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Concept #1: Microscopic View of Current

The equation is actually: *I = n * e * A * v _{d}*

This should be 7×10^{9}. The video is off by an order of magnitude.

**Transcript**

Hey guys. In this video we're going to be talking about the microscopic view of current. So, we want to look and see what's actually going on to these electrons as they pass through a conductor at the microscopic level. Alright, let's get to it. The speed of electrons through conductors is what we would call the drift velocity, okay? We know that this is going to be slower than if the electrons were free to move through a vacuum, okay? Because the electrons bounce around off of the different atoms. So, if there's an electric field E inside of the conductor, okay? And that electric field is due to the potential difference across the conductor then the drift velocity of these atoms, right? sorry, of these electrons as they bounce around inside of the conductor is going to be the charge of the electron E times that electric field over the mass of the electron times this new thing, which we give the Greek letter tau to, okay? Where tau is the average time between collisions, okay? So, maybe this time is longer than this time, which is longer than this time but shorter than that last time until the next collision, but on average we call the time between collisions the mean free time, right? Mean being average, free being when it's not in a collision, when the electron is in between atoms so the mean and free time, okay? Now, current can be calculated in this microscopic view with this equation, all I have to do is plug in the drift velocity, this is e, E squared times the electric field times tau over the mass times the cross-sectional area of the conductor, okay? Where n is the number of free electrons per cubic meter, okay? Called the free electron density. Now, in a conductor, right? A conductor is going to have some volume, it's going to have a bunch of atoms, okay? And each of those atoms are going to have electrons associated with them. So, there's going to be a certain number of electrons divided by the volume of this conductor. So, that's going to be the total electron density but in conductors a certain amount of those electrons are called free electrons because they are free to move about inside of this conductor. So, out of the total amount of electrons a small percentage of them are going to be free electrons, if we only count up the number of free electrons and divide that by the volume, that is the free electron density, okay? And the current density which is a value that we've seen before, just the current divided by the area of the conductor, is going to be this whole thing right here, divided by the area. So, we're going to lose the area. So, it's going to be n, e squared, Tao over m, and I've pulled the electric field to the right, okay? No big deal.

Alright, let's do an example. A conductor has 1 times 10 to the 20 electrons per cubic meter, 1% of which are free electrons, if the electric field inside the conductor is 5,000 news for Coulomb and the mean free time is 5 microseconds, what is the current density in the conductor, okay? We just saw that the current density was n, e squared, Tao over m times E, okay? Where n is the free electron density, okay? We're told, that in total is 1 times 10 to the 20, and in free is 1% of n total, okay? So, out of that one times 10 to the 20, 1 for every hundred electrons is a free electron, okay? So, this is just 1/100 of 1 times 10 to the 20, which is 1 times 10 to the 18, okay? You just divide that by 100, okay? So, you lose two exponents of 10, okay? Now, what we want to find is the current density. So, all we need to do is plug in these values, we know what n is, right? 1 times 10 to the 18, we know e 1.6 times 10 to the negative 19 squared, the mean free time is 5 microseconds or 5 times 10 to the negative 6 seconds, the mass of an electron is 9.11 times 10 to the negative 31 and the electric field is 5,000 Newtons per Coulomb, plugging all of this in, we get a current density of about seven times 10 to the 8 amps per square meter, okay? Now, we can find that the resistivity of a conductor by looking at this microscopic picture, the resistivity of this conductor is going to be given by this equation, okay? All the same things here, this is the mass of the electron, the number, sorry, the free electron density, so the number of free electrons per cubic meter, the electric charge squared and the mean free time. Now, we're going to define a new quantity related to the resistivity called the conductivity, if the resistivity is the inherent resistance to flow of electrons, right? To the flow of current then conductivity is the inherent benefit, right? The inherent strengths at which this conductor conducts current, okay? It's the opposite of resistivity and it's just one over the resistivity, okay? So, this is just going to be n, e squared, tau divided by the mass of the electron, okay? Let's do another example to wrap this up.

