Physics 1

• Electrostatics is the study of electricity in which electric charges are static (not moving)
• Atom


  Consists of a nucleus containing protons and neutrons, with electrons orbiting the nucleus

  
• Proton


  • Positively charged particle in the nucleus, with charge of 1.60 x 10^-19 C

  
• Neutron


  • Uncharged particle in the nucleus
• Electron


  • Negatively charged particle orbiting the nucleus, with mass about 1/1840th of the hydrogen atom and charge of -1.60 x 10^-19 C

  
• Normally, atoms are electrically neutral as the total negative charge on the electrons equals the positive charge on the nucleus
• Insulator (dielectric)


  Material in which electrons are firmly bound to the nucleus, so charges remain where placed

  
• Conductor


  Material in which electrons can freely move from one atom to another, allowing charges to flow
• Semiconductor


  Material intermediate between conductors and insulators, where charges can move but not as freely as in conductors
• Removing electrons from an atom creates a positively charged ion, and adding electrons creates a negatively charged ion

  
• Jumping of electrons from one body to another causes sparks
• Electrification by friction

  Electrons are transferred from one material to another when they are rubbed together
• Gold-leaf electroscope


  • Instrument for testing the polarity of charges, consisting of a metal rod with gold leaves that diverge when charged
• Electrostatic induction


  Inducing charges on a conductor without direct contact, by bringing a charged object near it
• Induced charge


  Charge left on a conductor after the inducing charge is removed, always opposite in sign to the inducing charge

  
• Electrostatic force vs gravitational force

  Electrostatic forces are much stronger than gravitational forces, and can be attractive or repulsive
• Electrostatic force calculations

  • Gravitational force between two 85 kg students 1 m apart is 4.82 x 10^-7 N, while electrostatic force between them with 2 x 10^-3 C charges is 36,000 N

  1. Two point charges of 1.8 x 10^-6 C and 2.4 x 10^-6 C produce a force of 2.2 x 10^-3 N, and are 4.2 m apart
  2. Net force on a 1.7 x 10^-6 C charge between a 2.5 x 10^-6 C charge 2 cm away and a -2 x 10^-6 C charge 3.5 cm away is 71 N (attractive - repulsive)
  3. Electrostatic force between an electron and proton 0.5 x 10^-10 m apart is 0.92 x 10^-9 N (attractive)

  5a) Two equal charges 50 cm apart repelling with 0.1 N force have a magnitude of 1.7 x 10^-6 C

  5b) In an insulating liquid with 10x the permittivity of vacuum, the charges would be 5.4 x 10^-6 C
• Electron and proton charges

  Equal magnitude but opposite sign of 1.6x10^-19 C
• Electric field


  Region where an electric force is experienced, mapped by electrostatic lines of force
• Electric field intensity (E)


  Force per unit charge exerted at a point
• Electric flux


  Product of electric field intensity E and area, represents total electric field lines passing through a surface
• Gauss's theorem


  Total flux through any closed surface equals Q/ε, where Q is the total charge enclosed
• Electric field outside charged sphere


  Behaves as if all charge is concentrated at the centre
• Electric field inside charged empty sphere

  Zero everywhere
• Electric potential

  Analogous to gravitational potential, work done per unit charge in moving a charge
• Potential differences between points is independent of path taken
• Positive charges move towards lower potential, negative charges move towards higher potential
• Work per unit coulomb

  The potential difference VAB between A and B
• VAB will be in volts if Q is in coulombs, a and b are in metres and ε0 is taken as 8.85× 10−12 or 1/4πε0 as 9× 109 approximately
• Potential energy is a scalar. Thus, the signs of charges are considered in the calculation
• Bringing charge near another positive charge requires input therefore the work is positive
• Bringing a charge near a positive charge releases energy therefore, work is negative
• The electric potential is defined in terms of moving of a positive charge. Therefore, + charges move towards low potential while – charges move towards high potential

  
• When we talk about potential at a point we are talking about the potential difference between that point and infinity, where the potential at infinity is ZERO
• The electric field due to a charged body near a conducting surface is complicated, hence difficult to calculate the potential of a point relative to the earth
• Theoretically, it is convenient to consider charges so far from the earth that the effect of the earth on their field is negligible, these are called isolated charges
• Potential at a point A

  V volts if V joules of work is done in bringing one coulomb of positive charge from infinity to A
• Potential difference VAB

  Potential at A - Potential at B = -δV
• The electric field surrounding a point charge is not uniform - that is varying strength and direction
• The electric field between charged plates is uniform in strength and direction