Current, Voltage and Batteries

[Jimmy87]

I have a bit of confusion over how a bulb lights up in a simple DC circuit. My book says it's because the electrons within the filament start moving as soon as a battery is connected. My book also says that voltage is the potential energy an electron has. It says it gains this energy as it moves through the battery (as this is the role of the battery). How can the electrons in the filament have energy and start moving as soon as you connect the circuit when they haven't been through the battery yet? Also, do the terminals of a battery refer to positive and negative charges or do they refer to differences in potential or both? Some sources I have read say that strictly speaking the terminals of a battery do not refer to net positive and negative charges but don't really explain why. I though redox reactions within the battery cause a difference in the number of electrons at the terminals and thus must refer to differences in net charge. Also, isn't a voltage directly caused by an electric field and electric field is caused by a net charge? Any help here would be greatly appreciated!

 

[Gliese123]

Like you said - the differences. The difference between the anode and cathode is the difference and as soon as the circuit is on, the flow of electrons starts. This will obviously continue as long as there is any difference to even out.
I'm not sure but I guess it refers to both negative & positive charges as well as the potential.

 

[SteamKing]

And remember, electrons travel sooo fast, you can't see them when they move.

 

[Drakkith]

I have a bit of confusion over how a bulb lights up in a simple DC circuit. My book says it's because the electrons within the filament start moving as soon as a battery is connected.

When the battery is connected electrons will move throughout the circuit, not just in the bulb.

My book also says that voltage is the potential energy an electron has. It says it gains this energy as it moves through the battery (as this is the role of the battery). How can the electrons in the filament have energy and start moving as soon as you connect the circuit when they haven't been through the battery yet?

Actually the electrons will not move through the battery. A very very basic explanation is that inside the battery a chemical reaction takes place at the cathode and anode of each cell that releases stored chemical energy. Electrons are removed from their atoms at one terminal and attempt to flow around the circuit to get to the other terminal where the atoms collect electrons. But, there are other electrons already in the circuit and the ones from the battery cause the ones in front of them to move as well, leading to a flow around the circuit. (Like water in a water circuit. The pump pushes water and that water pushes the water in front of it) There are a million webpages that explain how a battery works. Just google "how does a battery work" and you'll find them.

Also, do the terminals of a battery refer to positive and negative charges or do they refer to differences in potential or both? Some sources I have read say that strictly speaking the terminals of a battery do not refer to net positive and negative charges but don't really explain why. I though redox reactions within the battery cause a difference in the number of electrons at the terminals and thus must refer to differences in net charge.

I'm not sure. All I know is that electrons flow out of the - terminal and into the + terminal. If you were to measure voltage between a point in the circuit and one of the terminals, you would read a negative voltage on the negative terminal and a positive on the positive.

Also, isn't a voltage directly caused by an electric field and electric field is caused by a net charge? Any help here would be greatly appreciated!

Yes, but it's a bit more complicated than that. For example, a magnetic field can be used to create voltage in a circuit.

 

[davenn]

And remember, electrons travel sooo fast, you can't see them when they move.

ohhh yeah ? ;)

you don't believe in electron drift huh :)

Dave

 

[rcgldr]

The terminals of a battery have a relatively small amount of charge (compared to the voltage) on them (- on the negative terminal, + on the positive terminal) that the battery will mostly maintain when hooked up to a circuit (the amount of charge difference and voltage will decrease somewhat if the current is higher). The difference in charge is what drives the electrons through the circuit.

 

[DrZoidberg]

A voltage is always caused by an electric field. There are only two ways to create an e field - with charges or with a changing magnetic field. In a battery it's a charge imbalance between the terminals.
The voltage beween point A and B is equal to the integral of the e field over the distance from A to B. Every electron inside the field has potential energy relative to A and also potential energy relative to B just like an object close to a planet has potential energy relative to the surface due to gravity. The electrons don't have to get into contact with the battery, the presence of the field alone already means that they have potential energy. They start moving as soon as they are subjected to the field. They don't need to be pushed by other electrons.

 

[SteamKing]

ohhh yeah ? ;)

you don't believe in electron drift huh :)

Dave

Well, like, they drift pretty fast, too.

 

[davenn]

not that fast that they cause things to happen (near) instantly
That's the EM energy

Dave