184_notes:cap_charging

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184_notes:cap_charging [2018/09/27 13:15] – dmcpadden184_notes:cap_charging [2022/10/19 14:40] (current) – valen176
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 Section 19.1 in Matter and Interactions (4th edition) Section 19.1 in Matter and Interactions (4th edition)
  
-[[184_notes:cap_in_cir|Next Page: Capacitors in a Circuit]]+/*[[184_notes:cap_in_cir|Next Page: Capacitors in a Circuit]]
  
-[[184_notes:resistivity|Previous Page: Resistors and Conductivity]]+[[184_notes:resistivity|Previous Page: Resistors and Conductivity]]*/
  
 ===== Charging and Discharging Capacitors ===== ===== Charging and Discharging Capacitors =====
-Over the last two weeks we have been building a fairly robust model of what happens to the charges both on the surface of the wires and those moving through the wire. Now we are going to introduce a circuit element called a capacitor and see what changes about the electron current, the electric field and the surface charges. [[184_notes:motiv_movingq|A week ago]], we started talking about a two oppositely charged plates - so we'll start by returning to that example. +Over the last two weeks we have been building a fairly robust model of what happens to the charges both on the surface of the wires and those moving through the wire (through resistors). Now we are going to introduce a new circuit element called a capacitor and see what changes about the electron current, the electric field and the surface charges. [[184_notes:motiv_movingq|A week ago]], we started talking about a two oppositely charged plates - so we'll start by returning to that example. 
  
 {{youtube>GbOcSuFDEqs?large}} {{youtube>GbOcSuFDEqs?large}}
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 As this process continues, the capacitor plates lose more and more of their charge, so the electric field gets smaller and smaller causing the bulb to get dimmer and dimmer. Eventually, the capacitor plates lose all of their charge (both become neutral plates), so the electron current stops completely because there is no longer an electric field around the wire.  As this process continues, the capacitor plates lose more and more of their charge, so the electric field gets smaller and smaller causing the bulb to get dimmer and dimmer. Eventually, the capacitor plates lose all of their charge (both become neutral plates), so the electron current stops completely because there is no longer an electric field around the wire. 
  
-It is important to note here that //__this is no longer a steady-state current situation__//.  The current, potential difference across the capacitor, and the amount of charge on the capacitor are all decreasing as time goes on. Depending on the capacitor and resistor you have in your circuit, this process can take anywhere from fractions of a second up to minutes. The amount of time it takes a capacitor to discharge is slow compared to how long it takes to set up the electric field in the wire ([[184_projects:project_6|you found before that this takes nanoseconds]]). However, a capacitor is not capable of sustaining a constant current for long periods of time like a battery is. You might hear this referred to as a **quasi-static state**. +It is important to note here that //__this is no longer a steady-state current situation__//.  The current, potential difference across the capacitor, and the amount of charge on the capacitor are all decreasing as time goes on. Depending on the capacitor and resistor you have in your circuit, this process can take anywhere from fractions of a second up to minutes. The amount of time it takes a capacitor to discharge is slow compared to how long it takes to set up the electric field in the wire (this takes nanoseconds). However, a capacitor is not capable of sustaining a constant current for long periods of time like a battery is. You might hear this referred to as a **quasi-static state**. 
  
 ==== Charging a Capacitor ==== ==== Charging a Capacitor ====
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 [{{184_notes:chargingcap3.jpg?200|Charge distribution and electric field when the capacitor plates are fully charged.  }}] [{{184_notes:chargingcap3.jpg?200|Charge distribution and electric field when the capacitor plates are fully charged.  }}]
  
-=== Role of the Resistor === +==== Role of the Resistor ==== 
-When charging or discharging a capacitor, there is usually a resistor placed in the circuit (like a lightbulb or some other kind of resistor) because the resistor helps control both the maximum current possible and the time it takes to charge/discharge. The resistor does NOT impact how much charge the capacitor holds, just how fast the capacitor charges/discharges.+When charging or discharging a capacitor, there is usually a resistor placed in the circuit (like a lightbulb or some other kind of resistor) because the resistor helps control both the maximum current possible and the time it takes to charge/discharge. The resistor **does NOT impact how much charge the capacitor holds, just how fast the capacitor charges/discharges.**
  
-=== Video Demo ===+==== Video Demo ====
  
 Here is a video demonstration of a lightbulb and capacitor circuit, charging and discharging. Here is a video demonstration of a lightbulb and capacitor circuit, charging and discharging.
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 ==== Examples ==== ==== Examples ====
-[[:184_notes:examples:Week7_charging_capacitor|Looking at a Capacitor as it's Charging]]+  * [[:184_notes:examples:Week7_charging_capacitor|Looking at a Capacitor as it's Charging]] 
 +    * Example Video: Looking at a Capacitor as it's Charging 
 +{{youtube>Q34OmIW5rVU?large}}
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