How is current used, say, in a light bulb?

Question

I understand that current is the flow of charge, not necessarily of electrons (which drift slowly). The way I imagine this is a long tube filled with balls that just fit within the tube - when you push a ball at one end of the tube, the ball at the other end is pushed basically instantly, even though none of the balls move particularly much.

A filament light bulb works because a piece of wire with high resistance heats up and causes light to be seen. In the ball-in-tube analogy, I guess this would be like having three tubes connected to each other - one full of air, one full of oil and another full of air. All would hypothetically be connected to each other (pretend the oil had no way of filling into the air tubes).

Essentially:

[air][oil][air] = [wire][light bulb][wire]

I don't understand how moving one electron and essentially causing charge to be propagated throughout the circuit can cause a filament light bulb to work.

Can someone explain this using the ball-in-tube analogy, or if I have some conceptual misunderstanding in the analogy, correct me?

Answer 1

How is current used, say, in a light bulb?

It is not used. Current is constant in all parts and components along a path without branches. Any charge that enters the light bulb every second also has to exit it every second (otherwise charge is built up in there).

The thing that is "used" is not the current but the energy. In your analogy, try to replace the oil part with a mill-wheel and then just think of the wire as a tube of water.

• The water moves into the tube, reaches the mill-wheel, pushes it so it turns, and then moves out of the other end of the tube. Just as much water exits from as what enters, because where else would it go - it is not the water that is "used".
• Rather, the water is being pushed through this tube. Maybe there is a pump, maybe the tube is vertical so the push (pull) is gravity. This pressure on the water makes it move, and this pressure is higher before the mill-wheel than after. This pressure is being "used".

In electrical wires, this "pressure" is the voltage. Voltage is the amount of energy per charge. And this is being "used"; a part of this energy that every charge arrives with is delivered to the light-bulb and converted into heat and then light. The charges exit again in the same number but less energetic.

Conclusion: There is a difference between charge and energy; charge is not "used" or "taken away", but energy is. What you see as light from the bulb is not charges flying out, but energy radiating out.