4 ms·

> There is a simple solution to these problems: connect your system to the Earth. Drive some long metal rods into the dirt, and connect them to your wires. That

• sansnomme 7y ago
  Study the Maxwell equations and thou shalt be enlightened.
• mantap 7y ago
  Obviously rock doesn't conduct electricity but water+impurities does. When people talk about "ground" they are talking about the water in the ground.
  • saalweachter 7y ago
    Incidentally, this is why if you're running an electric fence, and you have rocky or dry soil, it is advisable to run alternating hot & ground wires, and to not rely on the ground to complete the circuit.
• ZainRiz 7y ago
  There are two different mental models I find really useful when thinking about electricity. The second one will answer your question more directly, but the first one helps set the stage better. Mental Model #1 (useful for most work with electricity): Think of electricity moving through circuits as water going through pipes. Electricity = water Voltage = water pressure Battery/Voltage Source = water pump Wires = Water pipes High/Low resistance = Wide/narrow water pipes Voltage is like water pressure. You have a pump creating high water pressure is like a power source with a high voltage. The pump is always on and creating water pressure, but there's a cap placed on it that blocks the water from escaping. Similarly, in an open circuit the air acts as this cap and blocks the electricity from proceeding. The wires in a circuit act as pipes channeling the water to different areas. Resistance in a circuit is analogous to the thickness of the pipes (low resistance == wide pipes, high resistance == thin pipes). The above analogy sets part of the mental model. The twist is that in reality electrons want to actually travel from a place of low voltage to a place of high voltage (classical electrical diagrams show this process in reverse). Which leads us to... Mental Model #2 Voltage sources are powerful electron vacuum cleaners trying to to suck up electrons. (You can apply this on top of mental model #1, just reverse the direction of the water flow) Open circuits generally don't conduct electricity since within a nano-second the voltage source will suck up all the free electrons in the circuit. The negative/ground pole in circuits are where the free electrons get supplied from for the voltage source to "suck in" the electrons from. The ground (by virtue of it being so large and having lots of moisture/materials which conduct electricity) happens to have a virtually limitless source of electrons available for the voltage source to suck into itself. This provides the circuit with all the electrons required to successfully operate. Hope that helps :)
  • chronolitus 7y ago
    Here's another mental model, the balls model, which I sometimes find useful, especially in this case. Here it is in case it helps someone: Imagine electricity as red balls, which can travel along conductors. Groups of red balls feel drawn strongly to places which have less balls, and away from places which have more balls, leading them to equalize quantities among all points in the circuit (let's assume for now cables can't store the red balls, only transport them. Points in the circuit are places that can store balls). if you have a battery, both the sides can store balls, but assuming it's charged, the plus side might have 12 red balls and the minus side none. The difference in red balls is 12! A quite strong force pushing red balls from the + to the -. (that's the voltage). When the circuit is closed with a wire, the balls will travel quickly and end up with 6 at both the + and -. How many of them travel at once (Intensity) depends on how large the tube is (Resistance). What if instead of 12 and 0 we have 112 at +, and a 100 balls at -? Well the force is the same as before (12), the whole circuit works the same way. 6 balls go from + to -, over the same amount of time (tubes have the same throughput), you then end up with 106 at both ends. You just have these 'residual' red balls hanging around, doing nothing special. However, if you suddenly connect this circuit with 212 balls to the first circuit with 12, the balls will quickly flow from from the first to the second, this time with a force of a 200! (The point here is that voltage is a relative measure, unlike charge.) You might have an isolated electronic circuit starting with 12 and 0 balls, and think all is well. You touch the circuit once to start it, the balls are flowing normally. You leave it working for a few hours and come back, it still seems to be fine. You touch it, and ZAP, a spark, burnt smell, you're wondering what happened. Well the thing is that external sources can sometimes add a red ball or two to your system. Over time, they might add up, and after a while you end up with hundreds. As soon as you touch the circuit, they all flow to somewhere that has fewer. Ok, where does the ground come into this? A 'ground' is like the - side of your battery. It's an object that can store balls the same way. However, instead of having one 'storage point' which can be filled with balls, it has a several. So take a metallic object, and say it has, for example, 11 storage 'points'. If you connect it to a single + point containing 12 electrons, the balls spread out so that there's the same amount of balls in each point (as we explained at the beginning). So after a while your 1 and 11 points, end up with one ball at each point. (Analog to the concept of capacity) So the natural earth turns out to behave like a huge metallic object. So big that it basically has an infinite amount of storage 'points'. You can pour red balls into it all day, and still end up at an average of zero balls per point. So now you take your 12 to 0 circuit and connect the - side to the earth. The difference between + and - is the same as before, 12. The flow from + to - is about the same (same resistance), so the circuit behaves almost the same. If however an external red ball is randomly added, it flows from the - to the earth, meaning that you never accumulate balls in the circuit. This allows you to set a reference amount of 0 balls in every circuit you work with. Which is great. The amount of red balls at the + can still be set to whatever you like, and they will flow nicely from + to - along whatever circuit you create (like before), but on top of that you never run the risk of having red balls jump from one circuit to another if they accidentally touch, because there is never a large difference in red balls as they all have the same reference amount. Anything big which is conductive enough can store a lot of red balls. So what qualifies? Soil usually works, water works to an extent (https://electronics.stackexchange.com/questions/164898/can-the-ocean-be-used-as-ground https://electronics.stackexchange.com/questions/164898/can-t...). When you can't connect to those a big object will have to do, like a car frame, it will have enough storage that over a short time it looks infinite, but if you really pushed and kept adding red balls at some point the reference level would change. With the earth you could pour red balls your entire life and barely make a difference to the reference level. (So now you're thinking, the red balls are electrons, right? Well sort of, but don't tell a physicist! Otherwise they'll get angry and start talking about flow direction being this or the other way. Or they'll say something about non-rigorous representations, wave-particle equivalence, the balls actually being a 'lack of electrons' and we'll all be confused.)
