Expert: George Maxwell - 8/4/2007

Question
QUESTION: This hopefully will be a general enough question that it will not require that you specialize in it. I was wondering how batteries work, and why they die? I was under the impression that it has to do with highly different metals inside, and a strong acid or something? Why aren't potato batteries infinate? I wouldn't think they would change much over time, or at least can easily be replaced.

ANSWER: Hi, and thank you for your question.

A battery is made up of two basic ingredients:

1) Two metal strips, each made of a different metal
2) A substance that can conduct electricity well.

When the two metals are put in a substance that conducts electricity (see note at the bottom), the more reactive metal starts to lose its electrons. These electrons travel accross the conducting material to the less reactive metal.

If the ends of the metal strips are connected to a circuit, the electrons will flow through the circuit: this flow of electrons is electricity.

You're right that, with some batteries (like car batteries) a strong acid (sulfuric) is used as the conducting substance, but the salty juice from a potato or the citric acid in an orange or apple may also work: it depends on the metals you're using.

Why do batteries run down? Well, in effect, what you're doing is breaking up the reactive metal to produce electrons: when this has gone on for a while, there won't be enough of the metal left to continue the process.

In reality, a battery runs down before all af the metal has been used because of other factors: it becomes more and more difficult to strip electrons from the metal as the reaction proceeds.

In your potato battery, you can

i) Replace the metal strips every so often.

ii) Soak the potato in salt-water every week or so to stop it drying out. Potatoes will lose lots of their salty water if left out for a small amount of time: it's this salty water that acts as the "conducting substance", so without it, your battery dies.

I realise this is a bit of a hit-and-miss answer, so please let me know if there's any particular detail you would like to know about: the topic is very wide!

Note: Actually, the substance is one that allows charged particles (ions) to pass through it, but this definition isn't important in a general answer.

Thank you again for your question.
Very best wishes.


---------- FOLLOW-UP ----------

QUESTION: I don't quite understand the dying part of it. How does the metal actually wear away? And if you picked strips of metal large enough couldn't you have one battery go for a very long time? How are batteries recharged?
ANSWER: Thanks for the follow-up.

Batteries run down because, as the reactive metal loses electrons, it's losing bits of itself. The best way to think of it might be to strike a match: the wood of the match provides the fuel for the flame, but as the flame burns, it uses up this fuel until it's gone, then the match goes out. Your reactive metal provides the "fuel" for electrons to flow round a circuit, but that fuel has to be used up if electrons are to flow. The metal "wears away" because you're ripping little bits of it out (the electrons)
to power the circuit, just like the match is using itself up to "power" a flame.

As I said before, though, it's not the wearing down that usually kills a battery: that's down to something else:

In normal conditions, a metal atom has a positively charged 'nucleus' surrounded by a cloud of negatively charged electrons. The positive and negative charges balance, and so the atom is held together. A strip of metal contains billions of these atoms, but overall, the charge is still balanced.

As the strip is used in a battery, electrons start to be lost from the metal atoms. This means there is now more positive charge than negative charge in this strip of metal. After a while, this positive charge gets so big that it is strong enough to stop any other electrons being removed (positive and negative charges pull towards each other like the north and south poles of a pair of magnets).
If the strip can't lose any more electrons, it stops providing an electric current.

There are several reasons why we don't go in for massive batteries / strips of metal:
1) Metals are expensive: it's much cheaper to use small amounts and replace them.
2) Batteries are meant as a portable power source, and you lose that advantage with bigger batteries.
3) Making the metal strips bigger might not improve the life of the battery anyway: bigger batteries means more resistance inside, and so power flow is reduced.
4) Some of the conducting substances (electrolytes) used are toxic, and are only safe in quite small amounts.

As for rechargeable batteries: if you use the right materials, you can get a battery that allows you to put electrons back in to the reactive metal after you've used it. This only works with a few metals, and makes the battery more expensive which is why there are still lots of non-rechargeable batteries on the market.

In a rechargeable battery, once the reactive metal has lost electrons, you hook it up to a power source that pumps electrons back in, ready to be used again when the battery is reconnected. It's exactly like a car in this respect: the "fuel" (electrons) is used up, so you fill the reactive strip up with more fuel, and it can then be used again.

