Chapter_25

# Chapter_25 - Chapter 25 Current Resistance and Electromotive Force In this chapter end of electrostatics Now ready to study charges in motion

This preview shows pages 1–4. Sign up to view the full content.

44 Chapter 25: Current, Resistance, and Electromotive Force In this chapter… ± end of electro statics ! Now ready to study charges in motion ± definition of electric current ± electrical resistance of conductors ± electromotive force and circuits Current (25-1) ± electric current is motion of charge from one place to another rate of charge flowing across some cross-sectional area if charge dQ passes through an area A during a small time interval dt , the current I is defined as ± SI unit: the ampere 1A = 1 C/s (André-Marie Ampère ~1820) ± current is a scalar quantity, but it can be positive or negative , depending on direction of flow defined as flow of positive charge, so electrons move in direction opposite to direction of current (due to Ben Franklin’s unfortunate choice of “positive”) ± examples: flow of electrons in a copper wire electron beam in an old-style video monitor or TV swarm of charged particles from sun analog: water flow in a pipe, measured in, say, gallons per minute ± steady-state currents in circuits of conducting wires in electro static equilibrium, 0 E = G inside conductors, and no current flows but certain “driving devices” (batteries, generators, Van de Graaff) can push charges around, making a non-zero E G field inside a conductor dQ I dt =

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
45 without a closed loop, eventually charge builds up, creating an electric field to cancel that of driving device, and total 0 E = G in conductor but if conductor forms a circuit (a loop), one gets a continuous flow of charges without charge build up Æ a steady current E G field also exists in surrounding air and non-conductors, but not generally useful or interesting ± motion of charge carriers in wire is complex: when no electric field, free electrons move by thermal agitation with large speeds 6 10 m/s v , but average velocity is zero frequently collide with ions in conducting material, resulting in random changes in direction of motion in an applied E G field, each free electron experiences a force eE G electrons experience accelerations in direction opposite to E G , but net effect is an average drift velocity d v G in direction opposite to E G with typical magnitude 4 10 m/s d v Æ current ! heating due to conversion of kinetic energy to random thermal motion drift velocity is very slow, but current begins to flow everywhere in a circuit immediately after switch closed Æ the driving electric field propagates along circuit elements at close to the speed of light Conductor Driving Device Conductor Conductor driving E G driving E G buildup E G buildup E G + + Driving Device driving E G Conductor driving E G + Conductor Net “drift” due to E field
46 ± different charge carriers

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## This note was uploaded on 07/14/2008 for the course PHYS 33104 taught by Professor Morningstar during the Spring '07 term at Carnegie Mellon.

### Page1 / 9

Chapter_25 - Chapter 25 Current Resistance and Electromotive Force In this chapter end of electrostatics Now ready to study charges in motion

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online