RRB ALP Physics Trade Notes on Current Electricity

Electric Current (I)

Electric Current Density (J)

• For irregular shapes of cross-section

I, J and A are all scalar quantities.

Electromotive Force

Drift Velocity of Free Electrons in a Metallic Conductor

• E is the applies field, e is the charge of an electron, m is the mass of an electron and t is the time interval between successive collisions (relaxation time)

Mobility

• The mobility μ of a charge carrier is defined as the drift velocity per unit electric field.

Ohm’s Law

• R = Resistance of conductor
• Unit of Resistance; Volt/ampere, (Ω ohm)

Resistance and Resistivity

• The inverse of resistance is called conductance G.

• S.I. unit Ω^-1 is called Siemens (S)
• The resistance of a conductor is directly proportional to its length l and inversely proportional to the area of cross-section A.

• The inverse of resistivity is called conductivity σ.

• S.I. unit is Ω^-1n^-1 or Sm^-1

Temperature Coefficient of Resistance

• R_T = Resistance at temperature T
• R₀ = Resistance at temperature T₀
• α = Temperature coefficient of resistance.

Temperature Coefficient of Resistivity (ρ)

• For conductors

• For insulators

• K = Boltzman constant
• T = Absolute temperature

Combination of Resistances

• Resistance In Series

• Resistance in Parallel

Internal Resistance of a Cell and Terminal Voltage

Kirchhoff's Rule

• First Rule (Junction Rule)

• Second Rule (Loop Rule)(Based on law of conservation of energy)
  • The algebraic sum of changes in potential around any closed loop must be zero.

• The metre bridge

Shunt Resistance Required to Convert a Galvanometer into an Ammeter of Desired range

• Resistance of ammeter so obtained

Series Resistance Required to Convert a Galvanometer into a Voltmeter of Desired Range.

  is desired range and R is additional series resistance.

Cell in Series and Parallel

Heating Effect of Current

• When a charge dq passes across a potential difference V, the work done dW is given by

  dW = V dq

• The flow of charge dq in time dt is equivalent to current I.

• According to Ohm’s Law V = Ri

Power

• Resistance of a bulb or other electrical appliances:

• And maximum current that can be passed without the appliance getting fused.

• Ratio of powers consumed in two resistors in series:

• Ratio of powers consumed in two resistors in parallel.

Potentiometer

Faraday’s Laws of Electrolysis

• First Law
  • The mass of a substance deposited or liberated at an electrode is proportional to the quantity of charge that passes through the electrolyte.

• Second Law
  • If the same quantity of charge is passed through several electrolytes, then the masses of various substances deposited at the respective electrodes are proportional to their chemical equivalents.

• The constant F is called Faraday.

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