Final answer:
The magnetic field increases as the number of loops in a current-carrying wire coil increases because the effects of each loop's field add together. If you reduce the number of loops from four to two, the magnetic field strength decreases to half of its initial value.
Step-by-step explanation:
When the number of loops in a current-carrying wire coil is increased, the magnetic field around the wire increases as well. This is because the magnetic fields generated by each loop add together, causing a stronger overall magnetic field. For example, if you choose a battery as a current source and set the number of wire loops to four, you will measure a certain magnetic field strength at a point. If you decrease the number of wire loops to two, the magnetic field strength at that point will decrease, specifically to half of its initial value, demonstrating the direct correlation between the number of loops and magnetic field strength.
The magnetic field in the center of a current loop can be described by Ampere's law, and it's directly proportional to the current and the number of turns in the coil. Moreover, the magnetic field generated is inversely proportional to the loop diameter. Therefore, if two loops carry identical currents but one has twice the diameter of the other, the one with the larger diameter will produce a magnetic field with half the strength at its center compared to the smaller loop.