Question

HELP: The heat developed in an electric wire varies jointly as the wire's resistance, the time the current flows, and the square of the current. In two minutes a current of 5 amps develops 1,200 heat units in a wire of 8 ohms resistance. What resistance does a similar wire have, which develops 6,000 heat units with a current of 10 amps in 5 minutes?
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Answer 1

H / (rtc^2) = k 

1200 / (8•2•5^2) = 6000 / (r•5•10^2) 

1200 / 400 = 6000 / 500r 
3/1 = 12/r 

3r = 12 
r = 4 ohms

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hope this helps