In a battery or a galvanic cell, the electric potential of the battery is due to a difference of charges between the two cells like in a capacitor? So it is the electric field due to this separation that is driving the electrons? if yes, why we call it electromotive force of a battery (EMF) ?
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Galvanic cells are driven by a chemical reaction known as a Redox reaction.
Schematically speaking the cell contains a oxidiser $O$ and a reducing agent $R$, separated by a conductive membrane.
When the oxidiser reacts it loses electrons:
$O \to O' + n e^-$ (where $O'$ is the reduced form of $O$)
When the reducing agent reacts it absorbs these electrons:
$R + n e^- \to R'$ (where $R'$ is the oxidised form of $R$)
It's these electrons that cause the potential to arise at the two electrodes and the cell to be able to provide current (a flow of electrons).
The overall Redox reaction is:
$O + R \to O' + R'$
A battery is usually (but not always) a number of the same cells connected in series to obtain the desired output voltage. The reactions take place only when the circuit is closed, so the electrons can flow from cathode to anode.
A typical system is the manganese dioxide ($MnO_2$), zinc ($Zn$) battery in which the oxidiser $MnO_2$ oxidises the $Zn$ metal. These batteries run out when either the oxidiser or reducing agent has been fully consumed in the redox reactions. In galvanic cells chemical energy is converted to electrical energy (when the cell is in use).
Another example of a galvanic (voltaic) cell is the $Zn/CuSO_4$ cell.
At the cathode $Zn$ is oxidised to $ZnSO_4$, while at the anode $CuSO_4$ is reduced to $Cu$. A permeable membrane allows transport of the sulphate ($SO_4^{2-}$) ions, while keeping the oxidation and reduction reactions separated.
The cell potential can be calculated as shown here.
Gert
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Ok, but if we connect a resistor to a battery, there is an electric field inside a resistor, from where it comes? from the chemical energy? and how a chemical energy can produce electric field if there's no charge separation? – Tonylb1 Sep 03 '15 at 01:37
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The oxidation potential pushes electrons to the cathode, even if the circuit is still closed (no electrons flow yet). So even with an open circuit there's voltage between cathode and anode. Yes, it's chemical energy that drives all that. I'll try and edit my answer tomorrow, illustrating that better. – Gert Sep 03 '15 at 02:10
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Ok I am waiting :p – Tonylb1 Sep 03 '15 at 07:16
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@Tonylb1 did you checked the video i posted in my post ?? – Ashok Varma Sep 03 '15 at 12:49
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1@Tonylb1. Two links have been added that hopefully increase your understanding of galvanic cells further. – Gert Sep 03 '15 at 14:12
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The electric field is established only when we connect +ve and -ve of a battery with some resistance between them.
There will be no electric field when the battery is in ideal state. But we need a measure for expressing the power of battery so our physicists introduced EMF, because measuring it with electric field doesn't make sense when battery is ideal.
How current flows in galvonic cells:
The current flows mainly beacuse of the wire connecting them. The wire will have free electrons since it is a conducter. When a wire is connected b/w two different potential chemicals. The first electron (excess electrons in the ion) transfers from -ve potential liquid to wire and then in wire the electrons flow and it passes one electron to the +ve potential liquid.
This link explains how electrons travel in galvonic cells
Extra Material : Check this to know about how electrons travel in wire
Ashok Varma
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Ok, but we know that only electric field and/or diffusion of charges is the reason of the establishment of a current in electric circuits. Then, how a chemical reaction can produce current without electric field in a circuit? – Tonylb1 Sep 03 '15 at 01:05
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In the chemicals the bond between the compounds break and they will divide in to ions or charged compounds.
The ions will have extra electrons or extra protons which make them +ve or -ve charged and they act as electrons and holes
– Ashok Varma Sep 03 '15 at 01:06 -
Ok, but if we connect a resistor to a battery, there is an electric field inside a resistor, from where it comes? from the chemical energy? and how a chemical energy can produce electric field if there's no charge separation? – Tonylb1 Sep 03 '15 at 01:18
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The transfer of electronics is responsible for electric current but not the electrons themselves. I will add a YouTube video it's difficult to explain in text, I am busy I will give link in two hours – Ashok Varma Sep 03 '15 at 01:24
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Added content to my answer check and ask if you have further doubts – Ashok Varma Sep 03 '15 at 03:08
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