Now consider a piece of copper wire, a very commonly used conductor. It is made of copper atoms (around 8.5 x 1019 per mm3). The nuclei, consisting of protons and neutrons, and most of the electrons are static, meaning that they keep their relative positions in the wire. They do not move. However, every copper atom has an electron that is able to escape from its atom. These electrons are called valence electrons. The little black dots in the figure below represent a few of these free electrons.
Now suppose that these electrons move through the wire at a certain speed, an electric current is said to go through the wire. By definition, an electric current is a displacement of a certain amount of electric charge per unit of time. So, the faster this charge moves, the higher the current is. For a constant current, the formula becomes:
I = ΔQ/Δt
In the numeric example below this formula is used to calculate the current in a typical situation.
The unit of current is the ampere, abbreviated with ‘A’. So, a current of 1 A passing through a wire corresponds to a charge of 1 C that passes through any point in the wire per second. The numeric example illustrates how slowly the electrons are moving (one tenth of a millimeter per second), even at a current of 1 A, which is considered as a very large current in electronics, where usually is dealt with milliamps (mA) or even less. Time for some common misconceptions.
Common misconception “I thought these electrons move at light speed.” No they do not. First of all, this is physically impossible since we know from Einstein’s relativistic theory that this would require an infinite amount of energy. Secondly, what is confused is the speed of the electrons with the speed of the effect when a current starts flowing. Indeed, when electrons start moving at one end of the conductor, a split second later the electrons at the other end start moving as well. The time it takes for these latter electrons to notice that the current has started equals the length of the wire divided by light speed.
Common misconception “Do these electrons nicely move in a straight line?” No they do not. Figure 1.1 is indeed misleading. What the arrows show is the net speed, but in reality these free electrons scatter in all directions. Even if the current is zero, these electrons are moving around in the wire, colliding with the atoms, and changing direction all the time. But if you sum up all these small displacements, the net result is zero. If the current is non-zero, then this net result is non-zero as well. The small speed in the numeric example above relates to this net result. The instantaneous speed of an individual electron is much higher.
Conventional current direction
A final word on the electric current. Since the electrons are moving in a certain direction (typically along the length of a wire), the current has a direction as well. Now, the direction of a positive current is opposite to that of the electron flow, because electrons have a negative charge. The direction of this positive current is called the conventional current direction.
What to remember
- The symbol for current is I.
- The unit of current is the ampere (A).
- 1 A corresponds to an electric charge of 1 C passing through a certain point during a 1 second interval.
- The strength of a current depends on the net speed of the electrons. The faster the electrons move, the higher the current.
- The conventional current direction is opposite to the direction of the electron flow.