Homework Help: physics

Posted by julie on Monday, March 14, 2011 at 7:02pm.

A small motor draws a current of 1.76 A from a 110 V line. The output power of the motor is 0.20 hp.
(a) At a rate of $0.060/kWh, what is the cost, in cents, of operating the motor for 4.0 h?
(b) What is the efficiency of the motor?

for (a) i am doing IV*hours*price and its coming up worng.
and for (b) i changed .2hp into Watts. and i got 149.2W.

im just not sure how to do this problem.

physics - Henry, Tuesday, March 15, 2011 at 10:57pm
Pin=VI=110 * 1.76=193.6W = 0.1936KW.

a. C = $0.060/kwh * 0.1936kw * 4h =
$0.0465 = 4.65 cents.

b. Po = 0.2hp * 746W/hp = 149.2W.

%Eff.=(Po/Pin)100%,
%Eff.=(149.2/193.6)100% = 77.1

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