A current of 1A flows in a wire carried by electrons. How many electrons pass through the a cross-section of the wire each second?

This is a quite simple but really difficult question to me right now, and I put this as the heading because it is one of the most basics of questions.

So the charge of an electron is -1.6 x 10^(-19) C. If we look at the definition of what current is, it is the rate at which charge is moving.

But how are we supposed to find the number of electrons, without any other information?

  • \$\begingroup\$ What do you know about the definition of an amp? \$\endgroup\$
    – user16324
    Nov 16, 2015 at 17:52
  • \$\begingroup\$ @BrianDrummond One Amp is the current that flows with electric charge of one Coulomb per second. I=Q/t. \$\endgroup\$
    – M.S.E
    Nov 16, 2015 at 17:55
  • \$\begingroup\$ So you know how many coulombs per second. Now, how many electrons make a Coulomb? \$\endgroup\$
    – user16324
    Nov 16, 2015 at 18:24
  • \$\begingroup\$ @BrianDrummond Thanks alot Brian :) I answered my own question you can see it under the Answers :) I understood. \$\endgroup\$
    – M.S.E
    Nov 16, 2015 at 18:25

2 Answers 2


I solved my own Question


Since I=1A and we are interested when t=1 second. Therefore:

$$Q=1\cdot 1=1C$$

We know that the charge of 1 electron has the magnitude 1.6 x 10^(-19) C. Therefore we are looking for the solution to this equation:

$$\text{# Number of electrons x }1.6 \text{ x }10^{-19}=1C$$

$$\implies \text{Number of electrons}=6.25 \text{ x } 10^{18}$$

  • \$\begingroup\$ ...and how much spaces occupies one ampere? \$\endgroup\$
    – GR Tech
    Nov 16, 2015 at 21:04
  • \$\begingroup\$ @GRTech how much space? Ummm no idea. \$\endgroup\$
    – M.S.E
    Nov 17, 2015 at 6:02

So the charge of an electron is -1.6 x 10^(-19) C. If we look at the definition of what current is, it is the rate at which charge is moving.

The current is the variation in the amount of charge vs. time, not the speed at which the load moves.
If you know the current, you can know how much power is transferred per unit time. TIP: make an analysis of the dimensions and units.

The current can be defined as

\$ i(t) = \dfrac{dq(t)}{dt} \$

that is, it is the variation of the charge \$q\$, with respect to time.
If we consider the cross section of a conductor, the variation of charge corresponds to the amount of charge passing through that surface.

  • \$\begingroup\$ Isn't Q=It, so therefore I=Q/t? What I meant by moving is rate of flow of charge. \$\endgroup\$
    – M.S.E
    Nov 16, 2015 at 17:43
  • \$\begingroup\$ @M.S.E look at the edit append to my answer. \$\endgroup\$ Nov 16, 2015 at 17:50
  • \$\begingroup\$ But isn't this exactly the same thing as what I said :( I still don't understand how this leads to an answer. \$\endgroup\$
    – M.S.E
    Nov 16, 2015 at 17:57
  • \$\begingroup\$ Suppose you have 100 kg of water flowing through a pipe each minute. How would you figure out how many liters that is? \$\endgroup\$
    – The Photon
    Nov 16, 2015 at 17:59
  • \$\begingroup\$ @ThePhoton Doesnt it depend on the duration. it would be 100L in one minute, 200L in two minutes, etc etc. So clearly the amount of litres depend on the time. \$\endgroup\$
    – M.S.E
    Nov 16, 2015 at 18:01

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