10/8/2008
“+” REMEMBER, THE ELECTRONS ARE ACTUALLY MOVING THE OTHER WAY! - -
Battery
A wire is a conductor We will assume that the conductor is essentially an equi-potential ◦ It really isn’t. Electrons are moving in a conductor if a current is flowing. ◦ This means that there must be an electric field in the conductor. ◦ This implies a difference in potential since E= V/d ◦ We assume that the difference in potential is small and that it can often be neglected. ◦ In this chapter, we will consider this difference and what causes it.
Current is the motion of POSITIVE CHARGE through a circuit. Physically, it is electrons that move but … Conducting material Q, t
Conducting material Q, t
A current of one coulomb per second is defined as ONE AMPERE.
A charged belt, 30 cm wide, travels at 40 m/s between a source of charge and a sphere. The belt carries charge into the sphere at a rate corresponding to 100 µA. Compute the surface charge density on the belt. [8.33e-06] C/m2
A small sphere that carries a charge q is whirled in a circle at the end of an insulating string. The angular frequency of rotation is ω. What average current does this rotating charge represent?
Today we continue on Resistance and Resistors. EXAM #2 is NEXT FRIDAY – the 17 th of October Covers Potential through Resistance Quiz Grades are being re-normalized. Next topic will be electric circuits
An electric current is given by the expression I(t) = 100 sin(120πt), where I is in amperes and t is in seconds. What is the total charge carried by the current from t = 0 to t = (1/240) s?
The figure represents a section of a circular conductor of non-uniform diameter carrying a current of 5.00 A. The radius of cross section A1 is cm. (a) What is the magnitude of the current density across A1? (b) If the current density across A2 is one-fourth the value across A1, what is the radius of the conductor at A2?
A particular object will resist the flow of current. It is found that for any conducting object, the current is proportional to the applied voltage. STATEMENT: V=IR R is called the resistance of the object. An object that allows a current flow of one ampere when one volt is applied to it has a resistance of one OHM.
Resistance Varies with Applied Voltage (actually with current)
Conduction is via electrons. They are weak and small and don’t exercise much. Positive charge is big and strong and doesn’t intimidate easily. It’s an ugly situation … something like ……
- +
We defined ◦ Current (and Amperes) ◦ Current Density We defined Resistance and Ohm’s Law
Electrons are going the opposite way from the current. (WHY?) They probably follow a path like … Average “drift” speed - v d OUT IN
v d average drift velocity of the electron nnumber of electrons (mobile) per unit volume. tinterval of time x average distance the electron moves in time t. Qtotal amount of CHARGE that goes through a surface of the conductor in time t.
Often a Vector The Diagram
Consider an electron. Assume that whenever it “bumps” into something it loses its momentum and comes to rest. It’s velocity therefore starts at zero, the electric field accelerates it until it has another debilitating collision with something else. During the time it accelerates, its velocity increases linearly. The average distance that the electron travels between collisions is called the “mean free path”.
Let n= number of charge carriers per unit volume (mobile electrons) We showed two slides ago: resistivity
The average drift velocity of an electron is about m/s
How can a current go through a resistor and generate heat (Power) without decreasing the current itself?
Loses Energy Gets it back Exit
In metals, the bigger the electric field at a point, the bigger the current density. is the conductivity of the material. =(1/ ) is the resistivity of the material
A conductor of uniform radius 1.20 cm carries a current of 3.00 A produced by an electric field of 120 V/m. What is the resistivity of the material?
TT
that’s it, Doc