• azalemeth 7y ago
  Whether we're talking electricity or gravity, potential differences are always defined _with respect to something_ -- where you put your zero is pretty arbitrary, and for some problems there's an obvious "right answer". For stuff to do with space, for example, we might want to put the zero of potential at the edge of the universe: at a glance, anything with positive energy therefore has enough to get there (and is "free" from any ground gravitating body) and anything with negative energy is "bound" to never get there (and typically bound into orbit around something else). It's the same idea for electricity. Potential differences are what drives currents: at some level, that boils down to more electrons over here, more electrons over there. The thing about the ground is that, usually, we're all on it. From a safety point of view, this matters, because we're slightly conductive bags of meat that rely upon electricity in certain bits to live. Bugger up the electrics, stop us living. As we always have one foot on the ground, the potential of the earth is a good reference point to chose, and it's the convention that for electrical thingywhatists, _it's_ the zero of potential, i.e. 0 V. By definition, therefore, electricity is anything that develops a potential difference with respect to this -- it will generate an electric field to move charges around, and the charges feel a force as a consequence. Conveniently, unless you live in a desert soil is itself quite conductive, and a long (typically >4 m long) rod of metal driven directly into it. Often your power company will do this for you as the niggly details about "how big", "how many" and "how wet is the soil" turn out to be quite important, and also quite complex. Concrete doesn't easily work as it's an insulator: wet, loamy soil works brilliantly. People have written whole books and PhD theses about how to deal with granite, sand, and similarly frustrating geology. Ordinarily, in most power distribution systems around the world, the flow of charges -- the electrical current -- doesn't actually use this ground all that much. You can think of it coming in via live and leaving via neutral, if you want. The permanent earth is a protective feature to ensure that a very large current _could_ flow to the ground if it got the opportunity to, and _not_ flow through you. This is a safety device in disguise, as it'd trip a circuit breaker and protect the wires in your house from catching fire. Mains electricity is a bit different: because of how generators work, it turns out to be easier to generate (pun not intended) three _different_ live wires all at the same time, called L1, L2, L3 or three different phases. They are all AC voltages, developed with respect to each other, 120º out of phase. At the substation that transforms these to lower voltages, the central tap of the transformer is hammered into the ground, and your neutral wire -- which is also connected to that central tap of the transformer -- therefore is about ground too. This provides a nice, handy, readily accessible common reference that both prevents you frying and lets your electric hob continue doing so at the same time.
• kd5bjo 7y ago
  Whenever you have two electrical conductors near each other, but not touching, you have a capacitor. Electrical transmission lines are obviously conductive, but so is the earth, with its large amounts of metal and salt water. When extra electrons collect in the transmission lines[1], they become negatively charged and repel electrons on the surface of the earth, leaving unpaired protons and a positive charge there. As there are two oppositely charged objects in proximity to each other, an attractive force exists between them. This force is generally referred to in terms of voltage because it’s very small in absolute terms, but electrons are also very light and thus strongly affected. The electrons can’t make it across the gap to the positively-charged Earth, however, because air is an insulator. That means that all of its protons and electrons are bound tightly to each other, so there’s no intermediate place for the electrons in the wire to jump to, and they have to make it all the way across the gap in one go, which is extremely unlikely. By driving a metal stake into the ground, you’ve provided a path along which the electrons can move little at a time in the direction that the electrical force is pulling them. When they arrive, they’ll pair up with a proton and reduce the attractive force a little bit. Over time, this prevents the large buildups that can become dangerous. [1] Electrical storms and lightning strikes are the most dramatic causes, but not the only ones.
• brandmeyer 7y ago
  I can answer a narrow part of your inquiry: Why do you need to sink the rod into the dirt, and merely touching it isn't good enough? Dry dirt is a very poor conductor. Wet dirt is pretty good, thanks to the dissolved salts, but still not great compared to copper. Most dirt is somewhere in-between, so the amount by which you must sink your ground rod to be safe partially depends on your local climate. Lets say you have a 3/16" grounding rod sunk three feet into the dirt. The cross section through the rod at the top of the rod is ~0.025 inch^2. But the total contact area on the local dirt is ~42 inch^2. That's an increase in area by a factor of over 1500x. The local dirt doesn't need to be particularly conductive to work as a reasonable conductor when you have that much contact area to work with. The total cross sectional area of the dirt-conductor around your ground rod also gets larger quickly, proportional to r^2. That term gets large rapidly with distance, such that almost all of the resistance between the planet as a whole and your ground rod is concentrated at its interface to the dirt.