Physics

• Materials based on conductivity

  • Conductors
  • Insulators
  • Semiconductors
• Doping


  The process of adding atoms of different elements to pure semiconductors to improve their conductivity
• Superconductor


  Materials that practically offer no resistance to the flow of electric charges below a critical temperature
• Superconductor


  • A material involving hydrogen sulfide which can conduct charges to a temperature of -70°C
• Charging


  The number of protons and electrons in an atom is equal, hence an atom is said to be neutral. However, an atom may gain or lose electrons. If the atom gains electrons, it becomes negatively charged; if it loses electrons, it becomes positively charged
• Charging by Friction


  Two neutral bodies are rubbed together. The material that will become positively or negatively charged will depend on its electron affinity
• Electron Affinity


  Measure of the attraction of an atom to an electron, the tendency of an atom to become negatively charged
• Materials in the Triboelectric Series

  • Air
  • Human Skin
  • Rabbit Fur
  • Glass
  • Human Hair
  • Nylon
  • Wool
  • Silk
  • Aluminum
  • Paper
  • Cotton
  • Steel
  • Wood
  • Hard Rubber
  • Nickel, Copper
  • Brass, Silver
  • Gold, Platinum
  • Acetate
  • Fiber/Rayan
  • Polyester
  • Cling Film
  • Polyethylene
  • PVC
  • Silicon
  • Teflon
• Sample Problem: The human hair is combed using a rubber comb. The hair becomes positively charged and the comb becomes negatively charged
• Sample Problem: When a glass rod is rubbed with a silk cloth, the glass rod loses electrons
• Sample Problem: The glass rod acquires a charge of magnitude 3.45 nC
• Sample Problem: The glass rod becomes positively charged
• Sample Problem: The number of excess protons in the glass rod is calculated to determine the number of electrons transferred
• The electron affinity of glass is lower than silk in the triboelectric series

  Glass rod loses electrons
• Magnitude of charge acquired by the glass rod = 3.45 nc or 3.45 X 10-9 C
• Since the glass rod becomes positively charged, the number of protons is greater than the number of electrons
• Number of excess protons = 3.45 X 10-9C divided by the charge of the proton = 2.15 X 1010
• The glass rod lost 2.15 X 1010 electrons
• The mass of the glass rod decreased by an amount equal to the mass of 2.15 X 1010 electrons
• The mass of an electron is 9.109 X 10-31 Kg
• Decrease in the mass of the glass rod = 1.96 X 10-20Kg
• The mass of the silk cloth increased by 1.96 X 10-20Kg
• Charging through conduction involves a neutral body (Body A) and a charging body (Body B)
• Charging body is highly negative or highly positive in conduction
• Charging by induction is the process whereby a neutral body becomes charged when brought near a charged body
• Negative charges on the neutral body are attracted towards the charging body if it is positive, and repelled if it is negative
• Earth is a huge reservoir of charges that can donate or accept electrons
• The Electroscope is a device used to determine the kind of electrical charge a body has
• The Gold Leaf Electroscope is composed of a glass case with an insulating stopper, a metal rod, a metal cap, and two thin leaves of gold
• The Electroscope must be given an initial charge before determining the charge of another body
• Table 1 shows how an electroscope determines the charge of a body
• Coulomb's Law states that the force of attraction between two small charged bodies is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them
• Coulomb's Law equation: F = k * q1 * q2 / r^2
• Coulomb's Law


  The force of attraction between two small charged bodies is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them
• Coulomb's Law equation


  F = (k * q1 * q2) / r^2; where F = Force, k = constant of proportionality (9 x 10^9 N.m^2/C^2), q1 and q2 = electrical charges of body 1 and 2 respectively, r = distance between the two charged bodies
• Superposition Principle


  Each charged body will exert a force on another charged body as if no other charges are present
• Sample Problem 1: Magnitude, direction, and nature of the force on a body of charge +4 x 10^-9 C that is 5 cm away from a second body of charge 5 x 10^-8 C
• Sample Problem 2: Magnitude, direction, and nature of the resultant electric force acting on q2 because of q1 and q3
• Sample Problem 3: Magnitude and direction of the electric force in an equilateral triangle with three identical point charges
• 2 and F3 on 2 going to the same direction

  FR = F1 on 2 - F3 on 2 = 5.74 x 10-1 N + 2.39 x 10-1 N = 0.81 N to the left