# Why is it called drift velocity and not drift speed?

Why is the average rate at which electrons cover the length of a conductor called drift velocity? Wouldn't it being called drift speed be more fitting as that would remove all notions of the conductor's shape and orientation? For example, if the electrons were moving through a bent wire of uniform cross-section, the drift velocity of the electrons would be different at the points before and after the bend, whereas the drift speed would be constant.

The technical definition often given is that $$j=(nq)u$$ where $$j$$ is the current density (which is a vector) and $$nq$$ together make up the charge density (which is a scalar) and with $$u$$ will be a vector velocity. So the idea is that the drift velocity within a material varies from point to point and time to time — in the general case. However, I would say that a lot of the time it is the magnitude of the drift velocity (say in a thin wire where the current is thought of as all in the same direction down the wire) that is computed, and you could argue that this is the drift speed. I think this is a case where people say — the meaning is clear from the context. Language is not always used in a simple manner. Details of the pragmatics and context often change the details of the meaning.

You need to consider the difference between velocity and speed. Speed is a scalar quantity and can be thought of as the rate at which an object covers distance. Velocity, on the other hand, is a vector quantity and can be thought of a the rate at which an object changes its position. The magnitude of the velocity of an object is its speed.

Without an electric field present in a conductor, the thermal motion of the free electrons is random. The individual electrons cover a great deal of distance per unit time, but over time their position does not change. So their speed is great but their velocity is zero. It's like you rapidly taking the same number of steps forward and backward. Your speed is great but your velocity (rate of change in position) is zero since your position over time does not change.

With an electric field present in a conductor, superimposed on the random motion of the electrons is the collective motion, or drift, of the electrons where their position changes in time in the direction opposite that of the electric field. The rate of change in position is the drift velocity. The drift velocity of the electrons is extremely slow compared to the speed of their thermal random motions. The drift speed is the magnitude of the drift velocity.

Hope this helps.

The simple answer is that even in physics we don't use English words to always mean exactly their technical definition. That's just the way it is...

While a speed is never used to refer to a vector quantity the term velocity is sometimes used to mean the magnitude of the rate of change of the position vector. Such is the case, for example, with drift velocity or terminal velocity.

If you want to know why we use imprecise language like that it'll be a question for linguists rather than physicist. (But given that physicists typically consider themselves to be qualified to comment on any field you might get a few answers from us here too!)

• And if you're wondering how we can cope with the ambiguity, it's worth considering that other languages don't distinguish between speed and velocity at all! And that even in English we often use the same term to refer to the vector quantity as well as its magnitude Take for example 'force'...! Oct 29, 2022 at 15:36