P=VxIxt is energy?

Tags:
1. Feb 5, 2016

GSK

The question:
Calculate the energy used by the lamp in 6 hours of use.

P=V.I.t
P= 240v X 0.2A X 6hrs X 3600s

P= 1036800W!!?

I thought energy was Joules??? Why has the answer been given in watts? Please help!

2. Feb 5, 2016

Staff: Mentor

Welcome to the PF.

Please be sure to use the Homework Help Template that you are provided when starting schoolwork threads here at the PF. It makes it much easier for us to help you.

On your question, yes, energy should be in units of Joules. Can you post the full question?

3. Feb 5, 2016

UchihaClan13

The answer is indeed in joules
You have done a mistake
Power does not equal the energy applied okay??
It's power ×time which is equal to the energy used up
The expression which you gave gives the amount of energy used by the lamp in 6hours
If I were you I would convert 1 volt
As 1 joule/1 coulomb, 1 ampere as 1 coulomb/1 second
And then multiply these two with the total no.of seconds
As you can then clearly see
You are only left with the unit joules in your answer
You can perform dimensional analysis to further verify your answer!:)

UchihaClan13

4. Feb 5, 2016

Staff: Mentor

BTW, when doing unit conversions, it's best to just multiply by "1" and cancel units. So using 1 = 3600s/hr, you should have written:

E= 240V X 0.2A X 6hrs X 3600s/hr

And your answer is in units of V * A * s = Joules.

5. Feb 5, 2016

GSK

I'm really sorry, but I don't understand what you mean?

Why does 's/hr' then mean it converts to joules from watts?

Apologies, u have a challenge on ur hands with me.

My problem isn't physics, but maths. You're gonna tell me they're the same huh?!

m really

6. Feb 5, 2016

cnh1995

s/hr means seconds/hour. You are just converting hr into s so that the unit of energy will be joule. In SI system, joule=V×A×s.