1. Physics 2113 Lecture 17 FRI 02 OCT Current & Resistance I Physics 2113 Jonathan Dowling Georg Simon Ohm (1789-1854) Resistance Is Futile!

2. Electric Current Current i is the Rate (charge/second) charge passes by a point. Positive Current is Direction of Positive Charge Flow!

3. ICPP: What is Direction and Magnitude of each current?

4. The figure shows a section of a conductor, part of a conducting loop in which current has been established. If charge dq passes through a hypothetical plane (such as aa’) in time dt, then the current i through that plane is defined as: The charge that passes through the plane in a time interval extending from 0 to t is: Under steady-state conditions, the current is the same for planes aa’, bb’, and cc’ and for all planes that pass completely through the conductor, no matter what their location or orientation. The SI unit for current is the coulomb per second, or the ampere (A): 26.2: Electric Current:

5. 26.2: Electric Current, Conservation of Charge, and Direction of Current: Kirchoff’s Law: Sum of Total Current Into a Junction Must Equal Sum of Total Current Out: Conservation of Charge! WHAT GOES IN MUST COME OUT! THINK OF GALLONS PER SECOND!

6. 26.2: Electric Current, Conservation of Charge, and Direction of Current: Fill in the blanks. Think water in hose!

7. E i J dA If surface is perpendicular to a constant electric field, then i=JA, or J=i/A Drift speed: v d :Velocity at which electrons move in order to establish a current. E i L A Charge q in the length L of conductor: n =density of electrons, e =electric charge Current Density J and Drift Speed v d The current is the flux of the current density! Units:

8. All quantities defined in terms of + charge movement! (a) right (c) right(b) right (d) right

9. Example, In a current, the conduction electrons move very slowly.: If electrons move so slowly, Why does a light come on the instant You throw the switch?

10. IPPC: Let the current density J remain constant. What happens to the current if we: (a)Double the Radius? (b) Double the Area? (c) Double the Length?

11. Example, Current Density, Uniform: SP-26.02 PAGE 750 What is total net current?

12. Example, Current Density Nonuniform, cont.:

13. Integration with Cylindrical Shells dr L i enc ≠ JA unless J is constant! J(r) = ar 2 is NOT a constant! i dA