# Why does a bulb not glow when one terminal of a bulb is connected to the negative terminal of a battery and other is connected to the ground?

I am a high school student and has been wondering a while about this:

Why does a bulb not glow when one terminal of a bulb is connected to the negative terminal of a battery and the other is connected to the ground?

My teacher said that since we don't have a closed circuit there wouldn't be any current and thus no light, but if there is no current how does grounding work?

Another situation that I am confused about is when a person with wet hands is holding a high-voltage battery and touching only one pole. The person is connected to the ground but is not electrocuted. Why? Isn't there any current moving from the battery to the ground? Would the person only get electrocuted when touching both ends?

• There's only current flowing from your HV battery to ground if it is connected to ground. Mar 19 at 22:19

## 9 Answers

Why does a bulb not glow when one terminal of a bulb is connected to the negative terminal of a battery and the other is connected to the ground?

Because this way, electric charge on the whole conducting path from the ground to the negative battery terminal redistributes so that the conducting path has the same electric potential everywhere. The resulting equilibrium state is such that negative terminal of the battery is grounded, both bulb terminals are grounded, and thus there is zero potential difference between the bulb terminals.

When one terminal of the bulb is connected to the negative battery terminal and the other bulb terminal is connected to the positive battery terminal, the battery maintains non-zero potential difference between those bulb terminals, and there is current flowing through the bulb.

The battery won't maintain potential difference on the bulb terminals, when just one bulb terminal is connected to the battery, and the other bulb terminal is on the ground.

Sometimes one pol of a batterie is called 0, and ground is also 0. but the zero at the batterie is not the zero of ground. The voltage of a batterie of say 3V just says one pol is 3V higher than the other, if you connect the + pol to ground the - pol is -3V to ground, if you connect the -pol to ground the other pol is+3V, if you do not connect one pol to ground there is no voltage between ground and one pol of the batterie.

"Ground" is one of those ambiguous words with a few different meanings.

It's simplest to think about static electricity. We know the planet has a pretty balanced electrical charge, neither positive nor negative (otherwise the solar wind would neutralize it, or we'd all float off into the air). Connecting something to the planet is a reasonable way to prevent it building up a static charge so it can't zap you. (You can still build up a charge and zap it, however, which feels exactly the same)

In a home electrical system "grounding" is used a different way. Metal objects (like the outside of metal computers) are connected to the ground wire. If a live wire touches a metal object, it sparks and trips the circuit breaker. We do this because that's better than the alternative. But this mainly works because the ground wire is connected to neutral inside the electrical panel - it's not only connected to ground. We're really talking about neutralling, not grounding. (So why don't they just use the neutral wire? Because that would be really unsafe if the neutral wire would get broken accidentally. A lot of the electrical code is about what if stuff goes wrong.)

Okay, so if we just need neutralling, why does it have to be connected to the ground with this big metal pole (grounding electrode)? It makes sure the electrical system's ground is about zero volts compared to the earth ground. You don't want to stand on the ground, and touch a metal computer, and get a fatal zap. (The electrical system is big with lots of capacitance, and uses AC which can leak through capacitance. It could hurt you more than static.)

What about battery-powered circuits then? Do they have ground? Actually, yes, kinda. You see, when engineers say "ground" it's often just shorthand for "zero volts". And zero volts is whatever you say it is. Usually in a battery-powered circuit it's the negative end of the battery, so in this case "connect to ground" just means "connect to the negative end of the battery".

To your other question: if you touch the negative terminal and the dirt-and-rocks ground why don't the electrons just flow out of the battery and into the ground?

They do... but only a few. Then the battery is positively charged which keeps any more electrons from going out. To make it keep going, you need to return the electrons to the positive side. In fact, so few electrons go out that you'd hardly notice it even if the battery was quite high-voltage.

• Why would the negative end of a battery be ground? Does this mean that touching the positive end (and not the negative one) would zap me? (I think you mean the right thing, but this paragraph is misleading.) Mar 19 at 23:19
• @PaŭloEbermann we call it that because we want to call it that because we're lazy and "ground" is 1 syllable while "zero volts" is 3. Mar 20 at 16:09
• @PaŭloEbermann you can get a zap from either terminal of a battery, and that's called "static electricity" and generally not harmful. Connecting either terminal to the planet will prevent the battery from building up static electricity, just like anything else. (However, you could still build up a charge and zap it). Mar 20 at 16:10

Current flows through the "ground" (which is more often metal than earth). So, you have to ground the positive terminal of the battery to make this work.

