Explain why the brightness of a filament lamp decreases when a second identical filament lamp is connected in series with it in a circuit containing a battery of fixed electromotive force (EMF) and negligible internal resistance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Connecting a second identical filament lamp in series increases the total resistance of the circuit.
Since the EMF of the battery is fixed, Ohm’s law (I=V/R) shows that the current in the circuit decreases.
In a series circuit the same current flows through each component, so the current through the original lamp is now lower.
The power dissipated by a filament lamp is P=I²R; with a reduced current the power – and therefore the light output – is lower, so the lamp is less bright.
Since the EMF of the battery is fixed, Ohm’s law (I=V/R) shows that the current in the circuit decreases.
In a series circuit the same current flows through each component, so the current through the original lamp is now lower.
The power dissipated by a filament lamp is P=I²R; with a reduced current the power – and therefore the light output – is lower, so the lamp is less bright.
Examiner tips
- State that resistance rises when a second lamp is added. Show the relationship I=V/R and explain the current drop. Explain that the same current flows through each lamp in series. Link lower current to lower power using P=I²R.
Common mistakes
- Failing to mention that the current is the same through all series components. Using P=IV instead of P=I²R and not showing the effect of current on power. Ignoring that the battery’s EMF is fixed and internal resistance is negligible.
Mark scheme (4 marks)
- Connecting a second lamp in series increases the total resistance of the circuit.
- Since the EMF is fixed and resistance increases, the current in the circuit decreases (by Ohm's law / I = V/R).
- In a series circuit the same (reduced) current flows through the original lamp.
- Lower current through the original lamp means less power dissipated in it (P = I²R), so it is less bright.
Key terms in this question
electromotive force (EMF) · resistance
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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