current electricity

• current in a wire can be defined in two ways

  1. instantaneous current : I=dq/dt
  2. average current : I(av) = Q(t2-t1)/(t2-t1)
• • charge is derived quantity
  • current is fundamental quality
• on the basis if rate of flow of charge, current is of two types

  1. uniform current : current same but current will not depend on time
  2. non uniform current : current depends on time
• • slope of Q-t graph gives us current ( slope : dy/dx)
  • area under I-t graph gives us charge flowed in a time interval
• thermal coefficient of resistance:

  relative charge produced in the resistance of the material per unit charge in the temperature
• if thermal coefficient of resistance is positive

  then temperature increase and resistance increase

  but in conductors, temperatures decrease and resistance also decrease
• if thermal coefficient of resistance is negative

  • temperature increase, resistance decrease
  • in semi conductors and insulators temperature decreases and resistance increase
• 1. thermal coefficient of resistance is small magnitude then on changing the temperature, resistance doesn't change significantly
  2. thermal coefficient of resistance is large magnitude, on changing the temperature, resistance changes significantly
• drift velocity(Vs) :

  velocity given to an electron after studying the combined motion of all the electrons in the wire
• combined motion of the electron doesn't have a direction, therefore current is zero

  • random motion of electron is brownian movement
• combined motion of the electron is directional, then current is not zero
• relaxation time :

  average time gap or interval between the successive collisions of an electron
• I = nAeVd

  • if n increases- more charge carriers can participate in current , therefore, current increases
  • if Area increase - it will be easier for the electron's to move forward from one point to another, therefore, current increases
  • if Vd increases - conductivity of material increases , therefore, current increases
• ohm's law :

  law that gives us the relationship between the current passing through a device and potential difference created across it
• series combination:

  in a series combination more potential drop is created across the resistor with high resistance
• parallel resistance:

  in a parallel resistance, more current will go through the arm having less resistance
• resistivity :

  • depends on the material
  • doesn't depend on dimensions of the wire
• kirchoff's law :

  • incoming current = outgoing current
  • the junction cannot create or destroy charges I.e conservation of charge

   
• loop rule :

  rule states that the potential difference created in a loop by batteries will be equal to the potential difference created across the resistor present in the loop
• In a closed loop there are two types of electric devices

  • one that creates potential difference in a loop, like, battery
  • one across which potential difference is created, like, resistor
• EMF :

  maximum potential difference that the cell have capacity to create (internally + externally)
• resistance :

  the resistance present inside the cell between it's plates
• terminal position : potential difference between the plates of a cell
• meter bridge :

  an electrical apparatus that works on the principle of balanced wheatstone bridge used to calculate resistance if an unknown resistor
• power of electrical instruments:

  rate of heat production in an electrical device when current passes through it
• power of bulb :

  reading a bulb

  1. value of V : V maximum that can be created across bulb before it fuses
  2. Value of P : p maximum they bulb can show before fusing
  3. P : power that the bulb exhibits when change in V =V is created across it
• delta V > 220V ; P exhibit > 40W = fuse
• delta V = 220V ; P exhibit max = 40W
• delta V < 220V ; P exhibit < Pmax ;

  Pexhibit < 40W
• 1. ideal cell : EMF = Vexternal ; Vinternal = 0
  2. non ideal cell : EMF > Vexternal ; Internal not ≠ 0