Electric current flows in circles: that's why we call electrical networks "circuits". No circle, no current flow.

• If you reach up and grab an overhead power line you might be shocked to death. Where's the circle? Mar 19 at 21:37
• @Ken-EnoughaboutMonica Outside your house, there is a ground stake. That connects to the tap on the pole transformer that is intended to be at zero potential. On the other side of the transformer, there is also a ground connection. This is repeated throughout the electric network. It's to control the potential of the various segments of the network and shunt unwanted current, including lightning, to ground. Otherwise, for example, a leaky transformer could inject 10kV relative to your grounded plumbing into your house, very dangerous. Mar 19 at 21:57
• @Ken-EnoughaboutMonica If you are standing on the ground and touch an HV line, the return current flows back into the network through these ground connections. Mar 19 at 21:58
• If you hang from the wire without touching anything else, you wouldn't be shocked, aside from a tiny parallel-path current through your hand. This is why birds are able to sit on power lines without dying. Mar 20 at 0:34

A battery is an electrochemical device that is designed to provide current from the positive terminal to the negative terminal of the battery at a certain potential difference. If one assumes the negative terminal to be at 0 volts, then the negative terminal would be ground for the circuit. But no current will flow unless the two are connected.

Why does a bulb not glow when one terminal of a bulb is connected to the negative terminal of a battery and the other is connected to the ground?

For the battery to deliver current to the bulb the other terminal of the bulb needs to be connected to the positive terminal of the battery instead of the ground to complete the circuit, i.e., to allow current to return to the positive terminal of the battery.

My teacher said that since we don't have a closed circuit there wouldn't be any current and thus no light, but if there is no current how does grounding work?

"Grounding" means different things and can serve different purposes. However I am not aware of the Earth being intentionally used to complete a circuit for the purpose of operating a load, such as a light.

Among the possible uses the term grounding are: (1) Connecting an electrical power system to the Earth for the purpose of limiting voltages on the power system due to events such as lightning (2) Connecting the metal housings of electrical equipment to earth to reduce the risk of electric shock, and (3) reference to a point in a circuit that is designated to be at zero or “Ground” electrical potential while having nothing to do with the Earth.

Another situation that I am confused about is when a person with wet hands is holding a high-voltage battery and touching only one pole. The person is connected to the ground but is not electrocuted. Why?

The other terminal of the battery is isolated. For current to flow through the person, the other terminal of the battery has to be connected to ground to complete the circuit through the Earth and back to the battery.

Would the person only get electrocuted when touching both ends?

Current will flow through the person if one hand is touching one battery terminal while the other hand is touching the other terminal because there would be a complete circuit involving the person and the battery.

Whether or not the person would get electrocuted would depend on the magnitude of the voltage, available current from the battery, condition of the skin (dry, wet, moist), contact area, etc..

Hope this helps.

• "single-wire ground return" is a real thing where the Earth is used to complete a circuit for the purpose of operating a load, etc. Mar 19 at 21:43
• @JamesK Can you give a specific example so I can include it. As I said, I am not aware of using the earth to carry load current. Mar 19 at 21:48
• Mar 19 at 22:09
• @JamesK Thanks for the link. Very interesting. However, the schematic of the SWER does not show the earth carrying load current, which is the current that would be drawn by the loads for final customer from the 240/480 vac secondary . It instead carries the current of the 19 Kv primary. For a 15A 480 vac load current the earth current of the primary of an ideal transformer would be 0.39 A . Mar 20 at 3:15

Electricity flows in a complete circuit. For a battery-powered circuit, that means between the negative and positive poles. For a mains AC powered circuit, that means either between two hot wires (240V in the US) or between a hot wire and a neutral wire (120V in the US).

There is a big exception: Natural electricity - from static electricity on up to lightning - makes a complete circuit by going to the actual physical "ground". That's why being in the wrong place in a thunderstorm is a problem - if you are tall then the lightning may hit you because you are a closer target than the ground. On the other hand, lightning rods work by being taller (and more conductive) than the nearby buildings.

