CURRENT ELECTRICITY |class12|physicsnotes|part1

CURRENT ELECTRICITY :

 

→Electric Current

→Drift Velocity of electrons and current

→Ohm’s Law

→Current Density

→Resistance, Resistivity, Conductance &
→Conductivity

→Temperature dependence of resistance

 →Color Codes for Carbon Resistors

→Series and Parallel combination of
→resistors

→EMF and Potential Difference of a cell

→Internal Resistance of a cell

→Series and Parallel combination of cells

  Electric Current:

 
The electric current is defined as the charge flowing through conductor in one second.

MATHEMATICALLY,current given by
I = q / t

 

Drift Velocity and Current:
 

Drift velocity is defined as  velocity with which the free electrons get drifted or moved

towards the positive terminal under the effect of the applied electric field.

mathematically,

drift velocity given by,

v d = - (eE / m) τ

 

current is given as,

I = n e A vd 

where,

vd = drift velocity,E =electric field,
e =electronic charge, m =mass of electron, n =number density of electrons, a = acceleration, τ =relaxation time,  l= length of the conductor and A =Area of cross-section

Resistance:

 
The resistance is defined as  the opposition offered by the conductor to the flow of electric current through it.

Mathematically,

by ohms law we have,V=IR,
R = V / I
 

Resistance mathematically given as
I = neA | vd |
I = neA (e |E| / m) τ    ( v d =  (e|E |/ m) τ)

I =ne2AτV/ml          (E=V/L)

 

V/I=ml/ne2τ A       (BY OHM LAWS, R=V/I)

 

R =ml/ne2τ A

R = m/ne2τ (l/A)

 

R = ρ (l/A)

where ρ = m/ne2τ ,is resistivity or specific resistance. IT IS PROPERTY OF MATERIAL.

Ohm’s Law:

when physical conditions such as temperature, mechanical
strain, etc. remain constant,

 then,electric current flowing through a conductor is directly proportional to the potential difference across the two ends of the conductor .

 Mathematically,

POTENTIAL DIFFERENCE GIVEN AS,

V=IR 

Series combination of resistors:

R = R1 + R2 + R3
R is greatest than all individual resistance, R1, R2, R3,etc  CONNECTED IN SERIES

Parallel combination of resistors:

1/R =1/R1 + 1/R2 + 1/R3

R is smallest than all individual resistance, R1, R2, R3,etc  CONNECTED IN parallel.

 emf( electro motive force):

The electro motive force is the maximum potential difference between the two electrodes of the cell when no current is drawn from the cell.
 

Continue reading with:

• Guass theorem
• Photoelectric effect
• Compton effect
• Semiconductor
• Electrostatics (physics class 12)
• transformer
• Solid state (chemistry class 12)
• Electromagnetic induction

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Comparison of EMF and Potential difference:

emf	Potential Difference
emf is the maximum potential difference between the two electrodes of the cell when no current is drawn from the cell i.e. when the circuit is open	P.D is the difference of potentials between any two points in a closed circuit
It is independent of the resistance of the circuit	It is proportional to the resistance between the given points or ends.
The term ‘emf’ is used only for the source of cells.	It is measured between any two points of the circuit