Amp-hours calculation

[JohnW2]

I find it quite easy to comprehend that an appliance might use 250kWh per year, and cost me £35 per year in electricity.

Needless to say, so do I - but eric does raise an (academically) interesting point about the units/dimensions. It all goes to show that, in such contexts, the word 'per', which makes total sense in terms of everyday language, does not have the mathematical implication that one might expect - even "250 kWh per hour" makes perfectly good 'everyday' sense, even though, viewed mathematically, it seems rather odd.

Kind Regards, John

 

[JohnD]

And a joule is one watt-second, so is certainly no better a unit than a kWh.

 

[JohnW2]

And a joule is one watt-second, so is certainly no better a unit than a kWh.

Sure, no better, just different. Joules would be too cumbersome, but we could just as easily use megaJoules as kWh (1 kWh = 3.6 MJ). We could always bring back BTUs

Kind Regards, John

 

[ban-all-sheds]

Or Therms as a billing unit for gas?

 

[JohnW2]

We could always bring back BTUs

Or Therms as a billing unit for gas?

IIRC, a Therm is/was nothing more than 100,000 BTUs, isn't/wasn't it? ... and it could be used for electricity, as well as gas, if you so wished

Kind Regards, John

 

[ban-all-sheds]

as you can see the numbers get rather large so we use the hour rather than second so there are 360 kJ = 1kWh

No. 1 J = 1 Ws So 1kWs = 1000 J and 1 kWh = 3,600 kJ or 3.6 MJ.

the unit is rather crazy as energy has no time

Well clearly it does. The Joule is the SI derived unit for energy and it is 1 Watt-second. Or 1 kg⋅m²⋅sˉ² if you prefer, but there's time again.

so the time in the watts cancels out the hours

No. kWh is a quantity of energy. It's 3.6MJ. If you transfer 1 kW for 1 hour you have transferred 1 kWh. If you transfer 500W for 2 hours you have transferred 1kWh.

It's like saying I have two eyes per hour and 48 eyes per day / hours.

No, it's like saying you have 2 eyes, and if you use them for 1 hour you have used 2 eye hours. Which could conceivably be a unit which makes sense. If we had humanoid robots which recorded things, then you could say that the latest Nexus 5 GTi has a recording capacity of 200 eye hours. Shut down one eye and it could record for 200 hours.

And just like (price-breaks and network capacity aside) the way that if you transfer 1,000 kWh from your electricity supplier they don't care if you use 1MW for 1 hour or 114W for a year, the bill is still £140, the Nexus 5 GTi could fill its memory in 4 days and 4 hours with both eyes running all the time or in 200 days with 1 eye working for 1 hour per day.

Energy is timeless if you lift 550 pounds one foot up it has energy only when you release that energy and allow it to return

No, it has (an extra 550 foot-pounds of) potential energy. The reason that it has that is because the 550 lb is related to gravitation acceleration, which has a time dimension. Were you to lift the same object up by 1 foot on the Moon you would have added 91 ft-lb, on Jupiter it would be 1300.

in say one second does it have power in that case one horse power.

Power is not the same as energy. Energy is the amount of work that can be performed by a force, power is the rate at which the work is performed.

A Watt is a unit of power. A Joule, or 1 Watt-second is a unit of energy.

A 10hp car weighing X and a 1000hp car weighing X can both get from A to B, but the 1000hp one, being more powerful, will do it in less time. Assuming 100% efficient engines they would both consume the same amount of fuel.

So look on the side of a freezer it says something like 250 kWh/annum clearly the hour and annum cancel them selves out so there is no time.

No they don't and yes there is.

It will use 250kWh of energy every year. It's a 28.52W load.

So it should say something like 90000 kJ.

In what time? It is a 28.52W load, so for it to use a given number of kJ it would have to run for a given amount of time. In this case, to use 90000 kJ that would be about 36.5 days

The label could accurately say 0.9GJ/annum, but since people are billed in watt-hours and not Joules that wouldn't be very helpful.

980 kilos in a Newton

No there aren't. 1 N = 1 kg·m/s². On Earth a mass of 1kg exerts a force of about 9.8 N (Earth's gravitational acceleration being about 9.8 m/s²). On Earth 980 kg would exert a force of about 9.6 kN. On Jupiter it would be about 22.7 kN.

The only way you could sort-of-reasonably say "there are X g in a Newton" (as a sort of sloppy shorthand for saying "a mass of X g exerts a force of 1 N) is to recognise that you have to have a particular gravity in mind. On Earth, BTW, there's about 102g "in" 1 N.

1 N = 1000 g·m/s². 1000/9.8 = 102

it would seem the computer world is returning to imperial with 32 pound in a slug etc.

A slug is an Imperial unit of mass. On Earth it weighs about 32.2lb (on Earth, gravitational acceleration is about 32.2 ft/s²). It's nothing to do with computing.

Any wonder kids to day get confused as to what to call things?

 

[ban-all-sheds]

IIRC, a Therm is/was nothing more than 100,000 BTUs, isn't/wasn't it?

Yup. Dimensionally equivalent to kWh which they use now.

and it could be used for electricity, as well as gas, if you so wished

You'd have to change all of the meters to measure in Therms rather than kWh. Or show the calculation on every bill, as they do now to convert from the ft³ recorded by the gas meter to kWh.

