Tuesday, 31 January 2017

Basic notes On Electricity | Physics



  • Electricity deals with the motion of the electric charge.
  • Electric charge is a scalar quantity is a scalar quantity its quantity is Coulomb.
  • Electricity produced by the friction between two bodies is called static electricity or frictional electricity e.g. generation of electric charge on rubbing the plastic comb with dry hair.
  • One body get positive charge and other body get negatively charged.

Coulomb Law:

The magnitude of the electrostatic force of interaction between two charge points is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distance between them.

Electric Field:

  • The space in the surrounding of any charge in which its influence can be experienced by other charge is called electric field.
  • Electric field intensity (E) at any point is defined as the electrostatic force acting per unit positive test charge at that point. Its unit is newton/coulomb.
  • E=F/q
  • Electric field intensity is inversely proportional to the square of the distance r from the point charge.

Electric Field Lines:

  • Electric field line is an imaginary lines or curve drawn through a region of space so that its tangent at any point is in the direction of the electric field vector at that point.
  • Two lines an never intersect, electric field always start from positive end and always ends on negative charge and do not start or stop in the mid.

Electric potential:

  • Electric potential at any point in an electric field is equal to the work done per unit charge in carrying at least a test charge from infinity to that point. Its unit is joule/coulomb.
  • Potential difference between two points in electric field is equal to the work done per unit charge in carrying a positive test charge from one point to the other point.
  • Potential difference decides the flow of charge between two points in electric field.
  • Positive charge always tends to move from higher potential towards lower potential.
  • Inside closed metallic body electric field inside it is zero.

Electric current:

  • Electric current is the flow of charge in respect to time.
  • Electric current=q/t
  • An electric current whose direction does not change with time is called direct current (D.C).
  • An electric current whose direction changes with time is called alternating current (A.C).
  • In solids current flow due to flow of electrons and in liquid current flow due to flow of ions as well as electrons and  in semiconductors due to flow of electrons and holes.


  • The resistance offered by any material in the flow of current is called as electrical resistance. Its S.I unit is ohm and [ML2T-3A-2] is its dimension.
  • R=PL/A
  • L=length of conductor
  • A=cross sectional area
  • P= resistivity of the material.
  • Conductance
  • Conductance or conductivity is the reciprocal of resistance and the resistivity of the material respectively. Its SI unit is mho.


  • The resistivity of a material is equal to the electrical resistance of its wire of unit length and of unit area of cross section. It unit is ohm-meter.
  • Resistivity of a material depends on the temperature and nature of material. It is independent of dimensions of the conductor, i.e. length, area of cross-section.
  • Resistivity of metals increases with increase in temperature.
  • Resistivity is low for metals, more for semiconductors and very high for alloys.

Combination of Resistances

  • Resistance can be connected in two ways i.e. in parallel and in series.
  • If resistance R1, R2 and R3 are connected in series their equivalent resistance is given by
R= R1+R2+R3
  • In series combination equal current flows through each resistors.
  • If resistance R1, R2 and R3 are connected in parallel then equivalent resistance is given by.
1/R =1/R1 +1/R2 +1/R3

Ohm’s Law

  • It states that if physical conditions of any conductor such as temperature, pressure etc. are remain unchanged then electric current(I) through  it is directly proportional to the potential difference(V) applied across its ends.
  •  V=IR

Kirchoff’s Law:

Kirchoff current law: states that  the net current on a junction in an electrical circuit will be zero. It is based on conservation of charge.
Kirchoff’s Voltage Law:  states that the algebraic sum of all potential difference along a closed loop is Zero. It is based on conservation of energy.

Electric Cell:

  • An electric cell is a device which converts chemical energy into electrical energy.
  • Electric cell are of two types:
  • Primary cell: cannot be charged. Voltaic, Daniell and Leclanche cells are primary cells.
  • Secondary Cell: can be charged again & again. Acid and alkali accumulators are secondary cells.

Joule’s Law of Heating

  • Joule heating describes the process where the energy of an electric current is converted into heat as it flows through a wire due to the resistance of wire. Heat produced in the conductor due to electric current in time “t” is given by
  • H=VIT=I2RT=V2t/r
  • Electric bulb and heater works on the basis of heating effect produced by the current.

Chemical Effect of Electric current:

  • When an electric current is passed through an acidic or basic solution, it decomposes into  its positive and negative ions. The positive ions collect at the negative electrode(Cathode) and the  negative ion is collected at the  positive electrode(anode).This phenomenon is called electrolysis.

Faraday’s Law of Electrolysis

First law:
  • The total mass (M) deposited at an electrode in the process of electrolysis is directly proportional to the total charge (q) passed through the electrolyte.
  • M=Zq, where Z is  electrochemical equivalent of the substance deposited at electrode.
Second law:
  • If same strength of electric current is allowed to passed through different electrolytes for the same time, then mass deposited at the electrodes is directly proportional to their chemical equivalent.
  • M1/M2 =E1/E2