The reasons related to the magnetic force of an electromagnet?

In physics, the influence of the magnetic force as below :

1、        It is related to the turning Number of the coil; the magnetic force varies for the Number of the coils.

2、        It is related to the magnitude of the current passing through the conductor. By sliding the rheostat, the current passing through the                conductor will be changed, and the current will be increased by increasing the number of batteries;

3、        The presence or absence of an iron core. When there is an iron core, the magnetism is strong, but when there is                 no iron core, the magnetism is weak;

4、        It is related to the permeability of the soft magnetic material of the iron core

   5          it is related to the cross-sectional connection of the iron core

       What we are learning is mainly related to the first three items above, and the others’ reason also related to the below:

        In our daily lives, we can make electromagnets and conduct experiments. The tools. we need include wires, nails, batteries, etc. The method of controlling variables can help us make one change while the other remain unchanged. For example, when studying the relationship between turns, two electromagnets can be connected in series in a circuit to ensure that the current and iron core do not change.