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I know that when AC is passed through the primary coil of a step-up transformer a higher emf is induced in the secondary coil (with more turns) of the transformer. Since energy is conserved, and P = VI, this would mean a drop in current. But I'm not able to visualize what is really happening to the electrons in the secondary coil. Does a higher voltage in the secondary coil mean that all electrons in the secondary coil are taken to a higher potential? And why is the current (flow of electrons) low when that happens?

What is really happening to the charges in the secondary coil?

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  • $\begingroup$ You're best thinking about the magnetic field the generated by the electrons in the loops of the primary coil, then how that magnetic field affects the electrons in the loops of the secondary. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2021 at 2:52
  • $\begingroup$ The “nanoscopic” level is not relevant at all for this subject. Especially not individual electrons. The interesting physics is in the fields at the level of Maxwell’s equations $\endgroup$
    – Dale
    Commented Jul 18, 2021 at 3:02
  • $\begingroup$ @Dale I don't understand what you mean by "not relevant". I'm not concerned about individual electrons, but the rate of flow of electrons as a whole seems to be decreased when a higher emf (Work done/charge) is induced in the secondary coil. I'm looking for the mechanics behind that. $\endgroup$
    – Sasikuttan
    Commented Jul 18, 2021 at 3:11
  • $\begingroup$ @DKNguyen The changing magnetic field in the primary coil creates a higher potential difference between the ends of the secondary coil, i.e. more work is done on each electron. But since energy is limited, this would mean that only fewer electrons will be flowing when the load is connected => Current is low. Does this make sense? $\endgroup$
    – Sasikuttan
    Commented Jul 18, 2021 at 3:47
  • $\begingroup$ If a potential difference is generated across just one loop from a given magnetic field, there is some potential difference and all the energy goes into that so more electrons can be moved (higher current). If the potential difference is generated across n loops, there is n times as much potential generated. Since energy is conserved that means only n times fewer electrons can be moved (lower current). You can dig a bit farther into the mechanism of the potential difference since I don't know it off the top of my head. Lenz's Law I beleve. $\endgroup$
    – DKNguyen
    Commented Jul 18, 2021 at 4:02

2 Answers 2

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Sorry for my poor english. French is my native language.

I don't think it is necessary to go to the microscopic level. it would be very complicated to study the radiation of electrons at this level !

Maybe it's more natural to reverse the reasoning by asking why the primary current increases when current flows in the secondary coil ? Imagine a transformer with the secondary coil in an open circuit: the current delivered by the secondary coil is zero. So, the current at the primary is very low (ideally zero) because the impedance of the primary circuit is very high (ideally zero if the permeability is infinite).

Now place a resistor at the secondary coil : current will flow under the effect of the induced emf. Why is the primary current going to increase? because the current flowing in the secondary coil will generate a flux through the primary coil. The generator supplying the primary coil no longer simply "sees" the input impedance of the single coil. The induced emf will modify this input impedance. The calculation shows that this leads to an increase in the current at the primary. And we understand this well by using the conservation of energy.

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  • $\begingroup$ I know that a transformer acts like a gearbox. While I can clearly and intuitively understand what's happening with velocity and torque in the case of a gearbox, the same cannot be done in the case of a transformer. All I can see is, when power is constant, if voltage increases, current decreases. But I cannot comprehend how the increase in voltage is causing a decrease in current in the secondary coil on a nanoscopic level. Low amperage means the rate of flow of electrons through a point is low. And this happens due to the increase in the energy or work done per unit charge (voltage). $\endgroup$
    – Sasikuttan
    Commented Jul 18, 2021 at 2:10
  • $\begingroup$ But I cannot connect both these ideas. $\endgroup$
    – Sasikuttan
    Commented Jul 18, 2021 at 2:13
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What happens in the primary coil

Electrons whose motion is forced into curved paths align their magnetic dipoles (and yes, each electron is also a tiny magnet). The core of the transformer directs this common magnetic field through the secondary coil.

What happens in the secondary coil

In the case that the magnetic field changes, the electrons in the secondary coil are aligned by this field and experience a shift (Lorentz force, Hall effect). Since for all electrons the magnetic dipole and the spin are related, all electrons are shifted in the same direction (an empirical fact that allows us to use inductive processes in everyday life).

Does a higher voltage in the secondary coil mean that all electrons in the secondary coil are taken to a higher potential?

In the secondary coil, the changing magnetic field induces electron displacement along the entire wire. If the length of the wire is short in relation to the primary coil, more electrons can be shifted per mm and this leads to a higher current. Since the power of the (ideal) transformer must be the same on both sides, the voltage on the second coil must be lower.
If on the other hand the secondary coil has more turns, fewer electrons per unit length can be displaced; the current is lower and the voltage higher.

And why is the current (flow of electrons) low when that happens?

The power on both sides of the transformer must be the same (without taking power losses into account). The power is the product of current and voltage P = U*I. If more electrons are affected in the secondary coil (higher current), the voltage is lower and vis-a-vis.

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  • $\begingroup$ //If the length of the wire is short in relation to the primary coil, more electrons can be shifted per mm// I'm not entirely sure about this, because it is not the length of the wire, but the number of turns that actually matters. Nevertheless, this is the exact point for which I'm looking an answer for. $\endgroup$
    – Sasikuttan
    Commented Jul 19, 2021 at 8:15

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