Three bulbs are rating 40W 60W and 100W designed to work on 220V mains. Which bulb will burn most brightly if they are connected in series across 220 V mains ?
Correct answer: A. 40 W bulb
- A. 40 W bulb
- B. 60 W blub
- C. 100 W blub
- D. all will burn equally brightly
Explanation
At the same rated voltage of 220 V, each bulb's resistance is R = V²/P, so the 40 W bulb has the greatest resistance. In series the current is the same, and power P = I²R, so the 40 W bulb dissipates the most power and glows brightest; option c is the likely mistake because it has the greatest rated power when used at its normal voltage.
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Resistance measures opposition to current in a conductor and follows R = V/I for an ohmic conductor, while resistivity is an intrinsic property of the material. Questions use R = ρL/A, examine effects of length, cross-sectional area and temperature, and distinguish resistance, resistivity, conductance and conductivity.
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