 

[JohnD]

the ft³ recorded by the gas meter to kWh.

m³

 

[JohnW2]

the ft³ recorded by the gas meter to kWh.

m³

I suspect that there are some very old gas meters still out there

Kind Regards, John

 

[ban-all-sheds]

Inc mine, obviously.

 

[GarethH]

the unit is rather crazy as energy has no time

Well clearly it does. The Joule is the SI derived unit for energy and it is 1 Watt-second. Or 1 kg⋅m²⋅sˉ² if you prefer, but there's time again.

SI could have chosen to define the Joule in terms of the amount of energy required to increase the temperature of X Kg of a substance (water?) by Y Kelvin. Both Kelvin and Kg are SI base units. There's no need for energy to be defined in terms of time; it's done that way for convenience.

Stating that a time components in KWh - (being 1000's of joules per second) times hours - cancel to yield Joules is perfectly reasonable and a good explanation, in my opinion. Those two time units do cancel.

A car travels at one mile per hour for one hour (London traffic). It travels one mile. The time component has gone. I expect you will agree with this is because distance is a SI base unit.

A LED uses one Watt (Joule per second) for one second, it has used one Joule. Now, according to your fallacious argument, the time component of the 1J/s doesn't cancel with the time component of the 1 second apparently because the Joule can be be defined in terms of other quantities that include time.

If you're going to be pedantic then at least be correct.

 

[ban-all-sheds]

SI could have chosen to define the Joule in terms of the amount of energy required to increase the temperature of X Kg of a substance (water?) by Y Kelvin. Both Kelvin and Kg are SI base units. There's no need for energy to be defined in terms of time; it's done that way for convenience.

Whatever the reason, they did not do it that way, so you can't argue that there isn't a time element. And even if they had defined it as you say, it would still equate to the amount of energy or work required to produce X watts of power for Y seconds. It would have to.

Stating that a time components in KWh - (being 1000's of joules per second) times hours - cancel to yield Joules is perfectly reasonable and a good explanation, in my opinion.

Maybe in your opinion it is.

I don't have a clue what you're on about.

kWh has a time component.

Joule has a time component. What time can possibly "cancel" to yield something which still has time in it?

A car travels at one mile per hour for one hour (London traffic). It travels one mile. The time component has gone. I expect you will agree with this is because distance is a SI base unit.

I do agree with that, but I fear that you have not grasped the fundamental and hugely important difference between 1 watt second and 1 watt per second.

If you consume 1 Joule per second for 1 second you have consumed 1 Joule. But Joule still has its time component.

If you consume 1 kW for 1 hour you have consumed 1 kWh.

A LED uses one Watt (Joule per second) for one second, it has used one Joule.

Yup. Or one watt second.

Now, according to your fallacious argument, the time component of the 1J/s doesn't cancel with the time component of the 1 second apparently because the Joule can be be defined in terms of other quantities that include time.

Forget the symbols J and s.

You're claiming that somewhere I argued that

a x b ≠ a
b

I don't think so.

1J x 1s = 1J
s

1 Ws x 1s = 1 Ws
s

If you're going to be pedantic then at least be correct.

If you're going to try and argue about this at least go away first and learn about it.

 

[GarethH]

Whatever the reason, they did not do it that way, so you can't argue that there isn't a time element. And even if they had defined it as you say, it would still equate to the amount of energy or work required to produce X watts of power for Y seconds. It would have to.

It doesn't matter what they (SI) did - that's the point. The concept of energy is independent to the concept of time.

You can express it in terms of a different quantity (such as Watts) and time (i.e. watts times time) and simultaneously introduce a new time unit on the numerator and denominator which cancel . That's because the concept of energy is independent of time unless you choose to relate it to or define it by other quantities that are dependent on time.

Joule has a time component. What time can possibly "cancel" to yield something which still has time in it?

It cancels because Joules are not related to time. You can introduce time by considering the consumption of energy for a length of time but you introduce the time unit into the numerator and denominator simultaneously which cancel because Energy is not related to time.

If you then choose to re-relate Joules to a consumption rate of Joules and a time-span you re-introduce time again on the numerator and denominator, which cancel again resulting in a fallacious and pointless circular argument.

This is how I see your "time doesn't cancel" argument:

1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J = 1W . 1s = 1(J/s).s = 1J

You can introduce an apparent dependence on time by relating the Joule to other quantities that are dependent on time but the Joule UNIT is INDEPENDENT OF TIME COMPLETELY. Once again, the derivation into SI base units is irrelevant and arbitrary.

A car travels at 1 mile per hour for one hour. It travels one mile. It makes no difference whatsoever than the resultant unit (distance) = speed times time. Time cancels.

A device consumes one Joule per second for one second. It consumes one Joule. It makes no difference that the resultant unit (energy) = power x time. Time cancels.

What's the difference?

We could refer to distance in Mile-per-hour-hours which is pretty similar to the KWh unit introduced to simplify the concept of energy to consumers of electricity. There's a reason you won't hear Physicists using KWh as a unit...

 

[JohnD]

<Stop watching this topic>

 

[ban-all-sheds]

We might as well.

GarethH is clearly ill equipped to contribute to it if he is going to claim that it's fallacious or pedantic to say that 1 Joule is 1 Watt second, (I look forward to him editing the relevant Wikipedia pages to remove their fallacious and pedantic statements), or that by equating 1 Joule to 1 Watt second I'm introducing a time component on the numerator and denominator, which cancel.

He just doesn't get it.


[edit]Corrected use of the wrong name[/edit]