MadSci Network: Engineering |
Irene, You have reached the correct conclusions concerning voltage and current (as we call it here) in series and parallel connections of batteries. Here's an analogy to help you understand why this is so. Think of electricity as water, wires as pipes, and batteries as water pumps. Then "water pressure" is analogous to "voltage" and "flow rate" is analogous to "current". Two measurements describe pumping ability: the first is the pressure rating of the pump, and the second is the maximum flow. For exampe, I recently bought an irrigation pump for my farm rated at 75 psi (pounds per square inch) at 60 gpm (gallons per minute). This means I can take in pond water at 0 psi, and discharge water at 60 gpm flow rate into a pipe at 75 psi pressure. If someday I need a larger flow rate, I can buy a second pump, and have it work in parallel with the first (i.e., both pumps take in pond water and both discharge into a single pipe), for a total combined flow rate of 120 gpm at 75 psi. This is similar to the case where you've connected your batteries in parallel: the current capacities (flow rates) add. On the other hand, if I need higher pressure, I could connect the inlet of a second pump to the outlet of the first, giving me 60 gpm at 150 psi. That is, the first pump takes in pond water at 0 psi, discharging it at 75 psi. The second pump takes in this water and raises the pressure by an additional 75 psi before discharging it, giving a net pressure rise of 150 psi. The flow rate stays at 60 gpm; the second pump can only discharge as much water as the first pump gives it -- it can't create more! Similarly, connecting batteries in series adds the voltages (pressures). In principle, I could go on connecting pumps in series this way to reach any high pressure I desire. The only thing that stops me from adding pumps this way are practical issues such as the ability of the shaft seals, pump casings, and pipes to withstand the high pressures (much as electrical circuits must have insulation strong enough to withstand the very high voltages involved). If we look a little closer at what's going on, we see that a pump works by adding energy to the molecules of water. Bernoulli's Law tells us this energy is divided between the potential energy of the water (proportional to its pressure and height above the pumping point) and the kinetic energy of the water(proportional to the square of the flow rate). Similarly, a battery works by adding energy to the electrons flowing through an electrical circuit. This increase in voltage instigates the flow of current through the circuit, much as a pump's increase in water pressure causes the water to flow from my pond to other useful places. Thus, combining batteries in series allows them to sequentially raise the electrons to higher and higher energy (voltage), while combining batteries in parallel allows more and more electrons to flow, with each battery contributing its current capacity to the total flow. By the way, the unit of current flow, the ampere, is defined in terms of the number of electrons per second moving past the measurement point; sounds like a flow rate, doesn't it? The unit of voltage, the volt, is defined in terms of force per unit charge, which reminds me of pressure, which is force per unit area. I hope this is of some help; good luck with your project! Steve P.S. A fine point for sticklers: The direction in which electrons actually move is opposite to the direction in which we say current flows. Thank Benjamin Franklin for this, who originated the convention of electrical current flowing from positive to negative; he didn't know about electrons, but the convention has stuck with us even after they were discovered. I say that this is a nuisance point of no consequence until you're ready to start designing solid state devices (e.g., transistors and integrated circuits). For now, you'll do fine simply remembering that current flows from positive to negative and not worrying about what the electrons are actually doing.
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