184_notes:current

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision		 Previous revision
184_notes:current [2017/09/24 17:51] – dmcpadden184_notes:current [2021/06/08 00:45] (current) – schram45
Line 1: Line 1:
 Sections 17.5 and 18.2 in Matter and Interactions (4th edition) Sections 17.5 and 18.2 in Matter and Interactions (4th edition)
 +
 +/*[[184_notes:resistors|Next Page: Resistors]]
 +
 +[[184_notes:defining_current|Previous Page: Defining Current]]*/
 +
 ===== Current in Wires ===== ===== Current in Wires =====
-We have already established that when connected to a battery there are surface charges in the wire that [[184_notes:q_in_wires|create a constant electric field through the wire]]. Because [[184_notes:pc_force|electric force is directly proportional to the electric field]] (→F=q→E), the electric field in the wire pushes the electrons from the negative plate of the battery to the positive plate of the battery causing an electron current through the wire. Rather than focusing on the surface charges, these notes will focus on describing the electron current that occurs in the wire.+In the last few pages of notes, we established that when connected to a battery there are surface charges in the wire that [[184_notes:q_in_wires|create a constant electric field through the wire]]. Because [[184_notes:pc_force|electric force is directly proportional to the electric field]] (→F=q→E), the electric field in the wire pushes the electrons from the negative plate of the battery to the positive plate of the battery causing an [[184_notes:defining_current|electron current]] through the wire. Rather than focusing on the surface charges, these notes will focus on describing the electron current that occurs in the wire and how we find the average speed of those electrons.
  
 +{{youtube>3D60Vme_tyg?large}}
 ==== Current in Different Parts of the Wire ==== ==== Current in Different Parts of the Wire ====
 Given what you know about the electric field in the wire, how would you expect the electron current to compare in different parts of the wire? If the electric field is constant along the wire, each electron would feel a constant force along the wire. For every electron that leaves the negative plate of the battery, there is one returning to the positive plate of the battery. Thus, **at every point along the wire, the electron current is the same**. Given what you know about the electric field in the wire, how would you expect the electron current to compare in different parts of the wire? If the electric field is constant along the wire, each electron would feel a constant force along the wire. For every electron that leaves the negative plate of the battery, there is one returning to the positive plate of the battery. Thus, **at every point along the wire, the electron current is the same**.
  
-What if you added a light bulb to the circuit, how would you expect the electron current to compare? Do the electrons get "used up" in the light bulb? It turns out that electrons transfer electric energy into heat and light at the light bulb (we will talk about this more [[184_notes:r_energy|next week]]), but the electrons are not destroyed or used up. We can justify this using the [[184_notes:charge|conservation of charge]]. A light bulb does not emit electrons, so this means that the amount of charge going into the light bulb must equal the amount of charge coming out of the light bulb. +What if you added a light bulb to the circuit, how would you expect the electron current to compare? Do the electrons get "used up" in the light bulb? It turns out that electrons transfer electric energy into heat and light at the light bulb (we will talk about this more [[184_notes:r_energy|next week]]), but **the electrons are not destroyed or used up**. We can justify this using the [[184_notes:charge|conservation of charge]]. A light bulb does not emit electrons (only light/heat), so this means that the amount of charge going into the light bulb must equal the amount of charge coming out of the light bulb. 
  
 //__In steady state__// we can rewrite the conservation of charge in terms of the electron current, called the "Current Node Rule": //__In steady state__// we can rewrite the conservation of charge in terms of the electron current, called the "Current Node Rule":
Line 18: Line 24:
  
 ==== Drift speed in wire ==== ==== Drift speed in wire ====
-{{  184_notes:vdrift.png?200}} +[{{  184_notes:vdrift.png?200|Graph of electron speed in a wire vs time}}] 
-Modeling all of these interactions for every electron in the electron current is quite complicated (or almost impossible). While there are several ways to model the electrons in the wire, we will use a model called the [[https://en.wikipedia.org/wiki/Drude_model|Drude Model]], which builds off of the idea that the electrons are interacting or bouncing off the positive nuclei in the wire. In this model, the electron will experience short periods of acceleration from the electric field, followed by periods where the electron drastically slows because of collision with a positive nuclei in the wire. The average speed of the electron in this stop/start motion is called the **drift velocity**, and we say that the electron "drifts" through the metal. The drift velocity of electrons in a wire is actually quite slow compared to the speed of the individual electrons (the same way that the wind has slow speed compared to the speed of the individual air molecules).+Modeling all of these interactions for every electron in the electron current is quite complicated (or almost impossible). While there are several ways to model the electrons in the wire, we will use a model called the [[https://en.wikipedia.org/wiki/Drude_model|Drude Model]], which builds off of the idea that the electrons are interacting or bouncing off the positive nuclei in the wire. In this model, the electron will experience short periods of acceleration from the electric field, followed by periods where the electron drastically slows because of collision with a positive nuclei in the wire. The average speed of the electron in this stop/start motion is called the **drift velocity** which has units of m/s, and we say that the electron "drifts" through the metal. The drift velocity of electrons in a wire is actually quite slow compared to the speed of the individual electrons (the same way that the wind has slow speed compared to the speed of the individual air molecules).
  
 Using the Drude Model, we can find the average drift velocity for the electrons in the wire. Starting with the momentum principle, we know Using the Drude Model, we can find the average drift velocity for the electrons in the wire. Starting with the momentum principle, we know
Line 39: Line 45:
 →vavg=u→E. →vavg=u→E.
  
-Likewise, we can combine this with the expression for electron current i=nAvavg to get:+Likewise, we can combine this with the [[184_notes:defining_current|expression for electron current]] i=nAvavg to get:
 i=nAuE i=nAuE
 +
 +====Examples====
 +  * [[:184_notes:examples:Week6_drift_speed|Drift Speed in Different Types of Wires]]
 +    * Video Example: Drift Speed in Different Types of Wires
 +  * [[:184_notes:examples:Week6_node_rule|Application of Node Rule]]
 +    * Video Example: Application of Node Rule
 +{{youtube>cxSQbMLoUk4?large}}
 +{{youtube>NFl0ZuWfkBc?large}}
  • 184_notes/current.1506275508.txt.gz
  • Last modified: 2017/09/24 17:51
  • by dmcpadden