I am having problems understanding why we do need salt added to the water to make this shown galvanic cell work.
I understand that Zinc wants to dissolve into the electrolyte, resulting in $\ce{Zn^2+}$. Since there is $\ce{Cl^-}$ present, it will bond with the Zinc.
At the cathode, the $\ce{H_2O}$ will be electrolysed, leaving $\ce{OH^-}$ behind. I assume the $\ce{OH^-}$ will bond with $\ce{Na^+}$.
I assume this is the reason, why we need the electrolyte: If there would be no electrolyte, $\ce{Zn^2+}$ would remain close to the anode, while $\ce{OH^-}$ would remain close to the cathode, resulting in an electric field that counteracts against the redox-reaction. I think, adding NaCl results in negative Cl-ions moving towards the anode and positive Na-ions moving towards the cathode, resulting in a closed electric circuit, keeping the whole thing running. If we wouldn't add salt, the electric current should disappear very quickly.
Please let me know if my assumptions are correct. I wouldn't mind if you would explain the whole thing as easy and detailed as possible, since I have obviously not studied chemistry.
Little update: I have just built that galvanic cell and let it sit there for 15min. Voltage was around 0.7V. Current was in the mA-range, drastically increasing when adding table salt.
As you can see, the Zinc-plate got some whitish residue on it. I am unable to scratch it off. Any idea what that is? Could be areas where Zinc-ions left the metal?
The copper plate also shows some change below the waterline, but I have no possible explanation why it did change.
I was also able to see that the current through the wire was decreasing steadily. After adding salt and stirring the water, it showed ~30mA. 15min later it showed ~3mA. When I was stirring the water again, the current was increasing again to ~30mA and was decreasing again thereafter.
I assume after some time cations accumulate around the anode and anions accumulate around the cathode, inhibiting further flow of electric charge.