Homework Help: Physics

Posted by Ricky on Sunday, December 5, 2010 at 6:33pm.

#1.) A wind turbine has a Cp of 0.42 and extracts 50 kW of power when the wind velocity is 25 m/sec. How long are the blades? (ρ = 1.2 kg/m3)

#2.) An electric hot water heater is used to heat 30 gallons of water from 20 °C to 45 °C.

a. What heat energy is required to increase its temperature?

b. What electric energy (still in Joules) is required if the efficiency is 98%?

c. If this heat energy flows into the water a constant rate over a 30 minute period, what power is
required over this period?

d. If this power is delivered by a resistive hot water heating element using a 240 V source, what must the resistance of the heating element be?

Physics - bobpursley, Sunday, December 5, 2010 at 7:02pm
heattoheatwater=masswater*specific*(45-20)

figure mass water from volume and denstity, look up specific heat.

if eff is .98, then energy=heat/.98

power=heateneryg/timeinseconds

R=V^2/energy

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