kids homework,kWh, power and energy?, duh

The SI unit for power is joules/second. I;m not sure what that means?

E=Pxt energy used equals power, times time
1000 hours in a kWh

question:headlight uses 30 amps @12 volts, power equals? She thinks 360 joules/second

Question: energy used by headlight for 3 hours (answer must be in kWh)

What's a joule?

How do you apply question one into question two?

How can she find kWh without finding watts?
(show your work)

Power = (volts * amps)/second
Power = (12 volts * 30 amps)/second
Power = 360 volts*amps/second

Energy = Power * Time
Energy = 360 volts*amps/second * 3 hours
Energy = 360 volts*amps/second * 3*60*60 seconds
Energy = 3888000 volts*amps
Energy = 3888000 volts*amps * (3600 seconds/3600 seconds)
Energy = (3888000 volts*amps / 3600 seconds) * 3600 seconds
Energy = 1080 volts*amps / second * 3600 seconds

For convenience we give a special name to volts*amps/second, we call it a watt. We call 1000 watts a kiloWatt, or kW.

Energy = 1080 watts * 3600 seconds
Energy = 1.08 kW * 3600 seconds
Energy = 1.08 kW * 3600 seconds * ( 1 hour/3600 seconds)
Energy = 1.08kW * hour

Power is measured in volt-amps, not volt-amps per second. (If you'd said "volt-coulombs per second", that would be correct.) A watt is a volt-amp (in the dc, at least).

Likewise, energy is in volt-amp-seconds or volt-amp-hours.

One way is as follows:

Compute the total amount of electric charge moved

I = Q / T (Amps = Coulomb / Second by definition)

So Q (charge) = I * T

t = 3 hours * 3600 seconds / hour

t = 10800 seconds

Q = 30 * 10800 = 324000 coulombs

Now compute the total amount of energy to move that charge:

E = Q * V

324000 coulombs * 12 volts = 3888000 joules

So we have the answer stated in terms of energy, just need to convert it to the desired units

1 kWH = 3600000 joules (unit conversion)

3888000 / 3600000 = 1.08 kwh

At no time did we compute the instantaneous power to arrive at the answer.

If the class has not yet covered the concept of "coulombs" then this might be considered a bit too cute.

power is energy/ time. unit of power is watt

Watt=J/s
360 W * ( 1000W/1 KW) *3hr= kWhr

360 Watts divde by 1000 W, multiply by 1 kW, this converts watts to kilowatts

to get kWh multiply by your hours

360W *(1 kW/1000W) * 3 hr= kWh

steveormary