Why do we need electricity for?
For instance, let us say for lighting.
The principle of the light bulb is that we send a flow of electrons on a metal wire that heats up and emits light. Particles bump on each other, absorb energy and release it as light. The first successful effort had used platinum, a material that does not melt at the temperature required (e.g. around 3000° Celsius). A glass bulb is placed around the metal wire.
Here is an interesting link: http://www.edisontechcenter.org/incandescent.html#works
How can we set up a continuous flow of electrons?
Here is the principle of the battery.
How can we extract electrons from a material, e.g. a metal?
Let us take a penny which is made mostly out of zinc. Immerse it in an acidic solution like lemon juice (or even a lemon). An acidic solution will have a lot of protons or H+.
Here is the relevant Wikipedia article: https://en.wikipedia.org/?title=Lemon_battery
"ZnCuVoltaicCell" by Easchiff - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:ZnCuVoltaicCell.svg#/media/File:ZnCuVoltaicCell.svg
Zinc is the atom made of 30 protons. Here is its electric configuration per shell: 2, 8, 18, 2.
Upon immersion, the two outer electrons of zinc will be relatively easily dissolved in the solution. The zinc having a deficit in two electrons will be positively charged like this: Zn2+. This is a spontaneous behavior for Zn which takes it to a lower and therefore more stable energy state. As a result, energy is released.
Therefore, we can easily get our electrons from zinc in the above way.
We now have to get them to flow onto a wire like a copper wire.
Generally speaking, what will happen if we put in contact a piece of zinc and a piece of copper?
Let us take a UK 5p which is made mostly out of copper.
Copper is the atom made of 29 protons. Here is its electric configuration per shell: 2, 8, 18, 1
Copper, in comparison to zinc, attracts electrons more strongly.
As we know, metals have electrons that are loosely attached and can move.
If we put in contact a piece of zinc and a piece of copper, electrons from zinc will be attracted more by copper and will move towards it. As the electrons will be concentrating on copper the new coming electrons will be repelled to an increasing degree and eventually the movement will stop.
What will happen if we immerse copper, the copper 5p that was mentioned, into the acidic solution?
Upon immersion, two electrons of copper will be released but with much more difficulty than zinc.
If we put the zinc and the copper (that are immersed in acid solution) in contact, for instance with the aid of a wire, then as copper attracts electrons more, the electrons would flow to copper; in this setting, as copper also loses electrons, we would no longer be in the case of the new coming electrons being increasingly repelled. As a result, a continuous flow from zinc to copper could be maintained.
This would give us the continuous electron flow that we need to light up a bulb.
The batteries are a similar setting of two electrodes immersed in an acidic solution.
This allows us to have electron generation (via extraction from metal) readily available, along with the mechanism that can set the extracted electrons in motion.