Is current used up when it passes through a bulb?
Short answer: Charge isn't consumed; energy is. Current is the same everywhere in a series loop.
The trap
The wrong idea
“Current is used up as it passes through a bulb.”
It sounds reasonable, and a lot of JEE/NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
In a series circuit with one bulb, current before the bulb vs after it is:
- More before
- Same
- More after
Show the answer
B. Same
Charge isn't consumed; energy is. Current is the same everywhere in a series loop.
The click
Electric current is the rate at which charge flows past a point, I = Q/t. Charge is conserved: the electrons that enter a bulb's filament must leave it, because in a steady circuit they have nowhere to pile up. Charge flows out of the bulb at the same rate it flows in, so the current before and after the bulb is identical.
What gets used up is energy. As charge passes through the bulb, it moves from higher to lower potential, and electrical energy is converted into heat and light. The potential difference across the bulb is the energy transferred per unit charge, V = W/Q, and the power is P = VI.
In a series loop there is only one path, so the same current flows through every component: the battery, the wires and the bulb. An ammeter placed anywhere in the loop reads the same value. In a parallel circuit the current splits between branches, but the branch currents add back up to the total (Kirchhoff's junction rule), so again nothing disappears.
A helpful analogy is a closed loop of water pipes with a pump. The pump (battery) gives the water energy and a turbine (bulb) takes energy out, but the same amount of water per second passes every point in the loop. The battery doesn't supply the electrons either; they are already in the wires.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.