Physics Asked by a3y3 on June 8, 2021
Say we have a straight wire connected across two terminals carrying electricity between them. If everything is kept the same (voltage across the two terminals, resistance of the wire, etc.) will there be ANY difference at all if the wire were to be, let’s say, tightly wound into a spiral as opposed to being straight?
I’m imagining there should be at least some difference because the spiral would make the electrons "bump" more along the edges. Is there any logic at all to this reasoning, or am I hopelessly illiterate about electricity?
Yes, there is a difference.
If you made the wire as you mentioned, into a spiral, like this:
then there is quite a big difference between this and a straight wire. The difference between a straight wire and a coil or spiral wire is that the spiral wire resists changes in current flow.
This is called an inductor or solenoid.
It resists changes in the current flow because when there is a changing current in the coil of wire, there is a back EMF that counters the EMF of the current entering the coil of wire. This makes it hard for current to pass (at least initially) through the coil of wire.
Inductors are characterized by the following equation:
$$v = L frac{dI}{dt}$$
Where $v$ is the voltage, $L$ is the inductance and $frac{dI}{dt}$ is the change in current with respect to time or how does current change when time ticks.
When the current is unchanging, the inductor behaves just like a normal wire. But when the current is changing, it behaves differently: the greater the change of the current, the greater the effect of the back EMF will be. At very high frequencies, the current will be mostly blocked, and theoretically, as the frequency approaches infinity, the resistance or impedance of the inductor will be infinite, and no current will pass.
Although straight wires also have inductance, it can be usually ignored because it is so small. Only in certain situations such as power line transmission, it becomes important to know the inductance of the straight wire where it cannot be ignored from the math when designing the power lines.
Correct answer by Tachyon on June 8, 2021
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