There are many factors that influence the strength of an electromagnet. what could be done to make the electromagnet stronger?

How do you tell if someone actually cares about you? and just say they do?

Ascientific blank must be testable and capable of being proven false?

The illuminance of a surface varies inversely with the square of its distance from the light source. if the illuminance of a surface is 120 lumens per square meter when its distance from a certain light source is 6 meters, by how many meters should the distance of the surface from the source be increased to reduce its illuminance to 30 lumens per square meter?