chapter 1 electric flied

• Electric charge

  The fundamental property of matter that causes it to experience a force when placed in an electromagnetic field
• Electric charges

  • There are two types: positive and negative
  • Like charges repel, unlike charges attract
• Thales of Miletus discovered that amber rubbed with wool or silk cloth attracts light objects, around 600 BC
• The name "electricity" is coined from the Greek word "elektron" meaning amber
• Materials that can be electrified by rubbing


  • Glass rods
  • Plastic rods
  • Silk
  • Fur
  • Pith balls
• Electrified

  A body that has acquired an electric charge
• Polarity of charge

  The property that differentiates the two kinds of charges
• When a glass rod is rubbed with silk, the rod acquires one kind of charge and the silk acquires the opposite kind of charge
• Electrically neutral

  An object that has no net electric charge
• Gold-leaf electroscope

  • A simple apparatus to detect charge on a body
  • Consists of a vertical metal rod with two thin gold leaves attached to its bottom end
  • When a charged object touches the metal knob, charge flows onto the leaves and they diverge
• Conductors

  Substances that readily allow the passage of electricity through them
• Insulators

  Substances that offer high resistance to the passage of electricity through them
• Charges on a conductor get distributed over the entire surface, while charges on an insulator stay at the same place
• Point charge


  A charged body whose size is very small compared to the distances between them
• Additivity of charges

  The total charge of a system is obtained by algebraically adding the individual charges
• Charge is conserved within an isolated system
• Quantisation of charge

  Electric charge is always an integral multiple of a basic unit of charge, the electron charge
• The value of the basic unit of charge (electron charge) is 1.602192 × 10^-19 C
• At the macroscopic level, the grainy nature of charge is lost and it appears to be continuous
• The only basic charges in the universe are integral multiples of the electronic charge e
• If a body contains n1 electrons and n2 protons, the total amount of charge on the body is (n2 - n1)e
• The charge on any body is always an integral multiple of e and can be increased or decreased also in steps of e
• Grainy nature of charge

  Charge can increase or decrease only in units of e, but at the macroscopic level this is not visible and charge appears continuous
• At the macroscopic level, the quantisation of charge has no practical consequence and can be ignored
• At the microscopic level, where the charges involved are of the order of a few tens or hundreds of e, they appear in discrete lumps and quantisation of charge cannot be ignored
• It takes approximately 200 years to collect a charge of 1 C if 10^9 electrons move out of a body every second
• A cubic piece of copper of side 1 cm contains about 2.5 x 10^24 electrons
• Coulomb's law is a quantitative statement about the force between two point charges
• Coulomb's law

  The magnitude of the force (F) between two point charges q1 and q2 separated by a distance r in vacuum is given by F = k(q1*q2)/r^2
• Coulomb arrived at Coulomb's law through experiments using a torsion balance
• Coulomb's law has been established down to the subatomic level (r ~ 10^-10 m)
• Permittivity of free space (ε0)

  The constant in Coulomb's law, with a value of 8.854 x 10^-12 C^2/N·m^2
• Coulomb's law can be written in vector form as F21 = (1/4πε0)(q1*q2/r21^2)r̂21
• The ratio of the magnitude of the electric force to the gravitational force between two protons is 1.3 x 10^36
• The acceleration of an electron due to the electrical force of attraction with a proton 1 Å apart is 2.5 x 10^22 m/s^2
• The Coulomb force is repulsive
• The electric force Fe between two protons inside a nucleus (distance between two protons is ~ 10-15 m) is ~230 N
• The gravitational force FG between two protons inside a nucleus (distance between two protons is ~ 10-15 m) is ~1.9 × 10–34 N
• The (dimensionless) ratio of the electric force to the gravitational force shows that electrical forces are enormously stronger than the gravitational forces
• The electric force F exerted by a proton on an electron is same in magnitude to the force exerted by an electron on a proton