Simple Nature - Light and Matter

Here are a few questions and answers about the voltage concept.Question: OK, so what is voltage, really?Answer: A device like a battery has positive and negative chargesinside it that push other charges around the outside circuit. Ahigher-voltage battery has denser charges in it, which will do morework on each charged particle that moves through the outside circuit.To use a gravitational analogy, we can put a paddlewheel at thebottom of either a tall waterfall or a short one, but a kg of waterthat falls through the greater gravitational energy difference willhave more energy to give up to the paddlewheel at the bottom.Question: Why do we define voltage as electrical energy divided bycharge, instead of just defining it as electrical energy?Answer: One answer is that it’s the only definition that makes theequation P = I∆V work. A more general answer is that we wantto be able to define a voltage difference between any two pointsin space without having to know in advance how much charge theparticles moving between them will have. If you put a nine-voltbattery on your tongue, then the charged particles that move acrossyour tongue and give you that tingly sensation are not electrons butions, which may have charges of +e, −2e, or practically anything.The manufacturer probably expected the battery to be used mostlyin circuits with metal wires, where the charged particles that flowedwould be electrons with charges of −e. If the ones flowing acrossyour tongue happen to have charges of −2e, the electrical energydifference for them will be twice as much, but dividing by theircharge of −2e in the definition of voltage will still give a result of 9V .Question: Are there two separate roles for the charged particles inthe circuit, a type that sits still and exerts the forces, and anotherthat moves under the influence of those forces?Answer: No. Every charged particle simultaneously plays bothroles. Newton’s third law says that any particle that has an electricalforce acting on it must also be exerting an electrical force back onthe other particle. There are no “designated movers” or “designatedforce-makers.”Question: Why does the definition of voltage only refer to voltagedifferences?Answer: It’s perfectly OK to define voltage as V = U elec /q. Butrecall that it is only differences in interaction energy, U, that havedirect physical meaning in physics. Similarly, voltage differences arereally more useful than absolute voltages. A voltmeter measuresvoltage differences, not absolute voltages.518 Chapter 9 Circuits