Chapter 31 #1-14

Current A wire will “appear” absolutely the same weather it is conducting electricity or not. When we control the motion of charges through a wire (bulb, microwave,LOAD) we create a current.

Signs that current is “flowing”…

Charges that move in a conductor are called “charge carriers” and can be either positive or negative. Charge carriers in metals are electrons, as proven by both JJ Thompson and the Toman-Stewart experiments. Charge carriers in semiconductors and ionic solutions may be different.

Electron Current & Drift Speed The available electrons in conducting metals are free to move around because there are not bound to the nucleus of a given atom. When an electric field in induced in a conducting metal, the sea of electrons (free electrons) move in one direction at a constant speed known as the drift speed (vd)…drift speeds are relatively low @ 10e-4m/s

Electron Current & Drift Speed cont. Once the sea of electrons begins to move, an electron current (# of electrons per second passing through a cross section of wire) is created. ie= Ne/t = neAvd Electron current is also equivalent to the electron density (ne) in an area of wire times drift speed. Electron density is based on the conduction levels of certain materials. (page 944)

Example 1: What is the electron current in a 3.5mm diameter gold wire if the electron drift speed is 1.0e-4 m/s?

From the previous example, electron density and cross-section area are properties of the wire we cannot change, the only variable we can control is drift speed. The drift speed is determined by the electric field inside of the wire.

Prove: ic> ib > ia > id The four wires are made of the same metal. The electron currents through the wires are ranked largest to smallest.

Chapter 31 #15-30

The wire in a circuit… is full of electrons just waiting to be “pushed” Gets warm because of millions of collisions between the electrons If maintaining a current, is not in equilibrium because a current is a non-equilibrium motion of charge created by an internal electric field

Establishing the electric field in a wire… only occurs if the “gap” or switch is closed occurs because of the non-uniform distribution of charges on the surface of the wire creates an electron current that flows in the opposite direction but an electric current in the same direction…LET’S DRAW!!

The acceleration of electrons in the wire can be calculated by the equation a=F/m or qE/m The first is obviously our old formula for acceleration. The second is new: e=1.6ee-19 E= electric field strength m=mass of the electron

These acceleration electrons transfer kinetic energy from one another during collisions and thus heat. Because the electric field is constant, once an electron or ion looses velocity during a collision it will re accelerate at a constant rate. These constant collisions create an avg. drift velocity.

E=electric field strength T=time between collisions Vd=eTE/m Vd is drift speed e=1.6ee-19 E=electric field strength T=time between collisions m= mass

Example: A 2.0 x 10-3V/m electric field crates a 3.5 x 1017electrons/seconds current in a 1.0 mm diameter aluminum wire. What are a) the drift speed and b) the mean time between collisions?

Chapter 31 31-62

Current is the rate of flow over time in the units of amps or C/s. Current density (A/m3) will control how fast a current flows in a certain type of conductor due to the high number of molecules in certain materials.

The ability to move a current is completely dependant on the “type” of material being used…in other words, “conductivity!!” Ѳ = nee2T/m

Kirchhoff’s Junction Law The number of electrons into a wire must equal the number of electrons out of the wire.

Conductivity vs. Resistivity Conductivity means to carry the current easily or allow the electrons to flow freely. Resistivity means to hinder the flow of electrons. Resistance is measured in ohms Ω

I=V/R Current=potential difference/resistance In order to preserve batteries, resistors are often introduced into circuits. Good conductors are known as ohmic while resistors are said to be nonohmic or diodes.

It is a wires’ resistivity and conductivity that that effect the amps carried in a circuit. σ = L/RA conductivity equals R is in ohms, area and length in meters ρ= 1/σ resistivity is inversely proportional to conductivity

Looking at these figures, what is the relationship between the battery, charge, potential difference, potential energy, current and electric field?