# If I connect the +ve terminal of one battery to a terminal of a bulb

In summary, when connecting the positive terminal of one battery to a terminal of a bulb and the other terminal of the bulb to the negative terminal of another battery, the bulb will not glow due to a lack of completed circuit. While there is a potential difference across the bulb's terminals, it will disappear quickly as electrons flow through the bulb, neutralizing the charges on the joined terminals of the batteries. This also applies to a single battery, as there will be excess electrons on the negative terminal and a deficit on the positive terminal. Additionally, even if the potential difference remains the same, the force between the two batteries would be too great for the bulb to light up. The idea of a capacitor also plays a role, as the unconnected terminals
If I connect the +ve terminal of one battery to a terminal of a bulb and the other terminal of the bulb to the -ve terminal of an other battery, why doesn't it glow? There is still potential difference across the bulb's terminals and so the charge should flow.

I'm thinking for the same reason water doesn't flow out of closed faucet/ hose. There's pressure pushing it, but if the path is blocked/ cut, then it's just not going to flow.

How does the current flow?

There will be excess electrons on the negative terminal of one battery, and a deficit of electrons on the positive terminal of the other battery. So when you connect the bulb across these terminals there will be a p.d. across the bulb. But this p.d. will disappear in a very short time as electrons flow through the bulb neutralising the charges on the batteries' joined terminals (those connected to the bulb). [If the batteries are identical a symmetry argument shows that there can be no net charge remaining on the joined terminals]. And that's it; there won't now be a p.d.across the bulb until the circuit is completed.

There will, of course, still be a p.d. across the terminals of one battery (i.e. between its connected terminal and its free terminal), and the same for the other battery. But it isn't either of these p.d.s which is being connected across the bulb.

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There will be excess electrons on the negative terminal of one battery, and a deficit of electrons on the positive terminal of the other battery. So when you connect the bulb across these terminals there will be a p.d. across the bulb. But this p.d. will disappear in a very short time as electrons flow through the bulb neutralising the charges on the batteries' joined terminals (those connected to the bulb). [If the batteries are identical a symmetry argument shows that there can be no net charge remaining on the joined terminals]. And that's it; there won't now be a p.d.across the bulb until the circuit is completed.

The same thing should happen if I connect the bulb to a single battery, since there too will be excess electrons on the -ve terminal and these will flow through the bulb to the +ve terminal.

There will, of course, still be a p.d. across the terminals of one battery (i.e. between its connected terminal and its free terminal), and the same for the other battery. But it isn't either of these p.d.s which is being connected across the bulb.

Let's say the potential of the positive terminal of a battery is V and the potential of the negative terminal is zero. So, the potential difference is V. If I take another battery, similar to this battery, and connect the bulb to the +ve of one of the batteries and - of the other. The potential difference across the bulb is still V and therefore it should glow.

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Calculate how much charge has moved when you've had the bulb lit for, I don't know, say a minute. Then calculate the force on the two batteries for the change imbalance. You'll quickly see why this doesn't happen.

Conversely, you could say that, at the instant of connecting things up, you may have had a slight potential difference between the tow battery terminals. Only a few picoCoulombs would have needed to flow for this PD to have reduced to zero. Not enough to make the bulb flash even for an instant.

As Vanadium says - the Force between a couple of 1Coulomb Charges held 1m apart is pretty fearsome!

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varadgautam: What is the basis of your claim that there is a p.d. across the bulb (even when there is not a complete circuit)? [Clearly I didn't manage to convince you that there wasn't (except for a very short time when the bulb is first connected)].

Here's the way I like to see it. Imagine that you connect two parallel plates to the unconnected ends of your batteries. Then charge will flow through the bulb until the potential across the two plates equals the sum of the potentials of the batteries. Then flow will cease, because that's what capacitors (in series) do to DC current. When flow ceases the light bulb will not shine.

Now imagine shrinking the plates to zero size. The capacitance will decrease, so that it won't take much charge to get the capacitor to the battery voltages.

So Kirchoff's rule says the the voltage of the batteries added to voltage drop across the capacitor is zero, so no current will flow.

The charges at the unconnected terminals of the battery act like a capacitor that stops flow.

RedX. Love it.

Got it, thanks for the help.

## 1. What will happen if I connect the +ve terminal of one battery to a terminal of a bulb?

Connecting the +ve terminal of a battery to a terminal of a bulb will create a closed circuit, allowing electricity to flow from the battery to the bulb. This will cause the bulb to light up.

## 2. Will the bulb light up if I connect the -ve terminal of a battery to a terminal of a bulb?

No, the bulb will not light up if you connect the -ve terminal of a battery to a terminal of a bulb. This is because the flow of electricity needs to have a complete path, and the -ve terminal of a battery is not connected to the positive side of the circuit.

## 3. Can I use any type of battery to power the bulb?

Yes, as long as the battery has enough voltage to power the bulb. However, it is important to use the correct type of battery for the specific bulb to ensure optimal performance and safety.

## 4. Can I connect multiple batteries to the bulb to make it brighter?

Yes, connecting multiple batteries in a series will increase the voltage and make the bulb brighter. However, connecting them in parallel will increase the current and may cause the bulb to burn out. It is important to use caution and follow proper electrical safety guidelines when using multiple batteries.

## 5. What happens if I connect the +ve terminal of a battery to both terminals of the bulb?

If you connect the +ve terminal of a battery to both terminals of the bulb, you are essentially creating a short circuit. This will allow a large amount of current to flow through the circuit, potentially causing the battery to overheat and potentially damage the bulb or other components in the circuit.

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