# Isaac Physics Bulbs &amp; Power - Level 2

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#1
Hi,

I am really struggling to complete this question on Isaac Physics. Can anyone point me in the right direction on how to work it out, please? I've looked at the video and its still not helping.
Thanks!
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4 weeks ago
#2
The objective is to find value of R in order for the power dissipated in the bulb to be 100 W. Do you know the formula for power dissipated in a resistor? This will allow you to find the current through the bulb when it is dissipating 100 W.

Then, you need to relate the current through the bulb to the current flowing through the main circuit. It may be helpful to know that the current through one branch of a pair of parallel resistors flows in proportion to the resistance of opposite branch.

The final step is to establish an equation for the current flow in the main circuit using the total resistance and the potential difference. This requires you to know the formulae for combining resistances in series and in parallel.

Combining all these together, you should be able to eliminate the unknown currents and obtain a numerical value for R.
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