1. Introduction: A Coil That Acts Like a Magnet
You already know two separate things. One — a wire carrying current has a magnetic field around it. Two — a bar magnet has a North pole and a South pole, with a field that loops from N to S.
Today these two ideas meet in one object.
We take a long wire, wind it into many round turns sitting side by side, and send current through it. Something surprising happens: the coil starts behaving exactly like a bar magnet — two poles, the same looping field, the power to pull iron.
Let me name the new thing right away. A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder. Picture wire wrapped tightly, turn after turn, around a belan (rolling pin), or the tight coil of a spring.
Stop scrolling. Picture it before reading on: many round loops, packed side by side, the same current running through all of them.
The question for today is simple to ask and worth the whole lesson to answer: what does the magnetic field around this coil look like — and why does it end up looking like a bar magnet?