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Capacitors, (L) Inductors and Resistors
 

 

Last updated: 21-03-11

 

Capacitor - Charging and Discharging

   
 
bulletWhat happens when a Capacitor being charged and discharged ?
bullet When the Capacitor connected to battery will a current flow and the result will be  a charge (Unit Coulomb) at the plates of the Capacitor.
bullet The Current will start at a high value and gradually be smaller (and become 0)
bullet The Voltage at the Plate given by:  Vcap = Q / C - where Q = Charge [Coulomb] and Capacitance [Farad] 
bullet When the Capacitor discharded through a Resitor will the Capacitor give back the Charge stored at the Plate.
bulletWhat's the time constant for the Circuit ?
bullet The time constant for a circuit with Resistors and a Capacitor = R * C = Tau
bullet What if theres more then one resistor ?
bulletHow will energy be stored in a Capacitor ?
bullet The Charge at the Capacitors plates will result in a Electric Field - which said to hold the Energy
   
   <= Click @ the figure for simulation
 
 
   
 
bulletWhat will be the voltage over the Cacitor one tau (time constant) after the voltage switched on ?
bullet Vc = (1- e-1 )* Vbattery  = (1- 0, 38) * 5 = 3,16 Volt
   
 
   
 
   

Capacitance  [Farad]   -     C = ɛ * A / d

   
 
   
 
bulletWhats the formula for Capacitance ?
bulletHow can the Capacitance be changed ?
bulletHow a change of Capcitance be used ?
bullet Microphone.
bullet Messurement of moisture (water) in grain and corn.
bullet Distance.
   

Inductor - Energized and De-Energized

   
 
bulletWhat happens when an Inductor being Energized and De-Energized ?
bullet When the Inductor connected to battery will a current start flowing and the result will be a magnetic field
bullet After a while (long time) will the Voltage across the Inductor become 0 Volt and the Current only limitated by the Resistor.
bullet When the Inductor disconneted will the Inductor like the current to continue flowing (at any price = high Voltage)
If a Resistor used to short-circuit the Inductor will the energy be "burned off" here.
bulletWhat's the time constant for the Circuit ?
bullet The time constant for a circuit with Resistors and an Inductor = L/ R = Tau
bullet What if theres more then one resistor ?
bulletHow will energy be stored in an Inductor ?
bullet The Current flowing in the Coil (inductor) will result in Magnetic Field
   
   <= Click @ the figure for simulation
   
 
   
 
   
 
   

       Inductor - Charging and Discharging

bulletWhat happens if an Inductor being switched of without no path for the Current ? (note the use of a Spark Gap)
   
   <= Click @ the figure for simulation
   
 
   
 
bulletHow can a Diode be used to protect a circuit from high Voltage when a Inductor being switched of ?
   
  <= Click @ the figure for simulation
   
 
   

The Self-inductance Coificient L of a coil - the unit called [Henry]
x x

The Self-Inductance Coificient L of coil depens on:

The physical dimensions - like length and Area

The number of turns N in the power of 2

The Permeability will depend on the material inside the Coil and hence will a Coil with a Iron core have a Self-inductance coff which up to 10.000 higher if the core was "air"

bulletHow could a change of the value L be useful ?
bullet Metaldetectors
bullet Inductive sensors ( instead of mechanical sensors)
bullet
   

Explain what happens when the Switches closed in the Circuits below.
   
 
 

 Click @ the figure for simulation - even its not exactly the same will it give some answers.

 
   

When the power turned on:

The Inductor will have a high Voltage over the terminals and low Current (0 Ampere).
Hence will it be possible to have a Current through the Lamp (it will light up)

The Capacitor will act as short circiut with zero Voltage over the terminals.
Hence will all current be used to Charge the Capacitor and none current will be avaliable for the Lamp in this circuit.

The Resistor and Lamp will acts as two resistors in parallel and share the current.
   

When the Power turned on a long time:

The Inductor will now be fully magnetised and the Voltage across the terminals will be 0.
Hence will the Lamb be turned off.

The Capcitor now fully charged and the Voltage across the terminals will depend of the Voltage across the Lamp (which light up)

The Resistor and Lamp will still share the Current.
   

When the being Turned off:

The Inductor holds energy stored in the Magnetic field of the Coil.
The current which creates the field will continue to flow through the coil but will now use the lamp as return path.
The Energi will soon be lost in the Lamp

The Capacitor holds energy stored as charge at the plates of the Capcitor. The charges creates an Electric field which drives a current through the Lamp.
The Energy will soon be lost in the Lamp.

The Resistor and Lamp will turned of the moment the switch opens.
However will the Lamp have an afterglow which dies out quickly.