# What is ground and what does it do?

I'm a bit confused about the concept of ground, and perhaps voltage as well, particularly when trying to analyze a circuit. When I learned about Ohm's law in grade school, I learned how to apply the law to calculate current, voltage, and resistance of simple circuits.

For instance, if we were given the following circuit:

We would be could be asked to calculate the current passing through the circuit. At the time, I'd simply compute (based on the rules given) 1.5V/1Ohms=1.5A.

Later on, however, I learned that the reason the voltage of the resistor would be 1.5V is because voltage is really the difference in potential between two points, and that the difference of the voltage across the battery would be the same as that of the resistor (correct me if I'm mistaken), or 1.5V. I got confused, however, after the introduction of the concept of ground.

The first time I tried to do the current calculation for a circuit similar to the previous circuit on a simulator, the program complained about not having a ground and "floating voltage sources". After a bit of searching, I learned that circuits need ground as a reference point or for safety reasons. It was mentioned in one explanation that one can pick any node for ground, although it's customary to design circuits so there is a "easy place" to pick ground.

Thus for this circuit

I picked ground at the bottom, but would it be okay to pick ground between the 7 ohm and 2 ohm resistor - or any other place? And what would be the difference when analyzing the circuit?

I've read that there are 3 typical ground symbols with different meanings - chassis ground, earth ground, and signal ground. A lot of circuits I've seen used in exercises either use earth ground or signal ground. What purpose is there in using earth ground? What is the signal ground connected to?

Another question: since the ground is at unknown potential, wouldn't there be current flowing to or from ground to the circuit? From what I've read we treat the ground as 0V, but wouldn't there be some sort of effect because of a difference in potential of the circuit and ground? Would the effect be different depending on what ground was used?

Finally: In nodal analysis, one customarily picks a ground at the negative terminal of the battery. However, when there are multiple voltage sources, some of them are "floating". What meaning does the voltage of a floating voltage source have?

• possible/partial duplicate: electronics.stackexchange.com/questions/51478/… Jan 26 '13 at 21:00
• Sorry - I tried to check for duplicates before asking this question but I missed that... Jan 26 '13 at 21:50
• Voltage is the DIFFERENCE in energy, therefore it doesn't matter which potential/energy is at ground. Jan 26 '13 at 21:55
• Voltage is the difference in potential. There's no energy involved until you bring a charged particle into the picture. Jan 26 '13 at 23:14

The first time I tried to do the current calculation for a circuit similar to the previous circuit on a simulator, the program complained about not having a ground and "floating voltage sources".

Your simulator wants to be able to do its calculations and report out the voltages of each node relative to some reference, rather than have to report the difference between every possible pair of nodes. It needs you to tell it which node is the reference node.

Other than that, for a well-designed circuit, the "ground" has no significance in the simulation. If you design a circuit where there is no dc path between two nodes, though, the circuit will be unsolvable. Typical SPICE-like simulators resolve this by connecting extra resistors, typically 1 GOhm, between every node and ground, so it is conceivable that the choice of ground node could artificially affect the results of a simulation of a very high-impedance circuit.

I picked ground at the bottom, but would it be okay to pick ground between the 7 ohm and 2 ohm resistor - or any other place? And what would be the difference when analyzing the circuit?

You can pick any node as your reference ground. Often we think ahead and pick a node that will eliminate terms for the equations (by setting them equal to 0), or simplify the schematic (by allowing us to indicate connections through a ground symbol instead of by a bunch of lines connecting together).

I've read that there are 3 typical ground symbols with different meanings - chassis ground, earth ground, and signal ground. A lot of circuits I've seen used in exercises use earth ground or signal ground. What purpose is there in using earth ground? What is the signal ground connected to?

Earth ground is used to indicate a connection to something that is physically connected to the ground beneath our feet. A wire leading through the building down to a copper rod driven into the ground, in a typical case. This ground is used for safety purposes. We assume that someone who handles our equipment will be connected to something like earth ground by their feet. So earth ground is the safest circuit node for them to touch, because it won't drive currents through their body.

