Current and Drift velocity

Monday 7-8-96

The relevant section in the textbook is 16.2

Main concepts:

• for a current to flow requires a complete circuit, which means the flowing charge has to be able to get back to where it starts
• current = flow of charge = I = q/t, with units of A=C/s. The current is the amount of charge flowing per unit time.
• even though we know that it is electrons which flow when we talk about currents, when analyzing circuits the direction of the current is taken to be the direction of the flow of positive charge, opposite to the direction the electrons go. We can blame Benjamin Franklin for this.
• in a current-carrying conductor, the charges (electrons) do not all flow in the same direction. In fact, in most cases the movement of the electrons is almost random, with a small net velocity, the drift velocity, in the direction opposite to the electric field.
• the drift velocity is typically 1 mm/s. The electric field, however, propagates much faster than this, more like 1 x 10^8 m/s. When a light switch is turned on, for example, it takes a long time for an electron to travel from the switch to the light, but a very short time for all the electrons in the wire to start moving in response to this field, making the light glow.