In an ideal transformer, power must be conserved: Vp Ip = Vs Is, and the voltages relate to turns as Vp/Vs = Np/Ns. Combine these two relations: Is/Ip = Vp/Vs = Np/Ns. So the secondary-to-primary current ratio equals the turns ratio, meaning Is/Ip = Np/Ns. This also matches the intuitive idea that increasing voltage on the secondary (more turns) comes with a proportional decrease in current, and vice versa, to keep power the same. For example, if Np is twice Ns, the secondary current is twice the primary current, since Is/Ip = Np/Ns = 2.

The key idea is overcurrent protection that sacrifices a part of itself to stop the current. A fuse uses a thin metal element that carries the circuit current. When the current exceeds its rating, the element heats up due to I^2R losses and melts, opening the circuit. This makes fuses single-use devices that must be replaced after they operate. Circuit breakers perform protection by tripping a switch—often via a bimetal thermal mechanism or a magnetic trip—so they interrupt the circuit without melting anything. They are resettable after the fault is cleared. Relays are control devices that open or close contacts in response to a signal, not primarily protective devices that melt to interrupt current. Transformers, on the other hand, are energy-transfer devices and do not function as protective elements that open circuits by melting.

Flux density measures how strong the magnetic field is at a point. The magnet’s field lines are crowded near the poles and spread out as you move away. Because the field is produced by a finite source, its strength diminishes with distance—roughly as the distance to the third power in the far field for a dipole. That means doubling how far you are from the magnet reduces the flux density by about eight times, and the general trend is a weaker field the farther you go. It doesn’t stay the same, increase, or oscillate with static distance. So the flux density decreases as distance from the magnet increases.

When current runs beyond what a breaker is designed to carry, the protective mechanism inside it trips and opens the electrical contacts. That action breaks the circuit and stops the flow of current, preventing overheating, insulation damage, or a fire. The triggering is typically thermal (a bimetal strip heated by excess current) or magnetic (a strong magnetic field from a short or surge pulls the contacts apart). After tripping, the circuit is opened and power must be reset to restore it. This explains why opening the circuit is the correct behavior, not shorting the load, not increasing current, and not staying closed during an overcurrent condition.