Copper has a conductivity of 5.8 times 10 to the 7 one over ohm, meters, if the density of free electrons in a copper conductor is 5 times 10 to the 17, what is the mean free time for the electrons, okay? We know the conductivity is n, sorry, not n squared, is n, e squared, tau over m, if we want to find the mean free time, we have to solve for tau. So, tau is just m times, sorry, Sigma divided by n, e squared, the mass of the electron is 9.11 times 10 to the negative 31, the conductivity is 5.8 times 10 to the 7, the free electron density, which is given to us, we don't need to calculate it, it's already given to us, is 5 times 10 to the 17, and the electric charge 1.6 times ten to the negative 19 squared, plugging all this in, we get 4.13 times 10 to the negative 3 seconds. So, about 4 milliseconds is the average time between collisions for these electrons, okay? This wraps up our discussion on the microscopic view of current in conductors, thanks for watching guys.

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Concept #1: Microscopic View of Current

In the circuit shown, two thick copper wires connect a 1.5 V battery to a Nichrome wire. Each copper wire has radius R = 8 mm and is L = 17 cm long. Copper has 8.4 × 10 28 mobile electrons per cubic meter and an electron mobility of 4.4×10 −3 (m/s)/(V/m). The Nichrome wire is l = 7 cm long and has radius r = 3 mm. Nichrome has 9 × 1028 mobile electrons/m3 and an electron mobility of 7 × 10−5 (m/s)/(V/m). What is the magnitude of the electric field in the copper wire?
1. 0.00315418
2. 0.0507735
3. 0.085777
4. 0.0399455
5. 0.00295718
6. 0.0775274
7. 0.0210359
8. 0.0670841
9. 0.0560538
10. 0.103878

The figure shows a metal wire and the direction of the electric current, I, in the wire. Which of the statements is correct?
A. Electrons in the wire move to the right towards a region of higher potential.
B. Electrons in the wire move to the right towards a region of lower potential.
C. Electrons in the wire move to the left towards a region of higher potential.
D. Positive ions in the wire move to the left towards a region of higher potential.
E. Positive ions in the wire move to the right towards a region of lower potential.

The electron density in iron is 1.7x1029 electrons/m3. When a 2.0 A current is present in a cylindrical iron wire with a cross-sectional radius of 2 mm, what is the electron's drift speed?

(a) In the attached figure, the (conventional) current through the resistor will go A. from a to b.B. from b to a.(b) Electrons moving through the circuit in the attached figure will go through the resistorA. go from b to a B. go from a to b

The biochemistry that takes place inside cells depends on various elements, such as sodium, potassium, and calcium, that are dissolved in water as ions. These ions enter cells through narrow pores in the cell membrane known as ion channels. Each ion channel, which is formed from a specialized protein molecule, is selective for one type of ion. Measurements with microelectrodes have shown that a 0.30-nm-diameter potassium ion (K+) channel carries a current of 1.8 pA. a) How many potassium ions pass through if the ion channel opens for 1.0 ms?b) What is the current density in the ion channel?

The electron drift speed in a typical current-carrying wire is a. Extremely slow (roughly 10 -4 m/s b. Moderate (roughly 1 m/s). c. Very fast (larger than 104 m/s) d. Could be any of the above. Submit

The electron drift speed in a 3.00-mm diameter gold wire is 5.00×10-5 m/s. How long does it take 1 mole of electrons to flow through a cross section of the wire?

A metallic wire has a diameter of 4.12mm. When the current in the wire is 8.00A, the drift velocity is 5.40 × 10−5m/s.What is the density of free electrons in the metal?

At t = 0 s, the current in the circuit in the figure is I0. At what time is the current ½ I0? Express your answer with the appropriate units.

The two wires in the figure (Figure 1) are made of the same material. (a) What is the current in the 2.0-mm-diameter segment of the wire? (b) What is the electron drift speed in the 2.0-mm-diameter segment of the wire? (in μm/s)

The wires in the figure are all made of the same material; the length and radius of each wire is noted.Rank in order, from largest to smallest, the resistances R1 to R5 of these wires. Rank from largest to smallest. To rank items as equivalent, overlap them.R1, R2, R3, R4, R5

True or False: Most conducting materials have different resistances at different temperatures.

When electric current is flowing in a metal, the electrons are movingA) at nearly the speed of lightB) at the speed of lightC) at the speed of sound in the metalD) at the speed of sound in airE) at none of the above speeds

What is the mean free time between collisions for electrons in an iron wire and an aluminum wire?

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