How to find out if the current is constant in a circuit.

Stack Exchange network includes Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

A question and answer site for electronics and electrical engineering professionals, students, and enthusiasts.It only takes a short time to sign up.

A battery creates an electric field and converts electric potential to energy.The current should be higher because the electrons have more energy near the positive terminal.

If you drop a ball from a building, the ball will speed up as it reaches the ground because more potential energy has been converted to kinetic energy.Shouldn't the electrons move faster as they approach the positive terminal since they have more energy?Shouldn't the current be higher?

The current is the most important thing in electrical circuits.If your current is constant, you have on average a constant kinetic energy.

Since the entire wire is filled with electrons, all the electrons must have the same velocities, and so the energy is equal.

The water molecule is equivalent to the electrons.The molecule at the start of the pump doesn't have a bigger current.

A train is a weaker analogy.Imagine the mechanism that applies the force to the rest of the carts as an engine.The carts in the train have the same speed.

There are some good answers here.I would like to explain from a different point of view.

I don't think of electrons flowing through wires as this implies that their mass and momentum is what's causing the transfer of power.You are often told to imagine a tube of ping-pong balls.This can also be misleading.Imagine a pipe filled with sand.If you force some sand in one end, some will come out the other, but velocity, mass, and momentum don't play much into it.

The energy transfer happens because of a wavefront of excited electrons.Not because of the electron mass.One millimeter per second is how fast the electron drift in a 1-mm thick copper conductor is.

The water analogy breaks down at that place.There isn't an electrical momentum based on mass.That's not absolutely correct, but it will serve you well.

You'll use an inductor if you want to add momentum to your circuit.The water analogy is useful again.

There is an excellent example of this.Check out this video of a Ram pump: http://www.youtube.com/watch?v=qWqDurunnK8.Many people have never heard of it.It turns out that it's the same thing.You will soon see boost converters.They're used in electrical circuits all over the place.

The Ram Pump works.To make it work in electronics, you need an inductor.It's great!There is a one-way valve and a pressure chamber.

It's not.What you are missing is the difference in electric potential between the top and bottom of the building and the ball.

The conductors have the same electric charge as a pipe full of water.If you put a charge in one end, it creates a higher pressure at the other end.Equalizing the pressure everywhere is achieved when a wave of force moves through the fluid.Waves move at the speed of sound in water.They are moving at the speed of light.

If your battery voltage isn't changing, eventually it will reach equilibrium and the current will be the same everywhere.Light is so fast when the circuit is small that it's reasonable to assume that the current is the same everywhere in the loop.

If this isn't the case, the circuit will likely be modeled with a transmission line and you will be entering the discipline of RF engineering.

You shouldn't think about electrons moving from the negative terminal to the positive terminal.Everything will be backwards because electrons are negative charge, and you will forget about half the charge in the universe: protons and other positive charge.Rarely is the motion of individual electrons relevant, and in many circuits (and certainly any circuit with a battery), electrons aren't the only charge carriers.We don't care about the forces transmitted by charge carriers.See:

When the battery is first connected, electrons are attracted to the positive terminal and repelled from the negative terminal.The wave of force travels through the wire until the circuit reaches equilibrium, when the current is flowing everywhere.

How does the current know how much to flow before you see it?

The speed of light and sound are the same.The Cherenkov radiation is something like the light of a sonic boom.

The energy from electron drift is very low.It's not significant in ordinary circuits when it appears as a kind of inductance at frequencies approaching daylight.

The electron drift is meters per hour.There are a lot of them.

The current is charge flow,quantized as so much charge per electron, nothing to do with kinetic energy, only how many electrons pass adivider per second.

When you drop a ball from a building, it doesn't have much stopping it until it hits the ground.In this thought experiment, air in the way represents a very small influence on the ball over the conditions one might imagine.

That is not the case with electrical circuits.The mass of the electrons is very small compared to everything else in the wire.The wire is full of electrons.You can't drop an electron because it will hit other electrons.Think of a sea of balls.What we care about is how we can exploit this invisible "fluid" to do work, not the individual balls.

The circuit you drew can't exist.The lines represent ideal "wires" that are infinitely conductive, which means the voltage is the same everywhere in them.There are a lot of ways to explain this.

The battery has a constant 9V between its terminals.The battery introduces a constraint if we call the potential at the positive terminal and the negative terminal the same.

The wire that connects the battery's terminals to the schematic wire must have no more than 0V across it.We have a system of equations.

If you try to build this circuit with a real wire, it will have resistance.It's $1Omega$.The math won't be hard because most short wires will be less.The equations are here.

In any real circuit, there must be some resistance between the battery terminals.If you want to make a comparison between resistance and other physical phenomena, you should use it.The energy from moving the charge from a high potential to a lower potential is converted to heat in the resistors.

Superconductors have inductance, but they have no resistance.An infinite current requires an infinite energy source if the battery can continue to supply energy.