Somewhat interactive site which intends to help with all sorts of physics homework and studying issues, and even allows you to email questions to the authors. Supported by the National Science Foundation. Provides solutions to electricity and magnetism physics problems normally found in common, undergraduate physics courses. A site to help you understand physics. Excellent list of formulas presented in a visual manner.
Small particles are difficult to measure, especially in bulk.
Here is a way to do it. An online book by D. Raymond. Life sciences, chemistry, physics, Earth and space sciences, inventions, and science fair resources. A resource for A Level Physics. Those doing high-school, O Level or first year university physics may also find this site helpful.