Exam-style question
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Which statement best describes the I‑V characteristic of a filament lamp when it is cold compared to when it is hot?.
- A.It is linear and passes through the origin in both states.
- B.It is linear when cold but becomes non‑linear when hot.
- C.It is non‑linear when cold but becomes linear when hot.
- D.It is non‑linear in both states and does not pass through the origin.
Model answer
What a good answer should say
- It is non‑linear when cold but becomes linear when hot.
Explanation
Why this works
A filament lamp contains a thin tungsten filament. When cold the filament has a very low resistance, so the I‑V curve is almost a straight line through the origin (ohmic behaviour).
As the filament heats up its resistance increases sharply, making the I‑V curve bend and become non‑linear. Thus the correct description is that the lamp is non‑linear when cold but becomes linear when hot.
Common mistake
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