7 ms·
It is true that those are all inter-related. The premise of his video is that the energy goes across to the bulb at the speed of light without using the circui
by binbag 5y ago
It is true that those are all inter-related.
The premise of his video is that the energy goes across to the bulb at the speed of light without using the circuit.
The real answer is there is a very small response at the speed of light because the moment the battery is connected there is effectively an antenna or capacitor set up between the two wires.
The light will turn on after the time it takes the energy to pass around the entirety of the circuit.
As for the Poynting vector 'demonstrating' that the energy goes straight across rather than being confined to the wires, there is a tiny bit of truth in that, but he fails to say (or to understand) that in reality the field lines running from battery to bulb would be SO ABSOLUTELY MINUTE compared to the very strong field lines confined to the circuit path.
So in short, there is a TINY amount of current flow at the speed of light because there is an antenna set up between the wires. It doesn't even matter if the wires are connected at their ends or not - you've built an antenna.
But this will NOT turn the light on because the energy received will be extremely small.
The light turns on after the energy has travelled the entire length of the circuit, and not before.
- topspin 5y ago> But this will NOT turn the light on because the energy received will be extremely small. It doesn't have to be very small. The pair of wires form a transmission line and if the characteristic impedance of that line is low enough the current will be much higher. You appear to understand that so I'm not telling you anything. But there is still value in proposing this because if one were to perform this experiment with really low Z coax (12 ohm or something) the bulb would instantly light to nearly full brightness, yet the 'mystery' would be dispelled or at least trivial to explain: the instantaneous current is supplied via the distributed LC circuit inherent to the transmission line.
- gus_massa 5y ago> with really low Z coax (12 ohm or something) To get this result, is it necessary that the wires are very close?
- topspin 5y agoYes. They use large center conductors with thin dielectric so the capacitance is high. Here[1] (PDF) is a data sheet on some low Z cables; ~10-26 ohm stuff. https://www.richardsonrfpd.com/Products/Product/TC-12 https://www.richardsonrfpd.com/Products/Product/TC-12