This doesn't work with normal batteries, as the metals inside them form non-reactive substances as they break down during use, and these substances don't respond to an input of electricity: they don't collect electrons ready to be re used.



---------- FOLLOW-UP ----------

QUESTION: I have a different question for you now. The physicists on this site have been unable to answer this question so maybe it is something a chemist would better know. I know that if you spinkly lead dust over a fire the fire will change green. Is there a safe way to play around with the colors of the flame. Like on bon fire or something that gives off a lot of smoke? I don't expect lead would be that safe. Are there other materials that can be burnt to alter the color of the flame? Maybe a hobby store would have something like this?
ANSWER: Hi, thanks for the follow-up.
You've asked the right person! Part of my work involves creating fire effects for TV, including coloured flames.

I will suggest a few household and a few lab chemicals that you might use. Normally, instead of sprinkling in the salt, you would dissolve it in alcohol (e.g. methylated spirits). Alcohol burns with a pale blue flame that is easily coloured. The salt-alcohol mix is put in a metal tin on a heatproof surface and then ignited.

Try:
Common Salt - this gives an intense yellow flame that looks particularly eerie if done in a dark room. Objects that appear red in normal light will look black under sodium light, so its often used in horror scenes on TV.

Low-sodium salt: if you can get hold of low-sodium salt that is 100% potassium chloride (check the label), it gives a purple flame.

Borax (the cleaning agent) - this gives a great green flame, but check on the packet: if the product contains sodium tetraborate, the flame will appear yellow instead of green.

Copper: you need a hot flame for this, but copper (e.g. coins) gives an intense blue - green light when heated.
You could dump lots of waste copper (like old pipes) in to your bonfire, and you'd get lots of flickering green in the middle of the blaze.

In the lab. you'd use the following chemicals:
For a:

Blue flame - Arsenic and antimony compounds
Red flame - Strontium compounds (e.g. chlorides).

Green flame - Boron compounds, like boron chloride.
         Zinc (metal) also burns with a green flame.
Yellow flame - Sodium compounds like sodium chloride.
Purple flame - Potassium compounds like potassium chloride.
Bright White flame - magnesium metal.

You might be able to get hold of some of these from a pharmacist or a hardware store.

Hope this helps: let me know if there's anything I haven't covered.

You might want to look up "flame test" on wikipedia for examples of the coloured flames that different metal compounds produce.
Thanks again.







---------- FOLLOW-UP ----------

QUESTION: None of these non-lab chemicals will produce any unhealthy fumes or toxins if used among many people? (Bon fire). And copper produces blue or green? You lost me there. You said it creates an intense blue, but green when heated? And produces a flickering green if I dumped coins into the fire? How can I get a certain blue color? I don't suppose there is anything that would create an ultraviolet light from a fire?

Answer
Thank you for the follow-up.

Re: copper - the colour of the copper flame depends on the flame temperature, so the flame is a mix of blue and green, changing randomly.

There is no common chemical that produces a pure blue flame when burned (think of how rarely you see a pure blue firework).The rare chemicals that do give a good blue flame are too unstable or toxic to use.

Alcohol burns with a pale bluish flame, but this is invisible in daylight, so it would have to be night for this to work.

Most of the chemicals (except salt / salt substitute) are toxic if ingested, but they don't decompose in to hazardous products when burned, so they're not a danger in use. Bearing in mind that you need a good air supply whenever you're using fire (to prevent a build-up of fumes), there won't be a large toxic hazard: these compounds are used in fireworks regularly without hazard.

I use all of the compounds I listed regularly, most often in small sets / stages, and I've never had any problems. The bigger danger is the fire hazard: If you're using the alcohol / tin method I mentioned before, you should have a CO2 or dry-chemical fire extinguisher standing by.

The energy provided by a fire (infra red) isn't enough to provoke UV emission from any chemical that I know of: any chemical that is that unstable would be radioactive anyway, and so you couldn't safely use it.

Hope this helps: again, please don't hesitate if there are further details that need clarification.
Thank you