Chassis ground is just the potential of the case or enclosure of your circuit. For safety purposes it's often best for this to be connected to earth ground. But calling it "chassis" instead of "earth" means you haven't assumed that it is connected.

Signal ground is often distinguished from earth ground (and partially isolated from it) to minimize the possibility that currents flowing through the earth ground wires will disturb measurements of the important signals.

Another question: since the ground is at unknown potential, wouldn't there be current flowing to or from ground to the circuit?

Remember, a complete circuit is required for current to flow. You would need connections to earth ground in two places for current to flow in and out of your circuit from earth ground. Realistically, you'd also need some kind of voltage source (a battery, or an antenna, or something) in one of those connection paths to have any sustained flow back and forth between your circuit and the earth.

However, when there are multiple voltage sources, some of them are "floating". What meaning does the voltage of a floating voltage source have?

If I have voltage source with value V between nodes a and b, it means that the voltage difference between a and b will be V volts. A perfect voltage source will generate whatever current is required to make this happen. If one of the nodes happens to be ground, that gives you immediately the value at the other node in your reference system. If neither of those nodes happens to be "ground" then you will need some other connections to establish the value of the voltages at a and b relative to ground.

• Thank you very much for the detailed response. One question about ground - if a circuit has multiple ground points, they are assumed to be at the same node. If I physically tried that (stuck some metal poles in the ground and formed a complete circuit), would the behavior exhibited be comparable to that of treating all points in ground as a single node? Jan 26 '13 at 22:34
• In most circuits, it's better to connect all those points with copper in your circuit itself. In some circuits its extremely important to have a very low-impedance connection between the ground points and then you need a full minimally-interrupted copper ground "plane" layer in your circuit board. For others, like possibly some machinery circuits, or power transmission, you might be able to get away with a connection through the earth. Jan 26 '13 at 23:12
• Some power distribution systems do just that - use a single wire and the earth as the return path. Jan 27 '13 at 0:18
• @inkyvoid: Multiple ground points are assumed to be at the same node, and this is used liberally to simplify schematics drawings. But it is important to keep in mind that this is an abstraction, which neglects resistance, inductance and capacitance between the different ground points. There are often real-world situations where the neglected parameters matter, and then the convenient abstraction starts to become untrue. At this point, ground actually becomes an electrical network whose parameters must be taken into account.
– sh-
Apr 11 '19 at 13:57

Sometimes people get confused just by the many definitions of the word.

ground
noun

1. the solid surface of the earth; firm or dry land: to fall to the ground
2. Often, grounds. the foundation or basis on which a belief or action rests; reason or cause: grounds for dismissal.

In the context of electronics, sometimes ground means sense 1 above. Earth is, after all, approximately a $6\cdot10^{24} kg$ ball of iron. Like everything else, it exists at some electric potential, and if you stick a long conductive rod in Earth, you can make other things connected to that rod at approximately the same potential:

Of course, Earth is really big. Not all of it is at the same potential. In fact, not even close. Earth's huge magnetic field is constantly changing, and induces currents all over Earth. Other people have their own rods stuck in Earth and put currents in Earth. Lightning moves tremendous current in Earth. Since Earth is not a perfect conductor, and by Ohm's law any current through any resistance much be accompanied by a voltage, the potential between two points on Earth is not the same, unless you are lucky, or the points are very near each other.

And, if you've ever operated a battery powered device, you know that electronic devices can function perfectly well without a connection to Earth. Yet, these devices do have a ground. So, this is probably not the sense of ground you should use for your basis of electrical understanding. The other sense, the basis on which a belief rests, is probably a better start.

It's a very astute observation that your confusion involves voltage, as well. Ground is, simply put, $0V$. But to understand what this really means, one must really understand voltage. Many people fall into the trap of thinking that since ground is $0V$, then ground is where there is no voltage. Thus, there must be voltage everywhere else. But, once you understand voltage, you see this can't be true.

