2.  Recall that electrons traveled from the cathode to the anode in the external circuit: Topic 5.1 Extended A – Current and drift velocity ---------- ---------------- ++++++++++++++++ ++++++++++ Electrolyte Anode Cathode The chemical cell - +  Note that it is the ELECTRONS that flow through the circuit, not the positive ions.  For historical reasons, circuit analysis is usually performed using what is called conventional current.  Conventional current is POSITIVE current flow. Mathematically, it is the same as negative current flow, except it travels in the opposite direction through the circuit.

3. Topic 5.1 Extended A – Current and drift velocity  This schematic diagram (symbolic representation of a circuit) shows electron flow:  Note that the ELECTRONS flow everywhere simultaneously. + -  This schematic shows conventional current flow: Electron Current + - Conventional Current  Note that the conventional current flows everywhere simultaneously. FYI: In order to help you understand why the electrons travel everywhere simultaneously in a complete circuit, visualize electrons as ping-pong balls in a tube: FYI: If the battery shoves an electron in the tube, all of the electrons are shoved along the tube to make room for it.  Conventional current flows in the direction opposite to the electron current.

4.  We define electric current I as the rate of flow of charge q past a point. Quantitatively, Topic 5.1 Extended A – Current and drift velocity I = qtqt  Or more simply I = qtqt Electric Current  The SI unit for current is given by coulomb second = CsCs = ampere (A)

5. Household circuits can carry up to 12 A of current. How many electrons would pass a point in a wire in 1 minute if there were 12 A in the wire? Topic 5.1 Extended A – Current and drift velocity Convert 1 minute into 60 second. Then I = qtqt q = It q = 12(60) q = 720 C 1 e - 1.6  10 -19 C = 4.5  10 21 e -

6. D RIFT V ELOCITY Topic 5.1 Extended A – Current and drift velocity  In a metal, free electrons move very rapidly, but collide constantly with the atoms in the lattice structure of the metal.  Note that through any cross-section of the conductor, the net current is zero. FYI: Thus, although the electrons have a very high velocity, the net result at the macroscopic level, is that there is no electron migration.

7. Topic 5.1 Extended A – Current and drift velocity D RIFT V ELOCITY  If we place that same portion of conductor under the influence of a potential difference, we have a slowish drifting of the velocities toward the lower potential:  Note that the net current is NOT zero in this case. FYI: The electrons still have a very high velocity, but this time the net migration is in the direction of lower potential. VV FYI: The speed of this net migration is called the drift velocity. FYI: The drift velocity is of the order of 10 -3 m/s. Thus it takes an electron about 1000 s (17 minutes) to travel 1 meter in house wiring! Question: If the wiring between the light switch and the bulb is 8 meters, it takes an electron about 133 minutes (2.2 hours) to travel from the switch to the bulb. Why does it light up instantly? Answer: The electric field travels at about the speed of light, causing all of the electrons in the conductor to drift virtually instantaneously!