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I connected a bulb to a battery positive terminals with positive and negative terminals with negative . It glows as it should but when i connect the positive terminal of the same bulb to the positive terminal of one battery and negative terminal of the same bulb to the negative terminal of another battery. The bulb does not glow.

Bulbs

What I want to ask here is that electric potential difference is being maintained then why does the bulb does not glow. NOTE : Batteries are not connected with each other. Both batteries are separately placed.

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  • $\begingroup$ Re, "electric potential difference is being maintained," Nothing in the second diagram maintains any electrical potential difference between the two ends of the bulb. There is an electrical potential difference between the two un-connected wires at the two sides of the picture, but the voltage across just the bulb is 0V. $\endgroup$
    – Solomon Slow
    Commented Aug 31, 2022 at 14:17

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It's the flow of a current that makes a bulb glow. In the second diagram there is no current flow because the current can't flow out of the end of the wires.

The following analogy may help. Electric current is like water flowing along a pipe. The wires correspond to the pipes. The end of a wire with air outside it corresponds to a pipe with a closed end because electric current can't flow out into the air (except a tiny bit which we are ignoring here). A battery corresponds to a device which creates pressure, allowing the water to be pushed up a slope to a higher level. Your second diagram is like a pipe running horizontally, then going up a slope, then horizontal again, then going up another slope. Both ends of the pipe are closed so there is no water flow.

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The potential difference exists between the two plates of the same battery. You don't know for sure if the potential difference exists between two plates from different batteries.

Additionally, you're not allowing any current to flow even if there is some potential difference. In other words, you have not closed the circuit. Current can't flow in a circuit that's not closed.

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  • $\begingroup$ If the bulb in the "no glow" diagram is an incandescent bulb, then we do know for sure the potential difference between the two batteries. It's the same as the potential difference across the bulb. It's 0V. An incandescent bulb acts like a resistor—it obeys Ohm's law, $I=V/R$. We know that no current can flow, $I=0$, because there is no complete circuit. If $I$ is zero, and if $R$ is finite and non-zero, then $V$ must equal zero. $\endgroup$
    – Solomon Slow
    Commented Sep 1, 2022 at 21:48
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@eqb is correct to some extent but the answer you want is as follows- In a battery when electrons reach the positive end it triggers a reaction which causes "release" of electrons at the negative end, now as you have connected the ends of two separate batteries when you take electrons from battery 1 and give to battery 2's positive end the new electrons are released on the negative end of the battery 2 but the negative end of battery 1 does not produce any electrons as for that to happen the electrons have to reach the positive end of the battery first

So an equilibrium is reached so no more current flows

Note this is an extreme simplification but I believe this is better in this context as you just want the plain reason

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