From Gabriel Carlyle, St Leonards-on-Sea, East Sussex, UK
Howard Bobry takes his local emergency preparedness meeting to task for assuring him that a discharged alkaline battery weighs less than a fresh one (Feedback, 7 July). But as Paul Hewitt notes in his textbook Conceptual Physics, a change of energy of any object at rest is accompanied by a change in its mass, so a hot cup of tea has more mass than it does cold. The catch, of course, is that the change in mass is tiny. I calculate that for a small battery with a full charge of 15,000 joules, that is equivalent to less than 0.2 nanograms.
