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I would like to know why does electrons flow through a wire connected to a battery as said in AC/DC: What's the Difference?.It tells that "the electrons that are stripped from the carbon electrode is collected on the zinc can" and then tells

The electrons at the negative terminal want to go to positive terminal, they just need a way to get there. In our light bulb circuit, the way to get there is through the wire.

Why couldn't the electrons go to the positive terminal through the electrolyte(potassium hydroxide) instead of going through the wire?

tryst with freedom
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justin
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  • The chemical reactions in a zinc-carbon cell are described here https://en.m.wikipedia.org/wiki/Zinc–carbon_battery#Chemical_reactions – Farcher Apr 06 '16 at 08:39
  • @Farcher:Sorry I couldn't see anything that tells about connecting a battery to wire even though the page explains about the working of a battery. – justin Apr 06 '16 at 08:45
  • By "electrolite" you mean the battery itself. You ask, "why don't stones fall upon the Earth in the battery, why do they raise up into the air?" This contradicts to the principle of potential energy minimization indeed. – Valentin Tihomirov Apr 06 '16 at 08:49
  • @ValentinTihomirov:No.I mean the pasty mixture in the battery. – justin Apr 06 '16 at 08:54

4 Answers4

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Batteries use a type of reaction called a redox reaction that involves the transport of electrons. Rather then the carbon zinc battery, which is a bit complicated consider the simpler example of a zinc copper battery as taught in school science lessons across the world. The reaction is:

$$ Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu $$

So the reaction dissolves the zinc electrode and produces copper metal at the copper electrode. The reaction goes this way because the overall free energy of the Zn/Cu system is reduced in doing so.

If we look more closely the reaction involves three steps:

  1. $Zn \rightarrow Zn^{2+} + 2e$

  2. transport of the electrons to the copper

  3. $Cu^{2+} + 2e \rightarrow Cu$

So as the reaction goes electrons flow from the zinc through the battery to the copper. In effect the reaction acts as an electron pump that pumps electrons from the zinc end to the copper end. So if you connect an external wire from the copper to the zinc the electrons flow out of the copper, through the wire and back to the zinc, then complete the loop by flowing from the zinc to the copper inside the battery. Electrons flow in that direction because the chemical reaction forces them to.

The traditional zinc-carbon battery uses a reaction between zinc and manganese - the carbon is actually just an electrode and doesn't take part in the reaction. While the reaction is more complicated the basic principle is the same. The zinc reacts to form $Zn^{2+}$ and electrons and the manganese absorbs the electrons. So just like the zinc-copper battery the electron flow is driven by the chemical reaction.

John Rennie
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  • :Okay.That's true.Is there anything that restricts the flow of electrons from zinc plate(-ve) to copper plate(+ve) through the electrolyte KOH instead of going through the wire? – justin Apr 06 '16 at 10:19
  • @justin: There is no $Cu^{2+}$ in the wire so the reaction can't occur in the wire. Good point though. If we have a hypothetical external wire made from copper sulphate solution it would be just the same as the cell and we wouldn't get electrons flowing in a loop. – John Rennie Apr 06 '16 at 10:24
  • :Sorry I'm not asking of the flow of electrons through the wire.I'm asking whether there's a flow of electrons from the zinc plate to the copper plate through the electrolyte. – justin Apr 06 '16 at 10:30
  • @justin: yes, sorry, I thought that was clear from my answer. Electrons flow from the zinc through the electrolyte to the copper than back through the external wire from the copper to the zinc. – John Rennie Apr 06 '16 at 10:34
  • :Do you mean to say that copper acts both as a negative and postive terminal at different times? – justin Apr 06 '16 at 10:37
  • @justin: No, because the definition of a positive terminal is that electrons flow from the negative terminal to the positive terminal through an external circuit. That makes the zinc the positive terminal and the copper the negative terminal. – John Rennie Apr 06 '16 at 10:51
  • :Oh but I couldn't get why did you say in your comment that they flow from zinc to copper and then from copper to zinc. – justin Apr 06 '16 at 11:14
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    @justin: the electrons flow from the copper to the zinc through the external wire, and they flow from the zinc to the copper through the electrolyte inside the battery. But this is a continuous process - at any time some electrons are flowing in the external wire while others are flowing inside the battery. The current in the external wire is always the same as the current flowing through the electrolyte. If this wasn't the case we'd get a traffic jam with electrons piling up at some point in the circuit. – John Rennie Apr 06 '16 at 11:17
  • :So do you mean to say that flow of electrons takes place both in the wire and the battery at the same time? – justin Apr 06 '16 at 11:18
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    @justin: yes, that's right. – John Rennie Apr 06 '16 at 11:19
  • :out of curiosity,could you tell is the rate of flow of electrons same in the wire and the battery. – justin Apr 07 '16 at 06:49
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    @justin: the flow in the wire and battery has to have the same magnitude because the electrons flow in a loop and the number of electrons is constant. Think of like water flowing through a pipe in a loop. The battery is analogous to the pump and the the pie is analogous to the external wire. So the water flows though the pump out into the looped pipe and eventually back to the other end of the pump. The water flow rate is obviously the same in the pump as in the pipe because water can't just disappear and appear. – John Rennie Apr 07 '16 at 08:06
  • :What is '1.5v' in essence?Is there any relation between the number of electrons pumped and the voltage in the battery? – justin Apr 07 '16 at 08:22
  • @justin: the battery voltage is related to the free energy change of the chemical reaction. The details get a bit involved, but basically the more energetic the reaction the higher the voltage. – John Rennie Apr 07 '16 at 08:47
  • :how do they specifically get this value like '1.5v' in battery when they just place an zinc and carbon/copper rode in KOH. – justin Apr 07 '16 at 08:50
  • @justin: this will need asking as a new question, but do some background reading first e.g. this article. – John Rennie Apr 07 '16 at 08:52
  • :I could get about DC flow by your comments but I'm stuck in AC flow.I even tried asking a question on reddit but no reply came.Since I'm afraid of dupe hammer I didn't ask on physics SE because a similar question exist here.Could you help me. – justin May 18 '16 at 09:38
  • :Could you tell whether in AC the energy(voltage) obtained from generators goes through the live wire and after it's use goes to the ground and then from ground it goes to live wire? – justin May 18 '16 at 10:28
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Via a series of chemical reactions a battery sets up a surplus of electrons on the zinc (negative) plate and a deficit of electrons (positive charges) on the carbon (positive) plate because it is energetically favourable to do that.
You can think of the reaction as a zinc atom producing a zinc ion and two electrons with the release of energy.

