When current flows out of one side of a power plant and back into the other energy is given out. That energy is turned into kinetic energy of the moving charges. The power plant must do work to supply this energy. There is a direct connection between each house and both sides of the power plant. When we turn on our lights more charge must flow from the power plant to give us what we want, therefore the power plant must do more work.
Much smaller currents can affect the electrical firing of the heart.
All paths across a voltage difference will see some current flow. Optics 16.1 Cameras Refraction is the bending of light when it crosses between two transparent materials. A lens is used to make an image on the wall.