General physics 2

• Electric charge or “charge” is a fundamental property of matter that determines how it reacts to electric and magnetic fields
• The SI unit of electric charge is coulombs (C) in honor of French physicist Charles de Coulomb
• The charge of one electron is 𝒆 − = − 𝟏. 𝟔𝟎 𝒙 𝟏𝟎^−𝟏𝟗 𝑪 while that of a proton is 𝒆 + = + 𝟏. 𝟔𝟎 𝒙 𝟏𝟎^−𝟏𝟗 𝑪
• The magnitude of the total charge of a particle such as a piece of paper is denoted by the symbol q or Q
• 600 B.C - Greeks were already aware that amber

  when rubbed with cloth could attract

  nearby objects
• William Gilbert (1600)– discover many other

  substances possess the same ability

  as that of amber
• • electrics - objects
  • electricity - ability
• Greek word "elektriks" which means amber
• ELECTROSTATICS - study of all phenomena associated with electric charges at rest
• LAW OF CONSERVATION OF ELECTRIC CHARGE

  "The total electric charge in an isolated system never changes"
• BENJAMIN FRANKLIN (1700's)

  • American physicist
  • Kite experiment
  • discovered that there are two types of charges

  positive charge (+)

  negative charge (-)

  • The atom that has lost an electron is now

  positively charged – it is a positive ion

  • The atom that has gained an electron is now

  negatively charged – it is a negative ion

  
  

  • like charges repel and unlike charges attract
• electric charge is

  • Quantized (Robert Millikan)
  • The smallest possible unit is the charge

  on one electron or one proton:

  e = 1.602X10⁻¹⁹ Coulombs

  • Conserved (Benjamin Franklin)
• STRUCTURE OF THE ATOM

  Atom – smallest particle of the

  element

  Elementary Charge

  proton (+e)

  electron (-e)
• PROPERTIES OF PROTON, NEUTRON, AND ELECTRON

  
• Conductors

  • materials through which charge can move freely

  Insulators

  • conduct charge poorly or do not charge at all

  Semi - conductors

  • electrical properties between those of insulators and conductors

  Superconductors

  • materials that are perfect conductors allowing charge to move without any hindrance below some critical temperatures
• Doping - process of adding some element to improve conductivity of semiconductors
• Heike Kamerlingh Onnes

  • dutch physicist
  • discovered superconductivity by cooling mercury to temperature of 4K or -269°C
• CHARGING BY FRICTION

  • Results when two different materials are rubbed together
• Electron Affinity

  • measure of the attraction of an atom to an electron
  • Tendency of an atom to become negatively charged
• Triboelectric Series

  • ranking of the materials based on electron affinity
  • the one that is higher on the list will become the positively charged
  • the one that is lower on the list will become the negatively charged
• CHARGING BY CONDUCTION

  • Requires physical contact between a charging body and a neutral body
  • The sign of the charge acquired by the neutral body is the same with that of the charged body
• CHARGING BY INDUCTION

  • A neutral body may also be charged without physical contact with a charged body
  • The body to be charged is brought very near the charging body
• Polarization – slight separation of charge makes one side of the atom somewhat positive and the opposite side somewhat negative
• FORCES BETWEEN TWO ELECTRICAL CHARGES

  • Directly proportional to the magnitude of each charge (q₁, q₂)
  • Inversely proportional to the square of the separation between their centers (r)
  • Directed along the line connecting their centers

  
• Charles Coulomb (1785)

  • measured the changes in the electric force as he varied the distance between two objects and the charges on them.
• COULOMB'S LAW

  F = force in Newtons (N)

  q = magnitude of the charges in Coulomb (C)

  r = distance in meters (m)

  K = Coulomb’s constant (8.99X10⁹ Nm²/C²)

  
• LAW OF UNIVERSAL GRAVITATION

  F = force in Newtons (N)

  m = magnitude of the masses in kilogram (kg)

  r = distance in meters (m)

  G = universal gravitational constant (6.674X10⁻¹¹ Nm²/kg²)

  
• Superposition principle

  • The resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are present
  • Remember to add the forces as vectors
  • The resultant force on q₁ is the vector sum of all the forces exerted on it by other charges:
• ELECTRIC FIELD

  • Space surrounding a charged body
  • Causes any charge to experience an electric force
• Michael Faraday (1791 - 1867) introduced the concept of electric field

  • He said that it is intrinsic property of nature that an electric field exists in the space around an electric charge.
  • He said electric field at a point in space is defined as a force field that exerts a force on charge placed at that field.
• LINE OF FORCE PROPERTIES:

  • Lines of force start from positively charged particles and end on negatively charged particles.
  • Lines of force neither intersect nor break as they pass from one charge to another.
  • The greater the number of lines of force, the stronger the electric field.

  Neutral Point

  • Point where no line of force pass.
  • Electric field is zero.
• CONCEPT OF A FIELD

  • A field is defined as a property of space in which a material object experiences a force.

  NO FORCE, NO FIELD; NO FIELD, NO FORCE

  • The direction of the field is determined by the force.
• ELECTRIC FIELD LINES

  • are imaginary lines drawn in such a way their direction at any point is the same as the direction of the field at the point.
  • Lines that indicate the strength and direction of the electric field.
  • The more dense the lines, the stronger the field.
  • As it moves closer to the charge, the electric field is stronger.
  • Electric field vectors are tangent to the curve.
• ELECTRIC FIELD LINE PATTERNS

  For a positive point charge the lines will radiate outward equally in all directions.

  • A positive test charge would be repelled away from the positive source charge.

  For a negative point charge the lines will point inward equally in all directions.

  • A positive test charge would be attracted toward the negative source charge.
• The magnitude and direction of the electric field are expressed by the value of E, called electric field strength or electric field intensity or simply electric field.
• ELECTRIC FIELD INTENSITY

  • It is a vector quantity; it has both magnitude and direction.
  • It is refers to the electric force experienced by a positive test charge at a point divided by the charge.
• The strength of the source charge's electric field could be measured by any other charge placed somewhere in its surrounding.

  Test charge

  • the charge that is used to measure the electric field strength.
  • When placed within the electric field, the test charge will experience an electric force either attractive or repulsive.
• The direction of electric field is in the direction of the test charge which is always taken as positive.
• ELECTRIC FLUX

  • Comes from the Latin word “fluxus” meaning flow
  • ɸ - symbol for electric flux
  • Measure of the number of field lines passing through a surface
• ɸ = EAcosƟ

  Where:

  ɸ = electric flux in N.m²/C

  E = electric field in N/C

  A = area vector in m²

  Ɵ = angle between the electric field and the area vector