So what is voltage? The more rigorous term for it is electric potential difference. All three words are part of the understanding of voltage. Electric is obvious.

What about potential? Potential has specific meaning in physics. Potential energy is the capacity for some arrangement of things to do work. For example, a compressed spring, a stretched bow, or a high-pressure tank of gas have the potential to do work, if released.

Imagine a ball at the top of a ramp. If the ball is allowed to roll down the ramp, at the bottom, it will be moving quite fast. It acquired this kinetic energy from the potential energy it had at the top of the ramp. If there were no other losses (friction, for example), then the kinetic energy gained by the ball is equal to the potential energy it lost, by the law of conservation of energy.

That's potential energy. Just potential by itself has a different definition: it is potential energy per unit of stuff at some point in a system. Obviously, a massive ball at the top of the ramp has more potential energy than a small ball at the top of the same ramp. So, the two balls have different potential energies at the top of the ramp, but they are at the same potential.

The relevant kind of stuff it is depends on the kind of potential. For gravity fields, the stuff is mass. For electric fields, the stuff is charge. Potential energy is measured in joules. Gravitational potential is measured in $J/kg$. Electric potential would then be measured in joules per coulomb ($J/C$), which actually, is exactly the definition of the volt.

So earlier we said voltage is electric potential difference. What's the difference? Imagine again our ramp. If you assume that gravity is equally strong anywhere on Earth (this is only approximately true, but is a valid simplifying assumption for much practical engineering), then does the location of the ramp matter? It could be in Death Valley or on Mount Everest: the ball, after rolling down the ramp, will have at the end the same kinetic energy. The potential at the top and bottom of the ramp is irrelevant; the important thing is the difference in potential between the top and the bottom. If we are assuming that Earth's gravity field is the same wherever we might take this ramp, than just the height of the ramp is relevant.

So, since voltage is a difference, we need two points to have a voltage. If we say some node in a circuit is $5V$, then we are saying it's $5V$ more than some other point. Ground is that other point, unless context says otherwise.

A similar convention exists with height. If I say the height of Mt. Everest is $8848 m$, you will assume I mean its height is $8848m$ more than sea level. I can also override this reference with explicit context. For example, I can say Mt. Everest is $237m$ higher than K2. The default reference can change, also. For example, if I say that Olympus Mons is $21229 m$, you probably don't assume this is above sea level, but instead some equivalent datum on Mars. There is no universal reference for elevation.

This is why ground is $0V$, just as sea level is $0m$. It's not that ground has no voltage, or sea level has no elevation: it's that these things are differences, and the difference between a thing and itself is $0$. Thus, there is no magic about ground. It doesn't do anything. It's just a node in the circuit, just like any other. It's only by definition that it is also $0V$, and this definition exists just as a convention to simplify our discussion of a circuit. There is no universal ground or $0V$ until we define something as such. Usually, it's just whatever we decide to stick the ground symbol on. We can put it anywhere we like, but we usually put it where it makes calculations easiest and discussion simplest.

Related questions:

• +1 An excellent answer, should be referenced by anyone looking for an explanation of voltage. Aug 14 '13 at 4:40
• Can you tell me why, "the two balls have different potential energies at the top of the ramp, but they are at the same potential." I thought they'd be different because of the mass. Oct 23 '16 at 18:35
• @johnny Their potential energy is different. Their potential (no energy, just potential) is the same. Oct 23 '16 at 22:23
• Very thorough and lucid answer. Sep 8 '17 at 10:19
• Saying "ground has no voltage" is meaningless. Has no voltage with respect to what? To itself, yes, but in general to any other point, no, it does have voltage. Feb 16 '20 at 18:03

See my answer here about what ground is and how the term "ground" is used in electronics. Significant parts of that answer are also relevant to the question here.

