Confused Battery Question

alan4cult

Ok I understand that the Earth is taken to have potential of 0 Volts. So if I connect one end of my voltmeter to the positive terminal of a 9V battery and the other end into the ground will it read 9 volts or am I confused?

p.s. ironically my multimeter is out of battery so I'm going to test my theory tomorrow but I'm not sure it's right?

alan4cult

Ok, it didn't seem to work. Can anybody explain it to me.

I thought that the ground was at 0 potential and that a 1.5V battery had a 1.5 potential at it's positive terminal so how come I cannot detect a potential drop between the positive terminal and the ground (an earth rod)?

spideog7

It's all relative. The positive terminal of your battery is at 9 volts relative to the negative terminal. Which is fine when you're dealing with batteries, however if you're dealing with a much bigger power system like for example the national grid then you need a more standard reference and that is where the earth comes in. The earth can sink an infinite amount of current in theory so it's a good one to use. So relative to earth the positive pin in your plug is at 220v. This basically means that the ground point of your circuit is in fact the ground and the "negative terminal" is connected to ground at the source so to connect it together you just connect the negative terminal at this end to ground too.

Confusingly the term ground is used interchangeable but it merely means the common reference in the system. So if you see a ground point in a circuit that's powered by a battery then that's the negative battery terminal.

It's a fairly basic concept so Wikipedia or even a real Encyclopedia will probably be able to help you out here.

alan4cult

^^ Thanks for your reply. I understand a bit better now but I have another question. If the electricity can travel through the earth what is the point in having a neutral wire? Could we not just let all the electricity flow back to the power station via the Earth (i.e the ground we stand on)?

spideog7

I don't know a whole lot about mains supply but AFAIK your "neutral" is your earth connection, the actual "earth" in your plug is there for safety reasons, to prevent voltage leaks, onto chassis etc. It is not feasible to have to make a ground connection every time you want to plug in an appliance therefore it's just done once (or perhaps several times) for your house and looped via the neutral.

The earth wire should never carry current whilst the neutral does. If current is detected in the earth wire your ELCB or GFCI will "trip" on your circuit board and disconnect power.

As you can imagine it's important to ensure that the neutral is properly earthed to ensure it's voltage doesn't rise (since the neutral does carry current), this is why there are regulations relating to the placing of earth rods and the size that they must be. You'll also notice frequent groundings along the electricity lines and at transformers.

Long story short don't mix up mains supply with battery supply they are essentially two very different systems. Even though the principles are similar, when you extrapolate the battery model out to a much larger system then you can no longer ignore certain aspects that had little or no effect on your small scale battery system (mains is also AC).

Hope this helps.

spideog7

I don't know if I actually answered your question as regard letting the current flow back through the actual ground. The earth isn't really the greatest conductor it's just a good reference point, it's much more convenient (or at least it works better) to have a 2 wire system with one of them referenced to ground.
I believe this also allows the system to be controlled a little better to prevent frequency variations etc.

Michael Collins

alan4cult wrote: »

Ok, it didn't seem to work. Can anybody explain it to me...

You need a complete circuit basically is the reason. If one end of the battery had been connected to ground (a good quality connection would need to be made - a stake driven a few metres into the gound at least) then you measured the voltage between the other end and ground (again, a good quality ground - touching it off the concrete probably won't do it) it would work - you now have a complete circuit.

What do you think would happen if you connect one end of your multimeter to the live wire coming into your house (~230 V) and make a good quality ground with the second probe? It will give a reading close to ~230 V all going well! This is because right outside your house, the neutral is driven straight into the ground, along with the earth wire. Hence you have a complete circuit again.

alan4cult wrote: »

^^ Thanks for your reply. I understand a bit better now but I have another question. If the electricity can travel through the earth what is the point in having a neutral wire? Could we not just let all the electricity flow back to the power station via the Earth (i.e the ground we stand on)?

It is done that way sometimes, particularly in Australia. It's called Single-Wire Earth Return (SWER). There's an article on wiki here. It saves the cost of a return wire obviously, but isn't quite as efficient as your standard metal neutral, and so some energy is lost in the ground.

watty

The Earth is very high resistance, especially if dry. Any power system needs two wires.

Using earth for return was done a bit in 19th century Telegraphs, but to get higher speed and distance they needed a much lower resistance, so the double expense of two wires was then the norm.

In UK they connect neutral (return wire) to earth mainly at the Electricy Substation (transformer from kV to 220V). In Ireland a ground spike (about 1.2m x 10mm) is buried/hammered in near the meter box and connected to Neutral wire. Two wires go to the substation transformer as in UK.

On building sites they use a 220V to 110V isolation transformer and earth the centre tap. This means the voltage to a worker from a damaged cable or tool is no more than 55V. It's illegal to operate otherwise on a building site.

In the USA they use 220V hardwired connections for water heaters, stoves, washing machines, Microwaves and often kettles. Only lower power devices can use the plug, which is a 110V system. I don't know how they do the earthing.

On power systems the Earth or Ground is for Safety only. The SWER is very rare and can only deliver low power. It needs a very high voltage isolation transformer at each end with a large array of very large earth spikes. Two wires at the same voltage can deliver 100s of times the number of users or power.

All wired communications systems use a balanced pair of wires for go & return. Only Radio Transmitters nowadays actually use an earth as part of the signalling/Transmission, and only at lower frequencies where a dipole is too big.

Bruthal

Yes as said the battery would need one terminal connected to a good quality ground for you to get a reading from the other terminal to earth.

As for using the actual earth as a return conductor, it is done in australia as i think was said by someone, and the earth is a poor conductor per square mm but as it has massive cross section area it can carry large currents. But its getting a good connection to the earth is a problem, large ammounts of earth rods are required.

So most places use a copper conductor as the neutral. And this neutral is earthed regularly along its length to give a fault path should live conductors contact the earth or earth connected equipment etc. In ireland the earth is connected to neutral at the consumer location to help keep earth fault impedence or resistance low and operate protection devices quickly, as an earth rod alone can have a relatively high resistance, especially if the ground is dry, or frozen.