Simple energy dissipated problem but I can't figure it out

[jauser]

Hi, I'm new, if I forgot something in asking my question the RIGHT way please direct me to the correct way.

Homework Statement

A current of 0.3A is passed through a lamp for 2 minutes using a 6V power supply. The energy dissipated by this lamp during the 2 minutes is:

Homework Equations

P=VI?

The Attempt at a Solution

So I took V*I=1.8W for Power
And took 1.8W dividing 120s to get 0.015J

BUT the correct answer is 216J, I don't know how that is. Please help, thanks.

 

[berkeman]

Hi, I'm new, if I forgot something in asking my question the RIGHT way please direct me to the correct way.

Homework Statement

A current of 0.3A is passed through a lamp for 2 minutes using a 6V power supply. The energy dissipated by this lamp during the 2 minutes is:

Homework Equations

P=VI?

The Attempt at a Solution

So I took V*I=1.8W for Power
And took 1.8W dividing 120s to get 0.015J

BUT the correct answer is 216J, I don't know how that is. Please help, thanks.

Welcome to the PF.

Energy is Power * Time. You have the correct equation for power; just fix what you did for the Energy calculation.

BTW, it's best to carry units along in your equations. That way you would have seen that the units weren't coming our right for Joules, and could have seen what you needed to do differently to get Energy units in the answer...

 

[jauser]

oh. That was simple. Thanks for the quick reply.

Love, =].

 

[berkeman]

BTW, this page shows you how units like Watts and Joules are made up of the fundamental SI units. You can put these fundamental units in your equations whenever you want to check your work:

http://physics.nist.gov/cuu/Units/units.html

.

 

[jauser]

sweet, that'll be very helpful, the Units definitely always mess up my work

 

[berkeman]

sweet, that'll be very helpful, the Units definitely always mess up my work

That was one of the most valuable lessons that I learned early in my undergrad. Get used to carrying units along in your equations, and cancel them out as appropriate just like you do variables (like meters/meters = 1). It helps your intuition about what variables go where (like velocity[m/s] = distance[m]/time), and helps you to catch errors early in equation manipulations.