# What happens when we connect battery to a circuit?

Example setup:

I have a piece if metal and I have 12v battery. When battery is connected to the metal current flows through the metal.

So, my question is, does electrons from battery move through out the entire circuit? If yes, what is velocity of electrons.

What I imagine when there is a current flowing through a circuit:

I imagine a bunch of electrons leaving negative terminal of battery and these electrons travel through the circuit and reach the positive terminal of the battery and how fast the electrons travel decides the value of current.

Is this correct? I'm confused.

So, my question is, does electrons from battery move through out the entire circuit? If yes, what is velocity of electrons.

Electron flow is actually pretty slow.

What is very fast is the onset of the flow; it's nearly instantaneous in all the metal.

Think about it like a water hose filled with (standing) water: Even if the water flows slowly through it, turning on the pump will nearly instantly make water come out of the end.

So, no. Current is not when the very same electrons leaving one terminal reach the other. Current is the amount of electrons flowing due to the electric field that the voltage causes.

So, while the speed of electrons is pretty slow, the speed at which a change in electrical field propagates is very high – in fact, it's the speed of light.

You can imagine the piece of metal as a resistor. Instead of being measured in ohms, the resistance would be more in the milliohm range. It would be the same as placing a resistor across the terminals of the battery, but much more current would flow.

The velocity of the electrons is determined by the cross sectional area and the amount of current. Because there is resistance, they are pretty slow. The electric field is what actually drives the current.

If you want to find the velocity of the electrons, use the calculator found here

• I think you mean cross-sectional area rather than surface area. – Transistor Jul 25 '19 at 20:02
• I think I mean that too, thanks – Voltage Spike Jul 25 '19 at 20:12

There are a lot of electrons in an ordinary wire. At ordinary currents, say 1A, the flow is 1 coulomb/second, which is about $$\6.24\times10^{18}\$$ electrons per second, which sounds like a lot, but it's not.

Wikipedia has a worked example of drift velocity showing a 2mm diameter (about AWG 12) copper wire carrying 1A has a drift velocity of $$\2.3\times10^{-5}\$$m/s. At room temperature the electrons are whizzing about at 1570m/s on average just from thermal effects, so if you attempted to observe the electrons' velocity alone you'd have to have a very accurate measurement to even notice the drift due to the 1A current.

To put it another way, to move the average electron in an AWG 12 wire a meter in a second you'd need a current of more than 40,000A. As it will fuse at around 240A that's not going to happen under plausible conditions.