A heating filament and an ideal resistor of fixed value are connected in series with a DC voltage source. After some time, the filament has reached its operating point so that the current in the circuit is steady. Which one of the following is correct?
- A.The steady current is larger than the initial value.
- B.The steady voltage across the ideal resistor decreases as compared to its initial value.
- C.Overall resistance of the circuit decreases.
- D.Power dissipation through the filament increases as compared to its initial value.
Correct answer
Explanation
The correct answer is B, The steady voltage across the ideal resistor decreases as compared to its initial value. A metal filament's resistance rises as it heats up, so the current falls and the resistor gets a smaller share of the voltage.
In a metal, hotter atoms vibrate more and scatter the moving electrons more, so resistance grows with temperature; a tungsten bulb filament is many times more resistive when glowing than when cold. In this series circuit the total resistance therefore increases, the current I = V/(R + Rf) decreases, and the voltage across the fixed resistor, IR, falls with it. This is also why bulbs often fail at the moment they are switched on, when the cold filament draws a surge of current.
A is wrong because higher total resistance means a smaller, not larger, steady current.
C is wrong because the filament's resistance has gone up, so the overall resistance increases.
D is not the right choice because the filament's power is I²Rf with a falling current, so it need not rise; only B follows in every case.
Exam tip: metals: resistance rises with temperature; semiconductors: resistance falls with temperature.
Practice Questions
View allRecently, there was a growing awareness in our country about the importance of Himalayan nettle (Girardinia diversifolia) because it is found to be a sustainable source of
- A.anti-malarial drug
- B.biodiesel
- C.pulp for paper industry
- D.textile fibre
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- A.1 and 2 only
- B.2 and 3 only
- C.1 and 3 only
- D.1,2 and 3
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- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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- A.1 only
- B.1 and 2 only
- C.3 only
- D.1, 2 and 3
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Read the full explanation →Which one of the following is NOT true for bleaching powder?
- A.It is used as a reducing agent in chemical industries
- B.It is used for bleaching wood pulp in paper factories
- C.It is used for disinfecting drinking water
- D.It is used for bleaching linen in textile industry
Show answer
Bleaching powder is an oxidising agent, not a reducing agent, so this statement is the false one. Bleaching powder, calcium oxychloride (CaOCl₂), is made by passing chlorine gas over dry slaked lime, Ca(OH)₂.
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