So what's the deal with wet hands? Dry skin is, fortunately, a good insulator. So is most dry clothing. But most wet things, including skin and clothing, are pretty good conductors. But even then, one wet hand won't do much, because if the electricity can get into you but can't complete a circuit then very little electricity will actually get into you. On the other hand (literally) if you have a second hand that is wet and it touches another part of a circuit (e.g., one hand touches AC hot and one touches AC neutral, or one touches battery negative and one touches battery positive) then you can complete a circuit. The problem is that if even a little bit of electricity goes from one hand to the other (or wet hand to wet foot on the ground) then it can stop your heart and kill you. That is what GFCI, Ground Fault Circuit Interrupters are all about, and water is why they have been required the longest in kitchens and bathrooms.

With building (mains) AC power, the ground (both the wires called "ground" and the actual ground via a "ground rod") are connected together and connected to neutral (in the US, at least). This has some major safety benefits, but it also means that while the ground is not a great conductor, it is able to conduct some, so wet hand on hot wire + wet feet on floor = some electricity can flow hot to ground to neutral and complete a circuit.

With a battery, even with wet hands and feet there may not be much of a way for current to get back to the battery. After all, if there was then the battery would be in constant use sending current through all sorts of things through the ground. As a result, touching one pole of a battery usually isn't very dangerous. In addition, most batteries that ordinary people encounter on a regular basis are low voltage (< 50V) and much less dangerous than higher voltage (AC or DC). That being said, there are a few exceptions:

• In almost any car, there is a 12V battery (but some are 6V or 24V) which has one of the poles (typically negative, but not always) connected to the metal frame of the car. This makes wiring easier as only one wire has to go to each device and the other half of the circuit is simply through the frame of the car. That doesn't work as well for AC or for high current DC, but for low current DC it works just fine. But that means that if you touch the other pole of the battery and any metal in the car and your hands are wet you will be in for a shocking experience. That is also why following the car manufacturer's instructions for jump-starting is critical - it is the only time most people, except mechanics, do anything dangerous with a car battery.
• Electric cars have much higher voltage batteries. They also have a low voltage battery, but the main battery (e.g., 50 kWh, 100 kWh or even more) is at a higher voltage and quite dangerous. But this battery is very well protected and you can't get at it easily under normal circumstances, except when charging. The charging cables are quite a bit more complex than a regular appliance power cable, which prevents significant current from flowing until after the connector is properly connected.
• Even some of the smallest batteries can cause problems. In particular, standard 9V batteries have the positive and negative terminals right next to each other. That is great for building compact electronic devices. But it also means that when thrown away they can much more easily short-circuit and start a fire than most other small batteries.

According to the conductor laws when a body is grounded there is charge distrobution i.e. charge may flow from the earth to the body or vica versa , such that there is constant potential everywhere.

When one terminal of the bulb is connected to the negative terminal of the battery and the other terminal of the battery is connected to the ground charge redistrobution takes place such there is no potential difference across the battery so the bulb does not glow

If you connect one terminal of a light bulb to the negative terminal of a battery and the other terminal of the light bulb to ground, electrical current will flow, but only for a tiny fraction of a second. As electrons flow out of the negative terminal of the battery, the battery acquires a positive charge since negative charges are leaving it. This positive charge attracts electrons, making it harder for more electrons to leave the battery. Eventually, the attractive force of the positively charged battery becomes equal to the batteries potential, so no more electrons can leave the battery, and the electrical current stops. This happens very quickly, since it takes relatively few electrons leaving the battery to build up a charge.

This is why the light bulb should be hooked up in a circuit. Returning electrons to the battery prevents the battery from acquiring a positive charge due to losing electrons. This is also why a person with wet hands is not shocked when touching a single terminal on a battery (and why the unpleasant way of testing a 9-volt battery requires licking both terminals).

Going back to the light bulb and battery, if you were to connect the positive terminal to the ground as well, the light bulb would stay lit. This is because the battery could pull electrons from the ground into the positive terminal to maintain its neutral charge and prevent the charge buildup that stops the electrical current through the light bulb. However, running current through ground is a bad idea. The situation is called a ground loop and wreaks havoc on systems more complicated than a light bulb and risks dangerous short circuits.