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An instant water heater (geyser) is rated 2 kW and converts electrical power to heat with 80% efficiency. Water flows through it at 100 cc per second and enters at 10 deg C. What is the outlet temperature? (Density of water 1 g/cc, specific heat 4.2 J/g/deg C.)

1. 12.2 deg C
2. 13.8 deg C
3. 20 deg C
4. 16.5 deg C

Correct answer: 13.8 deg C

Solution

Useful power 1600 W heats 100 g/s of water, raising it about 3.8 deg C, giving outlet near 13.8 deg C.

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