Domestic Wiring and Illumination Questions and Answers

Answer: D

The luminous efficiency is 15 lumens per watt.

Luminous efficiency is the light output divided by the electrical power taken, \(\eta = \Phi / P\), and the power is the product of voltage and current.

\[ \begin{aligned} P &= 250 \times 0.8 = 200\ \text{W} \\ \eta &= \frac{3000}{200} = 15\ \text{lm/W} \end{aligned} \]

12 lumens per watt comes from dividing the flux by the voltage alone (3000/250), and 240 and 9.6 do not follow from the given values.

Answer: C

Of these, Eureka has the lowest operating temperature.

Eureka is a copper-nickel alloy of the constantan type. Its resistance hardly changes with temperature, which suits resistors and instruments, but it oxidises at moderate temperatures, so as a heating element it is limited to low-temperature uses of the order of a few hundred degrees Celsius.

Nichrome (nickel-chromium) is the usual element for domestic heaters and works up to about 1100 to 1150°C. Kanthal (iron-chromium-aluminium) goes higher, to about 1300°C, and graphite elements can be run hotter still in a protective atmosphere.