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Formative assessment questions for teaching physics

A2L Item 225

  • Description: Identify the brighter of two bulbs having different wattages in a series circuit.
  • Goal: Reason about electric circuits
  • Source: 283-580 100-watt and 40-watt bulbs in series
  • Keywords: Circuits, Electromagnetism, Power

The question for students:

Two light bulbs are connected to a wall outlet as shown below. Bulb #1 is 100W and Bulb #2 is 40W. Which statement is true?

Item225_fig1.gif

  1. Both bulbs are at their normal brightness.
  2. The 100W bulb is brighter than the 40W bulb.
  3. The 40W bulb is brighter than the 100W bulb.
  4. Both bulbs are at equal brightness.
  5. Cannot determine their relative brightness.

Commentary for teachers:

Answer

(3) Since the resistance of the bulbs goes as the reciprocal of its wattage, students can reason that the most power is dropped in the 40W bulb. Another way to reason is this. Suppose the potential source was such that the potential difference over the 40W bulb was 120V, i.e. its normal operating condition. The potential difference over the 100W bulb is then 48V, which means that it is operating at (48/120)2 of its normal condition. The bulb is dissipating about 16W.