Why Is My Joules Calculation Incorrect?

[lettertwelve]

Homework Statement

A steam iron draws 8.0 A when connected to a potential difference of 120 V.
(a) What is the power rating of this iron? 960 W <<<<correct

(b) How many joules of energy are produced in 20.0 min?
320 J <<<incorrect why?

(c) How much does it cost to run the iron for 20.0 min at $0.010/kW·h?
.0032 $ <<<<correct

Homework Equations

for part b: 960*(20/60) = 320

whys it wrong?!

 

[dynamicsolo]

(b) How many joules of energy are produced in 20.0 min?

Homework Equations

for part b: 960*(20/60) = 320

whys it wrong?!

Look at your units: 960 W is 960 Joules per second. How many seconds are there is 20 minutes? This is the number of seconds you would multiply 960 W by (it is a rather larger number than 20/60).

If part (c) is correct, you managed to make a self-canceling error...

 

[lettertwelve]

too late now.

thanks anyway.

 

[mikelepore]

About the step for calculating joules... Perhaps, early in the course, the teacher mentioned a topic called dimensional analysis. When you're solving a problem, you write units next to your numbers. That includes conversion factors, such as 1 minute/60 seconds, or, if necessary to flip that upside down, that might be 60 seconds/1 minute. You see what units will cancel out. After you cancel out whatever you can, take a look: do you end up with joules in the numerator and nothing in the demonimator, as you were hoping for? If not, then you made an algebra mistake. That's what dimensional analysis is, more than anything else - it's a way to receive warnings about our algebra mistakes.