Direct Current

Parallel Bulbs


The shown circuit consists of an ideal battery (provides any current at its rated voltage), a flashlight bulb and a strip of coarse steel wool inserted in series with the bulb.  The first bulb lights as shown.  We then add additional bulbs as shown.  As we add more bulbs, the filament of steel wool heats up.  When a final bulb is added and the filament is already glowing red hot, the filament burns up and the circuit fails.

Q1: Why does the filament burn up, melt, or oxidize?

Q2: If the filament were not placed in the circuit, what would happen to the wires in the circuit?

Q3: How does the total flow of electrons change as more bulbs are added in this fashion?  Why?

Q4: How does the voltage across each bulb change as more bulbs are added in this fashion?  Why?

Q5: This circuit is analogous to a household circuit incorporating fuses or circuit breakers.  Explain this analogy, describing what the battery, bulbs and filament represent.

Q6: Under what circumstances does the circuit work?  When is it designed to fail and why?

Q7: How would you compare the lifetime of a battery used to light one bulb with the lifetime of a battery to light two bulbs in parallel?  Explain your response using electrical power.


Arons, A.B. (1994).  Homework and test questions for teaching introductory physics.  NY: Wiley, p112-113, Q6.6.

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