# Homework Help: How many electrons make up a charge of -90.0μC ?

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1. Mar 14, 2015

### Orhan

Now;
I know
1e = 1,6*10^-19C
-90μC=-9*10^-5C
N=q/e

so

(-9*10^-5C)/(1,6*10^-19C)
=-5,625E+14 or -5,625*10^14

but thats a computer program so i dont know what it really wants from me
i dont know my answer is true or not. i really need some help

2. Mar 14, 2015

### Staff: Mentor

Hello Orhan,
Welcome to Physics Forums.

In future, please use the formatting template provided to format your question. It's in the rules.

Note that the charge on an electron is negative. The e you've used is just the magnitude of that charge, so be sure to include the correct sign! You should end up with a positive number for the number of electrons.

Note also that they are asking for three significant figures. The value of e that you've used contains only two! So find (look up) a better value for e.

3. Mar 14, 2015

### Orhan

I searched from the net it says
1e = 1.60217646e-19C
so 1,60 is its 3 digit version and it equals 1,6. I thought that way. And yeah i fixed the -/+ problem but still not true

4. Mar 14, 2015

### Staff: Mentor

When you have constants in your calculations (like pi, or e, or the charge on an electron), it's always better to use values with more significant figures than the number of figures in your other data. Don't trim or round intermediate values n your calculation steps, otherwise rounding or truncation errors will creep into your significant digits.

Now, what do you mean by "still not true"? What is your current result?

5. Mar 14, 2015

### Orhan

Sorry for my bad English but i dont know if i understood true. u mean use "1e = 1.60217646e-19C" in your calculations and express your result after all the calculations finished, right ?
Edit : i just changed - to + nothing more :)

6. Mar 14, 2015

### Staff: Mentor

Well, if you're going for 3 significant figures in your you might want to use 5 for your constants.

The numerical value that you presented initially (ignoring the sign) was not specified to three significant figures (it was specified to four, and they were not accurate to four!).

Also, Orhan, please note that text-speak abbreviations like "u" for "you" are not allowed on the Forums.

7. Mar 14, 2015

### Orhan

sorry for abbreviation.
I'm doing everything and finding the result = 5,62*10^14 or 5,62E+14
and i just want to be sure 1,23 has 3 significant figures right? or should it be 1,234?

8. Mar 14, 2015

### Staff: Mentor

Three figures means three digits. So 1,23 has three significant digits.

Your value 5,62*10^14 looks good. Now, I don't know where the computer program that you're using was authored, but be sure to confirm that it accepts a comma as a decimal point for numeric values. You'll have to read its documentation to find out how to specify entries that require scientific notation.

9. Mar 14, 2015

### Orhan

Thanks for help i entered 999 times 5,62*10^14 because in lessons we use ",". But i changed with "." and it said to me correct.