

Direct Current on Atomic Scale
Name: Zelda
Status: student
Grade: 912
Country: Belarus
Date: Fall 2012
Question:
Does the potential difference or voltage drop across a load mean the loss of Potential Energy of one coulomb as charge moves across the load? Is it due to the applied electric field that permeates the wires? And when it is said that electrons "collide" with atoms does it really mean that electrons "fall" into the orbital of a metal ion and lose energy? What I am basically looking for is a detailed explanation of what exactly happens to electrons in a direct current and how this can explain voltage and resistance and the laws related to them.
Replies:
Hi, Zelda
Thank you for your questions. There are a few concepts which you need to get untangled. As an electrical engineer I will not be able to answer all of your questions but I hope I can help to explain some concepts that may be helpful.
At a beginning level I find it very useful to compare electricity and water.
A Coulomb is a unit of charge. This is a fixed (and very large) number of electrons, just like a liter is a very large number of molecules of water. The Coulomb is not a measure of energy as you mentioned.
Current describes rate of charge flow. One Ampere is one coulomb of charge in a second. This is comparable to some liters per second of water flowing through a pipe.
Voltage is a unit of electrical pressure. This is comparable to water pressure which can be expressed in pounds per square inch in English units or perhaps dynes per square centimeter in metric units.
Electrical Energy (in Joules) is charge in Coulombs multiplied by Voltage. 1 Joule = 1 volt x 1 Coulomb. Energy is closely related to mechanical "Work". One Coulomb x one volt = one Joule. With water, pumping one liter of water at a pressure of one dyne per square cm will require a fixed amount of energy or work.
Finally, Power in Watts is a rate of energy flow. One Joule per second is one Watt. A 2 watt motor can do work twice as fast as a 1 watt motor.
Now, copper wires have very low resistance so there is almost negligible electric field within them. Electric fields exist mostly in insulating materials such as a wool rug or the air between your finger and a metal appliance before you get a spark. That spark, by the way, was some small amount of charge (in Coulombs) which gets discharged at a very high voltage.
I wish you the best of success in your studies.
Bob Zwicker
Zelda,
The kinetic energy of electrons as they enter a resistive material is decreased as the electrons collide with the atoms of a resistive material. The atoms of the resistive material have fewer free electrons and therefore the number of electrons leaving the resistive material is less than what entered the material. Given that, the potential energy entering the resistive material is higher than the potential energy of the exit point. This is your potential difference and voltage drop.
As the electric field is applied to this resistive material, the electrons are "working" to move across the resistor, colliding with the atoms of the material. These collisions and Quantum vibrations are producing heat which is the thermal energy that is transformed from the loss in kinetic energy.
So the higher the number of atoms of few free electrons of a material, the higher the resistance. This higher resistance lowers the electrons' kinetic energy or current flow. In other words: I is inversely proportional to R, or I = V / R
Lastly, the higher the resistance, the bigger the potential difference or voltage drop. In other words: V is directly proportional to R, or V = I *R.
Hope that helps.
Alex Viray
Click here to return to the Physics Archives
 
Update: November 2011

