Why use cores in transformers?

Transformers often require/use iron cores because they operate on magnetic forces, which are difficult to understand when sharing certain characteristics with good old "electricity" (ohms, volts, amperes, etc.). Let's try some simplified ways to get the overall idea.

Start with a screwdriver - just a cylindrical coil. If we let the current flow through, a magnetic field (we call it the H field) is formed. The field depicted with the imagined field line flows up through the center of the coil, then disperses again after leaving the cylinder, then reassesses and re-enters the other end. You've seen the picture in the textbook. The magnetic field is strong and contained inside the cylinder (ID), while the magnetic field strength is weak outside (OD) because it diffuses in space. If the H magnetic field interacts with "anything" around the coil, whether it is vacuum, air or iron, it produces what we call a B magnetic induction field within the "material", the strength of which depends on the strength of the magnetic field. The properties of "matter" are called "permeability". For a given magnetic field strength H, vacuum or air forms a relatively weak induction field B, while iron forms a very strong sensing field (1000 times stronger).

If we make a second coil (solenoid valve) and parallel it to the first coil in the air, a portion of the weak air sensing field B flows through the center of the second coil. If we change the current in the first coil, its B field will change slightly, as will the B field flowing through the second coil (absolutely by a small margin). This is not only because the entire B magnetic field is weak, but also because only a portion of the entire B magnetic field actually passes through the second coil. Recall maxwell's equation, saying that the voltage sensed in the coil depends on the magnitude of the change through its B field. Therefore, in our case, since the B-field change through the second magnetic field is very small, we can expect only one weak voltage to be sensed in the second coil.

To make it better, we can place a piece of iron in the center of the first coil. This will make the B field in the iron stronger than the B field in the air. In addition, we can extend the iron sheet into a ring so that it passes through the second coil. (We've made a transformer core ). Most of the enhanced B magnetic field from the first coil now passes through the iron into the second coil, and the magnetic field change caused by the current change in the first coil is amplified, resulting in a greater inductive voltage in the second coil. Coil.

That's why we use iron core simplification in many, but not all, transformers.