Assume that the battery is not connected to an external circuit.

The chemical reaction is in effect forcing electrons to move from the positive carbon to the negative zinc.
The build up of charge on the carbon and zinc will continue until the electric field due to the charges on the zinc and carbon is such that the chemical reaction cannot move any more electrons from the positive carbon to the negative zinc.
This occurs when the potential difference across the terminal of the battery is about 1.5 volts.
The battery is like a pump taking electrons from the carbon electrode and depositing them on the zinc electrode, but like a water pump which can only pump water to a finite height the same is true of the electron pump which can only move electrons across a finite potential difference. About 1.5 volts in the case of a carbon-zinc battery.

Now when a external conducting path is added between the carbon electrode and the zinc electrode, electrons will flow from the zinc plate through the external conducting path to the carbon electrode.
Whilst this is happening the chemical reaction (electron pump) with move electrons inside the battery from the positive carbon electrode to the negative zinc electrode to maintain a potential difference of 1.5 V across the electrodes.

So the answer to your question is that it is not energetically favourable to move electrons from the zinc plate to the carbon plate.
The electrical circuit has electrons continuously flowing around it. Electrical energy comes from chemical energy in the battery and if the external circuit is a resistor then heat is produced from the electrical energy.

Farcher
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  • :Could you explain a bit more about why it's not energetically favourable to move electrons from zinc plate to the carbon plate. – justin Apr 06 '16 at 09:16
  • To get a proper answer you need to ask a chemist however here is a link to the HyperPhysics website which tells you that zinc is much more anxious to release its electrons and form a zinc ion than copper (the equivalent of carbon in the zinc carbon battery). http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrochem.html – Farcher Apr 06 '16 at 11:27
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Actually the flow of electrons is not that cause the light to glow. The electrons are just carriers of energy. I will make it clear. Consider a bulb connected to a battery. The wire is a conductive metal which means that it is in solid state. So the inter atomic spaces is very less. So the electrons can't move that much freely. There arises a no. of collisions with the atoms in the lattice. These collisions causes the electrons to lose their energy (since the collision is inelastic) and hence the kinetic energy of electron appears as lattice heat.
So the velocity of electrons get averaged. Theoretically, when a charge like an electron is placed in an electric field, it should accelerate. But the velocity of electron is averaged inside the wire. So if you switch on the current, the electron from the battery takes some sufficient time to get to the bulb. But that's not what happens in real life. The bulb glows instantly when you switch on. Then, that means it is not the flow of electrons that make the bulb glow. The explanation is like this. Consider an electron close to the battery. It absorbs electrical energy from it and hence moves. When it reaches in the vicinity of neighboring electron, there arises electrical repulsion between both. So one would expect that both will fly apart. But the first electron cannot go back as there is continuous energy from that direction. So the configuration holds some potential energy. Thus the neighboring electron starts to move. this happens through the entire wire. Even though electrons can move only at some average velocity, due to repulsion the effect of electrical energy get transferred instantaneously through the wire.
Now at the bulb, the bulb has some resistance. So the electron loses energy there which is picked by the bulb and convert it into light and heat energy.
There is no conduction possible through the internal arrangement of a battery because the two electrodes are separated by each other.

UKH
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The thing that most presentations forget to include is that pure water is an excellent electrical insulator. It lacks mobile electrons. Conduction in water (or any other useful electrolyte) is ionic. To drive an external electron current in a wire, the ions must react at the electrodes, losing electrons at the cathode, gaining them the anode. Those reactions produce a net transfer of chemical energy to electrical energy. The the flow of electrons balances charge, thus allowing the reactions to proceed. That flow must be through an external circuit, because electrons can't flow through the electrolyte.

John Doty
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