When you say that a transformer can change a small current with a high voltage i…

When you say that a transformer can change a small current with a high voltage into a large current with a low voltage, where do those extra charges come from?

A transformer involves two completely separate circuits: a primary circuit and a secondary circuit. Charges circulate within each circuit, but do not move from one circuit to the other. If the primary circuit of a transformer has a small current flowing through it and that current experiences a large voltage drop as it flows through the transformer’s primary coil, then the primary circuit current is transferring power to the transformer and that power is equal to the product of the primary circuit current times the voltage drop. The transformer transfers this power to the current flowing in the secondary circuit, which is an entirely separate current. That current may be quite large, in which case each charge only receives a modest amount of energy as it passes through the secondary coil. As a result, the voltage rise across the secondary coil is relatively small. The power the transformer is transferring to the secondary circuit current is equal to the product of the secondary circuit current times the voltage rise.

Where does the exact reversal occur in an alternating current circuit (where doe…

Where does the exact reversal occur in an alternating current circuit (where does the energy diminish completely and then turn the opposite way)?

The reversal of the current in an alternating current (AC) circuit occurs everywhere in the circuit at once. The whole current gradually slows to a stop and then heads backward. At the moment it comes to a complete stop, the electric power company isn’t supplying any power at all and the circuit isn’t consuming any. Because the power delivery pulses on and off in this manner, devices that operate on AC power are designed to store energy between reversals. Motors store their energy as rotational motion. Stereos store energy as separated electric charge in devices called capacitors, or as magnetic fields in devices called inductors.