⚙️: ELECTRIC CHARGES AND FIELDS

• Charge transfer when objects are rubbed together

  1. Charge is transferred from one object to the other

  2. Disturbs the electrical neutrality of both objects

  3. Object that loses electrons becomes positively charged

  4. Object that receives electrons becomes negatively charged
• Charge transfer


  • Glass rod rubbed on silk - positive charge on glass, negative charge on silk
  • Plastic/ebonite rod rubbed on fur - negative charge on plastic/ebonite, positive charge on fur
• Gold-leaf electroscope


  • Apparatus to detect charge on a body
  • Consists of vertical metal rod fixed in a box with two thin gold leaves attached to its bottom end
  • Charged object touches the metal knob on top, charge flows onto the leaves
  • Like charges repel, leaves diverge due to repulsive force
  • Degree of divergence indicates amount of charge
• Conductors


  • Substances that allow passage of electricity through them
  • Have free electrons that can move inside the material
  • Examples: metals, human/animal body, earth
• Insulators


  • Substances that do not allow passage of electricity through them
  • Do not have free electrons, offer high resistance to electricity
  • Examples: glass, plastic, wood, nylon
• Charge distribution on conductors and insulators

  1. Charge transferred to a conductor gets distributed to entire surface

  2. Charge transferred to an insulator remains at the same place
• Charged body brought in contact with earth

  Excess charge on the body flows to the ground
• Grounding/Earthing


  Process of sharing charge with the earth
• Earthing


  • Provides safety measure for electrical circuits and appliances
  • Thick metal plate buried in earth, connected to appliances by earthing wire
  • Electrical wiring has live, neutral and earth wires
  • Metallic bodies of appliances connected to earth wire
  • Charge flows to earth without damaging appliance or giving shock
• Charging by induction


  1. Positively charged rod brought near metal sphere A

  2. Free electrons in sphere A attracted towards rod, leaving excess positive charge on sphere B

  3. Sphere B separated from A without disturbing charged rod

  4. Charges get uniformly distributed on both spheres, equally and oppositely charged

  5. Charged rod does not lose any charge
• Electrified rod brought near light objects

  Objects get attracted due to induced opposite charge
• Point charge


  Charged bodies are treated as point charges if the sizes of charged bodies are very small as compared to the distance between them
• Basic properties of electric charge

  • Additive nature of charges
  • Conservation of electric charge
  • Quantization of electric charge
• Additive nature of charges

  The total electric charge of a body is the algebraic sum of charges located at different points on it
• Charge has magnitude but no direction, unlike mass which is always positive, charge can be positive or negative
• Conservation of electric charge


  The charge can neither be created nor destroyed but can be transferred from one body to another
• The total charge on an isolated system remains constant
• Charged particles can be created from neutral particles, e.g. a neutron turns into a proton and electron which have equal and opposite charge
• The total charge is zero before and after the reaction
• Quantization of electric charge

  The total charge on any body exists as some integral multiple of fundamental unit of charge
• Fundamental unit of charge


  Denoted by e, the charge on an electron or proton
• The value of the fundamental unit of charge is 1.6x10^-19 Coulomb
• Total charge on any object

  Q = ne, where n is the number of electrons deposited or removed
• Coulomb's law


  The magnitude of the electric force between two charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them
• The constant in Coulomb's law is 8.99x10^9 N·m²/C²
• The force on one charge due to another is equal and opposite to the force on the second charge due to the first
• The resultant force acting on any charge due to multiple charges can be found using the law of parallelogram of vector addition
• SI unit of charge

  1 Coulomb, the charge which when placed in vacuum at a distance of 1m from an identical charge repels it with a force of 9x10^9 Newton
• Electric field (Electric field intensity)

  The electric field at a point is the force per unit charge experienced by a small test charge placed at that point
• The magnitude of the electric field only depends on the distance from the charge and not the test charge
• For a positive charge, the electric field points radially outwards, while for a negative charge it points radially inwards
• Uniform electric field


  An electric field in which the electric field intensity is the same at all points
• Electric field


  • It accounts for the time delay between the cause (motion of charge) and effect (force on another charge)
  • It can transport energy
• Electric field lines


  Curves drawn to represent the direction of the electric field at each point
• Electric field line configurations

  • Single positive charge
  • Single negative charge
  • Two positive charges
  • Two negative charges
  • Two unlike charges
• Properties of electric field lines

  • They start from positive charges and end at negative charges
  • In a charge-free region, they are continuous curves without any breaks
• Two electric field lines can never cross each other. If they cross, the field at a point of intersection of the two lines will not have unique direction which is not possible.
• The electric field lines do not form any closed loop.
• The relative closeness of the electric field lines in a given region is proportional to the strength of the electric field in that region.
• Dipole


  In most of the molecules the centre of positive and negative charges coincides therefore their dipole moment is zero. However they develop a dipole moment when an electric field is applied such molecules are called non polar molecules e.g. CO, and CH, etc.