6 ms·

I liked the explanation, but I find the bit at the end confusing. Don’t the electric fields carrying energy need current in the wire to form? How are they carry

I liked the explanation, but I find the bit at the end confusing. Don’t the electric fields carrying energy need current in the wire to form? How are they carrying energy before the current propagates through the wire?

db48x 5y ago
Consider a point on a wire. As the current gets going, the current at that point will go from zero up to the final value over some (small) period of time. The magnetic field around the wire at that point must also grow from zero to it’s final strength over that period of time. The growing magnetic field causes a current in the wire flowing in the opposite direction as the first current. This slows the flow of current in the wire (so that it doesn’t happen instantaneously), and it’s also the source of the energy in the field; it’s called the self–inductance of the wire. An inductor in a circuit is just a coil of wire so that the magnetic fields from many parts of the wire all reinforce each other and cause a larger opposing current in the wire.
alok-g 5y ago
In the video itself, he stated that electricity propagates from the battery at the speed of light when he describes the diagram. I think what he has missed describing is that the wire folds carry capacitance (and inductance). The capacitance leads to current flow paths shorter than resistive paths (where resistance is assumed zero). Some commentors have asked what happens if the wire at the extreme end is cut. If it is not cut, then intially some current goes via capacitance, making the bulb light up sooner. Ultimately, assuming DC battery output, capacitive current decays down. However, the time-consuming resistive path establishes. If the wire is cut, capatance makes the bulb light up at first, however, the capacitive current soon decays without establishing of the resistive path, hence the bulb glow would decay. The 'electrical' model above does not contradict the 'energy flow via fields' description, as the two are one and the same thing at the basic level.
- alok-g 5y ago
  Adding more: It's a distributed R-C circuit, along with discrete R of the bulb. C per unit distance depends on the positioning of the wires. How fast current ramps up depends on the distributed C and the bulb's R (as wire's distributed R is ignored in the problem statement).
  - alok-g 5y ago
    L too. Distributed L and C, discrete R.

db48x 5y ago

Consider a point on a wire. As the current gets going, the current at that point will go from zero up to the final value over some (small) period of time. The magnetic field around the wire at that point must also grow from zero to it’s final strength over that period of time. The growing magnetic field causes a current in the wire flowing in the opposite direction as the first current. This slows the flow of current in the wire (so that it doesn’t happen instantaneously), and it’s also the source of the energy in the field; it’s called the self–inductance of the wire. An inductor in a circuit is just a coil of wire so that the magnetic fields from many parts of the wire all reinforce each other and cause a larger opposing current in the wire.

alok-g 5y ago

In the video itself, he stated that electricity propagates from the battery at the speed of light when he describes the diagram. I think what he has missed describing is that the wire folds carry capacitance (and inductance). The capacitance leads to current flow paths shorter than resistive paths (where resistance is assumed zero). Some commentors have asked what happens if the wire at the extreme end is cut. If it is not cut, then intially some current goes via capacitance, making the bulb light up sooner. Ultimately, assuming DC battery output, capacitive current decays down. However, the time-consuming resistive path establishes. If the wire is cut, capatance makes the bulb light up at first, however, the capacitive current soon decays without establishing of the resistive path, hence the bulb glow would decay. The 'electrical' model above does not contradict the 'energy flow via fields' description, as the two are one and the same thing at the basic level.

alok-g 5y ago
Adding more: It's a distributed R-C circuit, along with discrete R of the bulb. C per unit distance depends on the positioning of the wires. How fast current ramps up depends on the distributed C and the bulb's R (as wire's distributed R is ignored in the problem statement).
- alok-g 5y ago
  L too. Distributed L and C, discrete R.