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Making connections: take-home investigation—filament observations

Find a lightbulb with a filament. Look carefully at the filament and describe its structure. To what points is the filament connected?

We can obtain an expression for the relationship between current and drift velocity by considering the number of free charges in a segment of wire, as illustrated in [link] . The number of free charges per unit volume is given the symbol n size 12{n} {} and depends on the material. The shaded segment has a volume Ax size 12{ ital "Ax"} {} , so that the number of free charges in it is nAx size 12{ ital "nAx"} {} . The charge Δ Q size 12{DQ} {} in this segment is thus qnAx size 12{ ital "qnAx"} {} , where q size 12{q} {} is the amount of charge on each carrier. (Recall that for electrons, q size 12{q} {} is 1 . 60 × 10 19 C size 12{ - 1 "." "60" times "10" rSup { size 8{ - "19"} } "C"} {} .) Current is charge moved per unit time; thus, if all the original charges move out of this segment in time Δ t size 12{Dt} {} , the current is

I = Δ Q Δ t = qnAx Δ t . size 12{I = { {ΔQ} over {Δt} } = { { ital "qnAx"} over {Δt} } "."} {}

Note that x / Δ t size 12{x/Δt} {} is the magnitude of the drift velocity, v d , since the charges move an average distance x size 12{x} {} in a time Δ t size 12{Dt} {} . Rearranging terms gives

I = nqAv d , size 12{I= ital "nqAv" rSub { size 8{"d"} } } {}

where I size 12{I } {} is the current through a wire of cross-sectional area A size 12{A} {} made of a material with a free charge density n size 12{n} {} . The carriers of the current each have charge q size 12{q} {} and move with a drift velocity of magnitude v d size 12{v rSub { size 8{d} } } {} .

Charges are shown moving through a section of a conducting wire. The charges have a drift velocity v sub d along the length of the wire, shown by an arrow pointing to the right. The volume of a segment of the wire is equal to A times x, where x equals the product of the drift velocity, v sub d, and time t. A cross section of the wire is marked as A, and the length of the section is x.
All the charges in the shaded volume of this wire move out in a time t size 12{t} {} , having a drift velocity of magnitude v d = x / t size 12{v rSub { size 8{d} } =x/t} {} . See text for further discussion.

Note that simple drift velocity is not the entire story. The speed of an electron is much greater than its drift velocity. In addition, not all of the electrons in a conductor can move freely, and those that do might move somewhat faster or slower than the drift velocity. So what do we mean by free electrons? Atoms in a metallic conductor are packed in the form of a lattice structure. Some electrons are far enough away from the atomic nuclei that they do not experience the attraction of the nuclei as much as the inner electrons do. These are the free electrons. They are not bound to a single atom but can instead move freely among the atoms in a “sea” of electrons. These free electrons respond by accelerating when an electric field is applied. Of course as they move they collide with the atoms in the lattice and other electrons, generating thermal energy, and the conductor gets warmer. In an insulator, the organization of the atoms and the structure do not allow for such free electrons.

Calculating drift velocity in a common wire

Calculate the drift velocity of electrons in a 12-gauge copper wire (which has a diameter of 2.053 mm) carrying a 20.0-A current, given that there is one free electron per copper atom. (Household wiring often contains 12-gauge copper wire, and the maximum current allowed in such wire is usually 20 A.) The density of copper is 8 . 80 × 10 3 kg/m 3 size 12{8 "." "80" times "10" rSup { size 8{3} } `"kg/m" rSup { size 8{3} } } {} .


We can calculate the drift velocity using the equation I = nqAv d . The current I = 20.0 A is given, and q = 1.60 × 10 19 C is the charge of an electron. We can calculate the area of a cross-section of the wire using the formula A = π r 2 , where r is one-half the given diameter, 2.053 mm. We are given the density of copper, 8.80 × 10 3 kg/m 3 , and the periodic table shows that the atomic mass of copper is 63.54 g/mol. We can use these two quantities along with Avogadro’s number, 6.02 × 10 23 atoms/mol , to determine n , the number of free electrons per cubic meter.


First, calculate the density of free electrons in copper. There is one free electron per copper atom. Therefore, is the same as the number of copper atoms per m 3 . We can now find n as follows:

n = 1 e atom × 6 . 02 × 10 23 atoms mol × 1 mol 63 . 54 g × 1000 g kg × 8.80 × 10 3 kg 1 m 3 = 8 . 342 × 10 28 e /m 3 .

The cross-sectional area of the wire is

A = π r 2 = π 2.053 × 10 −3 m 2 2 = 3.310 × 10 –6 m 2 .

Rearranging I = n q A v d to isolate drift velocity gives

v d = I nqA = 20.0 A ( 8 . 342 × 10 28 /m 3 ) ( –1 . 60 × 10 –19 C ) ( 3 . 310 × 10 –6 m 2 ) = –4 . 53 × 10 –4 m/s.


The minus sign indicates that the negative charges are moving in the direction opposite to conventional current. The small value for drift velocity (on the order of 10 4 m/s size 12{"10" rSup { size 8{ - 4} } `"m/s"} {} ) confirms that the signal moves on the order of 10 12 size 12{"10" rSup { size 8{"12"} } } {} times faster (about 10 8 m/s size 12{"10" rSup { size 8{8} } `"m/s"} {} ) than the charges that carry it.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
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Source:  OpenStax, General physics ii phy2202ca. OpenStax CNX. Jul 05, 2013 Download for free at http://legacy.cnx.org/content/col11538/1.2
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