A $2$ kW electric heater is used for $3$ hours. How much energy does it consume, in joules?
A$3.6 \times 10^6$ J
B$2.16 \times 10^7$ JCORRECT
C$6000$ J
D$6$ J
JUPEBPhysicsWork, Energy and Power
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Why the answer is B, and why the others tempt you.
Energy $= Pt = 2000 \text{ W} \times (3 \times 3600) \text{ s} = 2000 \times 10800 = 2.16 \times 10^7$ J, which is $6$ kWh. The value $3.6 \times 10^6$ J is one kilowatt-hour, and 6 without conversion is the answer in kWh, not joules.
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