes in current, while capacitance stores and releases charge, both of which affect how energy is transferred through the circuit, particularly in the case of long wires where delays and reflections occur.

What is a 'transmission line' in the context of this discussion?

A transmission line refers to a long electrical conductor, such as a wire, that carries alternating current or electromagnetic waves. In the context of the video, the transmission line model helps explain how the energy propagates through long wires with delays, reflections, and impedance matching.

Why does the author emphasize the importance of the frame of reference in understanding energy transfer?

The author emphasizes the frame of reference because different perspectives (such as whether you're looking at the electrons in the wire or the electromagnetic fields around it) can lead to different interpretations of how energy is transferred in a circuit. This is particularly important when discussing the flow of energy in AC circuits.

What does the simulation show regarding voltage across the lamp in the scenario with 20 kilometers of wire?

The simulation shows that when the circuit is turned on, the voltage across the lamp rises slowly and takes several milliseconds to settle to the battery voltage. This is because of the inductive and capacitive effects in the transmission line, causing the voltage to increase in steps rather than instantly.

How would matching the impedance of the transmission line with the lamp affect the voltage in the circuit?

If the impedance of the transmission line is matched with the lamp's impedance, the voltage across the lamp would jump immediately to a decent level, without any ringing or delays. This is the ideal case for energy transfer, as it allows maximum power to be transferred efficiently.