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Here is the problem: A 40 W appliance runs on 12 V. What current does it draw and what is its resistance?

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Use the electric power formula, rearranged for the unknown, in SI units.

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Formula: electric power.

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Rearrange for current I.

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Substitute the known values.

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Resistance (Ohm prime s law).

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So the answer is current I equals 3.33333 A; R equals 3.6 Ω.

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Electric power is the rate at which energy is transferred: P equals VI. Combined with Ohm prime s law it can also be written P equals I squaredR equals V squared/R. A 60 W bulb on 120 V draws 0.5 A.

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Substituted every value back into the formula: both sides agree, which checks out.