Conceptual Physics

current is actually the movement of negative charge carriers (electrons), the positive charge flow convention had already been established, and it remains in place to this day. In basic configurations like the segment of conducting wire shown to the right, electric current is stated as a positive quantity and its direction is indicated with an arrow. In other circumstances, such as alternating current circuits where the direction of current flow changes constantly, signs can be used to indicate the direction of current.

I = current

ǻq = charge passing through surface

ǻt = elapsed time

Units: amperes (A)

In 2.0 seconds, 3.0 coulombs of

charge flow past this charge

counter. What is the current?

I = 1.5 A

25.2 - Drift speed

Drift speed: Average speed of charge carriers

that make up a current.

Whether or not there is an electric current in a wire, the electrons in the wire move randomly at high speed, on the order of a million meters per second, due to their thermal energy. This random motion does not create a current because it causes no net motion of charge over time. When there is an electric current, there is a net motion of charge. The average speed at which charge moves in a current is called its drift speed. To illustrate this concept, we use the simplified diagram of the wiring in a car shown in Concept 1. In the diagram, a battery is shown; the potential difference between its terminals causes electrons to move counterclockwise along the wire. The average speed at which they move along the wire is the drift speed. The magnitude of the drift speed may surprise you; it is on the order of 10í^4 m/s. That is less than a meter per hour, slower than a snail slimes across the ground. This may seem surprising because when you flip the car’s headlight switch, the lights illuminate almost instantaneously. It may seem as if electrons are flowing from near the battery to the headlights at an incredible speed. Why do the lights go on so quickly? When the switch is flipped, the electric field created by the car’s battery travels through the wires at nearly the speed of light, 3.00×10^8 m/s. It causes electrons everywhere in the wire, including those in the filaments of the headlights, to start to move almost immediately. It is like when you turn on a water faucet: water flows out very quickly. You receive water near the faucet; you do not have to wait for water to flow all the way from the reservoir to your tap. Why do the electrons move along the wire so slowly? If the electrons were in a vacuum, the electric field would accelerate them until they moved at near the speed of light, since there would be no force opposing their motion. But in a wire, the electrons collide with the atoms that make up the wire. The combination of the collisions and the electric field cause them to move in the erratic zigzag pattern you see in Concept 2. (We overstate the curved element of their paths to emphasize the effect of the field on their motion.)

Drift speed

Average speed of charges in current In typical wiring, about 0.1 mm/s

Electrons zigzag ...

... but move along wire