This preview shows pages 1–4. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentThis preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: Drift Velocity &#2; If we apply an electric field to a wire, and can monitor the velocity of a single electron, how fast does it travel over the length of the cylinder? &#2; Possible answers: 3 x 10 8 m/sec. (speed of light in a vacuum) Somewhat less than 3 x 10 8 m/sec. since its traveling in a conductor Slower than running water in your home. Lets calculate the drift velocity u (it helps to draw pictures) Volume of cylinder = AL, the cross sectional area times the length Number of free electrons in cylinder = nAL where n = the density of free electrons per cubic meter Charge carried by electrons in cylinder = nALq e where q = charge on an electron Length of cylinder = ut = L (so we can substitute this for L above) where u = drift velocity of the electrons in m/s t = time in seconds Charge carried by electrons in cylinder = nALq e = nAutq e Current = charge/time = i = nAuq e or u = i/(nAq e ) where i = current in Amperes and u is the speed of e 1 L Crosssectional Area A Drift velocity v Drift Velocity &#2; Assume we have a current of 5 A in a copper wire of crosssectional area 2 mm 2 (~14 gauge wire). &#2; n = 8.5 x 10 28 electrons/m 3 for copper, q = 1.6 x 1019 C &#2; Solving, u = 5/(8.5 x 10 28 x 2 x 106 x 1.6 x 1019 ) = 0.18 x 10 3 m/s = 0.18 mm/s &#2; Two Observations: Closing a switch results in electrical effects occurring at the speed of light in a conductor. The drift velocity for an electron is much different. Assuming the distance from the electric meter on your house to your room is 15 m, our electron will take 15/(1.8 x 104 ) = 8.3 x 10 4 seconds = 23 hours or about a day to travel this distance, slower than running water. Questions? 2 Vs. Voltage &#2; Energy is expended to move the charges in an electric circuit &#2; Voltage is the energy per unit of charge associated with the motion of charge between two points ....
View Full
Document
 Spring '06
 MCCANN

Click to edit the document details