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What is the resistance of a bulb if the current in it is 1.25 A when a 4 V voltage supply is connected to it? If the voltage supply is increased to 7 V, what will be the current in the bulb?
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Final Answer

$R = 3.2 \Omega$

$I = 2.2 \textrm{ A}$

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OpenStax College Physics for AP® Courses Solution, Chapter 20, Problem 7 (Test Prep for AP® Courses) (0:48)

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Video Transcript

This is College Physics Answers with Shaun Dychko. Ohm's Law says voltage is current times resistance and so we can solve this for resistance <i>R</i> by dividing both sides by the current <i>I</i>. So <i>R</i> is voltage divided by current. So that's four volts divided by one and a quarter amps, giving us 3.2 ohms is the resistance. Then if you take the same resistor, the same bulb, and apply seven volts across it, what would the current be? So current is voltage divided by resistance. We can rearrange this formula to solve for current as well, by dividing both sides by <i>R</i> and we get voltage divided by resistance. That's seven volts divided by the same resistance of 3.2 ohms which is 2.2 amps.