My calc yielded 37.97705305W, Oh, there is a simple mistake in 60 X 12 X 0.053. It is 38.16W.

Are you really claiming that you computed the power to ten sig figs?

It's not obvious where the discrepancy between your two answers come from since the reported 0.053 has two sig figs and your answers agree to better than 2.5 sig figs. You are talking about a discrepancy at the end of 3 sig figs and approaching the 4th.

 

I can't tell that 60.12 is to be read as 60 multiplied by 12.

I guess then your answer of 30.16 is to be read as 30 multiplied by 16?

Do you see how using a decimal point to indicated multiplication in one place and the separation of the whole part of a number from the fractional part can be just a tad confusing?

Sorry, I didn't think about that. I should use other symbol such as * for multiplication.

And, had you included the units I would have been able to divine your meaning a whole lot better.

Yes, I thought about that but I feel it is a bit confusing because sometimes I even don't know where is unit and where is variable.
For example, in your example above 120π r/s*t, I sometimes misread and consider r/s are variables.
But this maybe only my problem.

 

Sorry, I didn't think about that. I should use other symbol such as * for multiplication.

Yes, I thought about that but I feel it is a bit confusing because sometimes I even don't know where is unit and where is variable.
For example, in your example above 120π r/s*t, I sometimes misread and consider r/s are variables.
But this maybe only my problem.

Now, telling which is a variable and which is a unit can be confusing. There are two effective ways of dealing with this. First, put values into parens, so your example would be

(120π r/s)t or even (120π r/s)(t)

Of course, if you are reading what someone else wrote, then you have to decipher it a bit.

The better and more effective way is to get the values, especially those that have units, out of the equation and replace it with a parameter until the point at which you are really to plug in numbers for all of the variables. So do something like:

Vbatt = 12V
Vd = 0.7V
Vs = 60V (Vpeak)
R = 4.26Ω

Then work the problem with the symbols. At the end, you can plug in the values. In this case, by carrying the symbols through, changing the Vd from 0.7V to 0V was simply a matter of plugging a different value into parts of the final couple lines. Simimilarly, you could have quickly changed your Vs from 60V to 60V*sqrt(2) = 84.85V if you wanted to see what the result would be if the given supply voltage was RMS and not the amplitude.

It also makes sanity checking the work and verifying that the units work out so much easier and it makes finding and correcting mistakes so much quicker and efficient.

 

FYI: You image insertions didn't work.

Should this be in Homework Help?

Unlike a resistor, a current in a battery one direction is putting energy into it while a current in the other direction is taking energy out of it. In a resistor, current in either direction is putting energy into it.

The reason is that the voltage that the current is going through is constant. If this were a resistor, the voltage would be a proportional to i(t) and so you would end up with i^2 that you must find the average of. But since V is constant, it can be factored out and now you just need to find the average of i(t).

When trying to figure these things out, don't be scared to go back to the basics and see what the math has to say.

THANKS Alot for this very helpful reply.

 

Are you really claiming that you computed the power to ten sig figs?

It's not obvious where the discrepancy between your two answers come from since the reported 0.053 has two sig figs and your answers agree to better than 2.5 sig figs. You are talking about a discrepancy at the end of 3 sig figs and approaching the 4th.

Oh, not really, that is I am not reporting the error due to significant digits.
The figure was quoted as 30.16 in the posts, and all I wanted to say was it is 38.16. Well the no. of significant digits came from an online integrator, excel and cut/paste, and was not thinking about it at all. Sorry, I did not make this clear.