A heating element is marked 210 V, 630 W. The current drawn by the element and the resistance of the element when connected to 210 V DC mains would respectively be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( 3\,\mathrm{A},\ 70\,\Omega \)
The marking gives the rated power at the rated voltage. The element is run at exactly its rated 210 V, so \( P = VI \) gives the current and Ohm’s law gives the resistance.
\( \displaystyle I = \frac{P}{V} = \frac{630\,\mathrm{W}}{210\,\mathrm{V}} = 3\,\mathrm{A} \)
\( \displaystyle R = \frac{V}{I} = \frac{210\,\mathrm{V}}{3\,\mathrm{A}} = 70\,\Omega \)