thanks all for the pointers: they led me to an explanation from Feynman, which I'd not found before. And hadn’t seen in high school. https://www.feynmanlectur

thanks all for the pointers: they led me to an explanation from Feynman, which I'd not found before. And hadn’t seen in high school. https://www.feynmanlectures.caltech.edu/I_31.html https://www.feynmanlectures.caltech.edu/I_31.html