# Various definitions of ground

I'll assume you know the difference between electric potential energy, electric potential, and voltage/electric tension/electric pressure/electric potential difference.

The word ground is used for many different things. I think the following is an exhaustive list:

1. In electronics and circuit theory/analysis, the ground is the node with respect to which all other nodal voltages are measured, regardless of what is the electric potential of the ground node. In this context, it’s better to call this node as the reference node/point or the common node/point, but unfortunately hardly anyone calls them like that (I say it's unfortunate because it gives the word ground even more meanings).

Choosing this reference node is what we do in nodal analysis as first step, and what we do when measuring voltages of a physical circuit with an oscilloscope, and what all or most circuit simulators (LTspice, PSpice, Multisim, etc.) do or require us to do (by placing a ground symbol and connecting it to at least one node).

Unfortunately, many textbooks on circuit analysis and probably all circuit simulators use the ground symbol to indicate the reference node, even if such node is not grounded (read definition #4; the simulators should use a different symbol which we should call the reference node symbol, to distinguish it from actual ground (read definitions #2 and #3).

2. The ground is the node or point in space defined as zero volts of electric potential (not zero volts of voltage). In electromagnetic theory, it is necessary to choose a point and define as 0 V of electric potential, before calculating the potential at any other point in space. Usually, we choose the Earth ground (read definition #3) or a point infinitely far from the region of space under study.

By connecting or bonding one terminal of a wire or conductor of small resistance or impedance to the ground node, the other terminal of such wire will be very close to the same potential as the ground wire. (It is not exactly at the ground potential because real wires have resistance/impedance, unlike ideal wires and superconductors.)

Note that definition #2 says ground is the point defined as zero volts of electric potential, not of voltage. The reason is because saying “this arbitrary node has zero voltage” doesn’t make sense from that sentence alone. Voltage is a measure between two points in space (and in general the path between the two points), but if you say “the ground node has zero voltage”, the follow-up question is “with respect to what other node?”, because it makes no sense to talk about the voltage at a single point (similar to height at a single point). If your answer to the previous question is “with respect to itself”, then that definition of ground wouldn't be useful, because all nodes have zero voltage with respect to themselves, so according to that definition all nodes are ground. So don't say the ground has zero voltage, it has zero electric potential. If you’re in the US and measure the voltage of the live wire with respect to itself, the voltmeter will read zero volts, not 120 V RMS nor 240 V RMS.

3. The ground is the ground of the planet Earth, as in where we stand; the soil. This is also called AC ground.

4. The verb to ground (grounding) (US terminology) or to earth (earthing) (UK terminology), which means to connect/bond a device/load/component to the Earth ground (definition #3).

5. In the US, the main breaker panel of a house has a ground busbar, connected to the neutral busbar, and separated from the neutral busbar in subpanels. Read this answer.

6. The ground busbar (read definition #5) of the main panel of the circuit breaker is grounded (read definition #4) by using a ground rod outside the house/building. Power systems and substations also have ground rods.

7. Appliances have a ground/grounding wire (US terminology), besides the two live/hot/active wires (for 240-V appliances) or besides the live wire and the neutral/grounded wire (for 120-V appliances) in the US. For this definition as well as #5 and #6, watch this video, this video and this video.

8. There is a condition in a circuit known as a ground loop. Watch this video, this video and this video. Because in the US many distribution transformers have the middle terminal of the low-voltage split-phase winding grounded (by using a ground rod), then technically speaking there are many ground loops in the US power grid.

9. The Earth ground symbol (for the definition #3), which has three horizontal parallel line segments of decreasing length; the chassis ground symbol (for the chassis or enclosure of an equipment or the car, even if it's not grounded [read definition #4]), which has diagonal parallel line segments; the *ground symbol/signal ground symbol (for the definition #1, even if it's not grounded), which is a hollow or filled triangle. Read this answer and this webpage.

Using multiple signal ground symbols in a circuit diagram/schematic allows to draw the diagram with less wires, making it easier and faster to read.

