# Turn on an LED using opposite sides of 2 different batteries

I know this is a very beginner question but I'm not even sure what to lookup to learn about its details, so here it goes.

Why won't this turn the LED on? does't the current flow from positive to negative?
Why can't positive side of a battery move to the negative of another battery?

• I could swear I saw someone else ask this same question the other day. Jan 15, 2022 at 4:54

## 5 Answers

simulate this circuit – Schematic created using CircuitLab

1. Two independent batteries floating in space. There is no reference between them. No current can flow. Connecting a meter between the batteries on either end will give a reading of zero volts.
2. The batteries are connected in series. There is a 3 V difference between the top and bottom. In your proposal the batteries should go flat immediately because the the charge would somehow go from one to the other.
3. Here's the same situation but rather than a direct connection a lamp has been wired between the cells. Since no current flows the voltmeter reads 0 V.
4. To allow current to flow a closed loop must be formed. Here when SW1 is closed current can flow and the lamp will light.
5. The circuit of Figure 4 rearranged in the more conventional flashlight arrangement.

Why can't positive side of a battery move to the negative of another battery?

Because the overall charge in the battery must remain the same. If current leaves one terminal (forget about electrons - just think of conventional current flow from positive to negative) then the same current must come in the other terminal. With any break in the circuit that can't happen.

If what you proposed could happen then the batteries in a flashlight would go flat very quickly after insertion even if the switch was open.

• is it safe to say that electrons don't really "flow" freely on their own like water? They move like a chain. If they are not connected in a loop the chain wont move as explained in this video for AC current: youtu.be/bHIhgxav9LY?t=87 Jan 17, 2022 at 4:34
• I didn't watch the video but yes, that's the idea. Again, forget about electrons. In some situations charge moves by positive ions. Jan 17, 2022 at 7:17

Current flows in closed loops.

You need to create a closed loop for current to flow.

You can connect things, one to the other, end to end (in 'series'), all day long, but until you close the loop no current flows (ideally).

• @gedako, that closed loop is called a circuit, that's why that term is used. Upvoted for keeping it simple and for not trying to make a parallel with water, which doesn't work here. Jan 15, 2022 at 11:42
• is it safe to say that electrons don't really "flow" freely on their own like water? They move like a chain. If they are not connected in a loop the chain wont move as explained in this video for AC current: youtu.be/bHIhgxav9LY?t=87 Jan 15, 2022 at 17:52

What you are doing is in violation of Kirchhoff's circuit laws. You need to create a closed circuit for electrons to move or for current to flow. Which means you have to connect the negative side of the right battery to positive side of the left battery. This way current can flow. Something like this:

• right, but the question is why wont it flow in mine? how does the electron know the other end is closed or not? doesn't it just want to move to a negative side? Jan 15, 2022 at 5:00
• @gedako You need a reference node (GND) so other nodes can be defined by them. In your circuit you can't really say which node has a higher value in volt since the circuit isn't closed.
– Momo
Jan 15, 2022 at 5:26
• @gedako Maybe this can help you: electronics.stackexchange.com/q/2440/303288
– Momo
Jan 15, 2022 at 5:29

Energy (stored in a battery) is not a stack of electrons you can pour. Electrons unleash energy when they move, but to do that you must give them a way to run in.

I'll try to give an example with my bad english.

Think at water mills (those big wheels partly immersed in water). They produce energy because the water is flowing and make them turn, but you must have a channel for the water. If you dip your mill wheel in a mountain lake, it will not spin. A mountain lake has a lot of energy, but it is quiescent (just like a battery). Your circuit is the same as taking two mountain lakes at the same altitude, connect them with a channel and hoping the water goes from one lake to the other. It will work only if, in one of the lakes, you make a channel that voids it: the other lake will then fill the one that is emptying, and the wheel will turn.

You can think at a battery like two bottles put together, one full of water and the other one empty. Settled the right way, you can make water flow from the full bottle to the empty one and, doing so, the flowing water can spin a turbine. But the water has to go into the empty bottle of the same battery, to make it work.

• is it safe to say that electrons don't really "flow" freely on their own like water? They move like a chain. If they are not connected in a loop the chain wont move as explained in this video for AC current: youtu.be/bHIhgxav9LY?t=87 – Jan 16, 2022 at 23:57
• @gedako thanks. One can also think at a battery like a pump which can push water through + pole, but only if other water enters from then - pole. Jan 17, 2022 at 8:30

The dumbed down answer is that the two unconnected batteries have no relative voltage difference. The positive of one battery and the negative of the other battery are at the same voltage potential, so there is no movement of electrons. They do not know they are connected because they both see everything equally.

When you finish the connection, a difference is created and current exists due to the different voltage levels.

• You show 3V for the battery cells and a 2V red LED. Without a resistor limiting the current when the loop is completed then the LED will very quickly burn out. Jan 15, 2022 at 17:14
• @audioguru sure if this wasn't a hypothetical about current flow Jan 15, 2022 at 17:20
• I'll ask the same question here, is it safe to say that electrons don't really "flow" freely on their own like water? They move like a chain. If they are not connected in a loop the chain wont move as explained in this video for AC current: youtu.be/bHIhgxav9LY?t=87 Jan 15, 2022 at 18:48