Energy Calculations: P=VxIxt - Why Watts & not Joules?

In summary: So, you need to convert all the units into these basic units in order to get the right unit for energy. Watts is a measure of power, which is the rate at which energy is used, not the total amount of energy. To get the total energy used, you need to multiply by time, which is why the units of seconds were introduced in the conversion. In summary, in order to calculate the energy used by a lamp in 6 hours of use, you need to use the formula P=V*I*t and convert all units to their basic units of volts, amperes, and seconds. This will result in the answer being in units of joules, which is the correct unit for energy.
  • #1
GSK
2
0
The question:
Calculate the energy used by the lamp in 6 hours of use.

The answer:
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!
 
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  • #2
GSK said:
The question:
Calculate the energy used by the lamp in 6 hours of use.

The answer:
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!
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
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
GSK said:
P= 240v X 0.2A X 6hrs X 3600s
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. :smile:
 
  • #5
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 going to tell me they're the same huh?!
berkeman said:
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. :smile:
m really
 
  • #6
GSK said:
Why does 's/hr' then mean it converts to joules from watts?
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.
 
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FAQ: Energy Calculations: P=VxIxt - Why Watts & not Joules?

What is the formula for calculating energy using watts?

The formula for calculating energy using watts is P=VxIxt, where P represents power in watts, V represents voltage in volts, I represents current in amps, and t represents time in seconds.

Why do we use watts instead of joules in energy calculations?

Watts are used in energy calculations because they measure the rate of energy transfer, while joules measure the total amount of energy. Since many devices operate at different rates, watts provide a more accurate measurement of energy usage. Additionally, watts are a more practical unit for everyday use as they are smaller and easier to work with compared to joules.

Can watts and joules be converted into each other?

Yes, watts and joules can be converted into each other using the formula 1 watt = 1 joule/second. This means that 1 watt is equivalent to 1 joule of energy being transferred per second.

Is power consumption the same as energy usage?

No, power consumption and energy usage are not the same. Power consumption refers to the rate at which energy is used, while energy usage refers to the total amount of energy used over a period of time. For example, a device that consumes 100 watts of power for 1 hour will have a power consumption of 100 watt-hours, but an energy usage of 100 watt-hours x 1 hour = 100 watt-hours.

How can energy calculations be useful in everyday life?

Energy calculations can be useful in everyday life for understanding and managing energy usage. By calculating the power consumption of devices and appliances, individuals can make informed decisions about which ones to use and for how long in order to conserve energy. Additionally, energy calculations are used by utility companies to measure and bill for energy usage, and by engineers to design and improve energy-efficient technologies.

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