10. In electronics, the ground node, ground rail or just ground is the node or rail of the circuit where the negative terminal of the DC supply or battery is connected, even if such node is not grounded (read definition #4). It usually has one terminal of many devices/components connected to it. It is also called DC ground. Unfortunately, we may use the ground symbol (read definition #9) to indicate such node (as well as in nodal analysis and circuit simulators for the reference node [read definition #1]), which may give the impression that such node is grounded, when In reality it may not be.

11. The ground pin of an integrated chip.

Note the definitions that use the word ground alone which can be confused are definitions #1, #2, #3 and #10.

The first time I tried to do the current calculation for a circuit similar to the previous circuit on a simulator, the program complained about not having a ground and "floating voltage sources". After a bit of searching, I learned that circuits need ground as a reference point or for safety reasons. It was mentioned in one explanation that one can pick any node for ground, although it's customary to design circuits so there is a "easy place" to pick ground.

Correct. In the above sentence, your first use of ground (in "as a reference point) is as defined in def. #1, so I'll instead call it reference node; your second use (in "or for safety reasons") can be as defined in defs. #5, #6 and #7 (they're related; watch the videos in def. #7).

I picked ground at the bottom, but would it be okay to pick ground between the 7 ohm and 2 ohm resistor - or any other place? And what would be the difference when analyzing the circuit?

Yes, it would be okay. In general, all nodal voltages would change, but the element voltages (i.e. voltages across devices, such as all types of ideal independent and dependent sources, resistors, capacitors, inductors, diodes, etc.)

The general approach for the analysis would be the same (apply KCL at all nodes except the reference node, use elements' voltage-current equations, derive auxiliary equations for dependent sources and supernodes, write element voltages in terms of nodal voltages, solve), except the fact that the specific expressions (or numerical values) would be different.

I've read that there are 3 typical ground symbols with different meanings - chassis ground, earth ground, and signal ground. A lot of circuits I've seen used in exercises either use earth ground or signal ground. What purpose is there in using earth ground? What is the signal ground connected to?

As I said in def. #9:

• The earth ground symbol is used for def. #3 (i.e. indicating a connection of the node with the Earth ground).

• The chassis ground symbol is used for indicating a connection of the node with the chassis or enclosure of an equipment or the car, even if it's not grounded (read def. #4).

• The ground symbol/signal ground symbol is used for def. #1 (i.e. for indicating the reference node for nodal analysis, circuit simulators and measuring voltages with an oscilloscope), even if it's not grounded.

Another question: since the ground is at unknown potential, wouldn't there be current flowing to or from ground to the circuit? From what I've read we treat the ground as 0V, but wouldn't there be some sort of effect because of a difference in potential of the circuit and ground? Would the effect be different depending on what ground was used?

Whether current (charge or charged particles, to be accurate) flows through the ground symbol in a circuit diagram depends on how the circuit is drawn. A given circuit can be drawn in different ways, and in some ways current will flow through the ground symbol while in others it won't. Read this answer I wrote for an example and illustrations.

Finally: In nodal analysis, one customarily picks a ground at the negative terminal of the battery. However, when there are multiple voltage sources, some of them are "floating". What meaning does the voltage of a floating voltage source have?

In the above sentence, you used ground as defined in def. #1, so I'll instead call it reference node.

Consider two super simple circuits (one voltage source in series/parallel with a resistor, for each circuit), which are conductively isolated from each other (i.e. there are no wires connecting any nodes between them). Suppose you choose the node of the negative terminal of the voltage source of one of the circuits as reference node (read definition #1), as shown in the following figure. Then, it is impossible to determine the nodal voltages of the other circuit. You or a circuit simulator wouldn't be able to solve the equations for the nodal voltages, because there wouldn't be any known equations relating the voltages or currents between both circuits.

To solve this problem, you could connect the node of the negative terminal of the voltage source of the second circuit to that of the first circuit, as shown in the following figure.