26.2:
Drift Velocity
Free electrons in a conductor travel in random paths and collide with other electrons and particles.
In an electric field, the randomly moving electrons gradually drift in the direction opposite to the applied electric field.
Thus, the average velocity of free-charged particles in a material due to an electric field is known as the drift velocity. It is the ratio of the current in a conductor to the product of the concentration of charged particles, the magnitude of charge of each particle, and the cross-sectional area of the conductor. The SI unit of drift velocity is meters per second.
Consider a copper wire with a diameter of 1 mm. What would be the mean drift velocity of the electrons in the wire when a current of 10 A passes through it?
To begin, calculate the density of free electrons in the copper wire. The number of free electrons in copper equals the number of copper atoms per cubic meter. Second, compute the cross-sectional area of the wire.
By rearranging, and substituting the terms, the drift velocity can be determined. The negative sign indicates that the charges move in the opposite direction to the conventional current.
26.2:
Drift Velocity
The high speed of electrical signals results from the fact that the force between charges acts rapidly at a distance. Thus, when a free charge is forced into a wire, the incoming charge pushes other charges ahead due to the repulsive force between like charges. These moving charges move the charges farther down the line. The density of charge in a system cannot easily be increased, so the signal is passed on rapidly. The resulting electrical shock wave moves through the system at nearly the speed of light. To be precise, this fast-moving signal, or shock wave, is a rapidly propagating change in the electrical field. Good conductors have large numbers of free charges. In metals, the free charges are free electrons. The distance that an individual electron can move between collisions with atoms or other electrons is quite small. The electron paths thus appear nearly random, like the motion of atoms in a gas. But there is an electrical field in the conductor that causes the electrons to drift in the opposite direction (opposite to the field, since they are negatively charged). The average velocity of the free charges is known as the drift velocity and is represented in meters per second. Drift velocity is quite small, in the order of 10−4 m/s, since there are so many free charges. Free-electron collisions transfer energy to the atoms of the conductor. The electrical field does work in moving the electrons over a distance, but that work does not increase the kinetic energy of the electrons.
The charges of the moving particles may be positive or negative depending on the type of material. In metals, the moving charges are negative (electrons), while in an ionized gas (plasma), the moving charges may include both electrons and positively charged ions. In the case of a semiconductor like silicon, conduction is partly by electrons and holes. Holes are some vacancy sites of missing electrons, which act like positive charges. The conventional current is treated as a flow of positive charges, regardless of whether the free charges in the conductor are positive, negative, or both; whereas, in a metallic conductor, the moving charges are electrons, but the current still points in the direction of positive charges would flow.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version], Pg 95- 96. Retrieved from https://openstax.org/books/university-physics-volume-2/pages/9-1-electrical-current