Physics · Common misconception
Is current used up by a bulb?
Track charge around a simple circuit and distinguish it from energy transfer.
The short answer
Current is the rate of flow of electric charge. In a steady series circuit, the current is the same at every point. A bulb transfers energy electrically to other stores and surroundings; it does not use up the charge flowing through it.
Conceptual illustration. Colours distinguish parts of the model.
TRY A SMALL CHANGE
Predict first. Then move the control.
Where the confusion starts
A dimmer bulb does not imply that less charge comes out than goes in. A sustained difference between incoming and outgoing current would mean charge was accumulating. Real circuits can have brief changes while switching, but the steady-state model has equal series current.
WORK IT THROUGH
A steady series circuit has a current of 0.30 A.
- 0.30 A means 0.30 coulombs pass a point each second.
- In 10 seconds, Q = It = 0.30 × 10 = 3.0 C.
- The same 3.0 C passes a point after the bulb during those 10 seconds. Energy has been transferred, while charge is conserved.
CHECK YOUR UNDERSTANDING
An ammeter before a bulb reads 0.40 A in a steady series circuit. What should one after it read?
Make a prediction or write an answer before opening the explanation.
Reveal the explanation
0.40 A. The bulb transfers energy, not a disappearing supply of charge. This comparison assumes the two ammeters are in the same series branch.
SEE IT. CHANGE IT. EXPLAIN IT.
Take the next step in the lesson.
Use the visual explanation in the lesson, then choose Practice to test the idea with another question.
Spotted something unclear? Send feedback. These are original teaching explanations, not predictions of future exam questions.