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Physics 16: Electric Charge and Electric Field. Christopher Chui. Static Electricity, Electric Charge. A piece of rubber, a glass rod, or a plastic ruler rubbed with a cloth will display static electricity

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physics 16 electric charge and electric field

Physics 16: Electric Charge and Electric Field

Christopher Chui

Physics 16: Electric Charge and Electric Field - Christopher Chui

static electricity electric charge
Static Electricity, Electric Charge
  • A piece of rubber, a glass rod, or a plastic ruler rubbed with a cloth will display static electricity
  • An object becomes charged due to a rubbing process, and is said to have a net electric charge
  • Unlike charges attract; like charges repel
  • Franklin (1706-1790) named charges to be either +ve or –ve. They must be conserved
  • The net amount of electric charge produced in any process is zero
  • Neutral atoms contain equal numbers of protons and electrons; if an atom gain or lose electrons, it is an ion
  • Molecules may be polar or non-polar
  • Metals conduct electricity and are called conductors
  • Nonmetals do not conduct electricity and are called insulators

Physics 16: Electric Charge and Electric Field - Christopher Chui

induced charge the electroscope
Induced Charge—the Electroscope
  • When a +ve charged object is brought close to a neutral metal object but not touching it, a charge will be induced at the end of the rod
  • An electroscope is a device that can be used for detecting charge
  • Inside the electroscope are 2 movable gold leaves. The leaves are connected by a conductor to a metal ball on the outside of the case, but are insulated from the case itself
  • Modern electrometers are based on the principle of electroscope

Physics 16: Electric Charge and Electric Field - Christopher Chui

coulomb s law
Coulomb’s Law
  • Coulomb (1736-1806) investigated electric forces using a torsion balance
  • Coulomb’s law: F = k Q1Q2/r2 ; k=9x109 N-m2/C2
  • The constant k = 1/4peo ; eo = permittivity of free space
  • 1 C is the amount of charge, if placed on each of 2 point objects 1 m apart, will produce a force of 9x109 N on the other
  • The charge of an electron = 1.6 x 10-19 C
  • Electric charge is quantized
  • Electric forces add as vectors
  • Electrostatic force is also called as Coulomb force

Physics 16: Electric Charge and Electric Field - Christopher Chui

electric field
Electric Field
  • Faraday (1791-1867) proposed an electric field extends outward from every charge and permeates all of space
  • Definition of electric field, E = F/q, E is in N/C
  • Electric field due to 1 point charge, E=kQ/r2 = 1/4peoQ/r2
  • Superposition principle for electric fields, E=E1+E2+…
  • In calculations, first ignore signs of charges and calculate magnitudes. Always draw a diagram to show directions. Use symmetry to save work

Physics 16: Electric Charge and Electric Field - Christopher Chui

field lines
Field Lines
  • Electric field lines are drawn to indicate the direction of the force due to the given field on a +ve charge
  • The number of lines starting on a +ve charge, or ending on a –ve charge, is proportional to the magnitude of the charge
  • The closer the lines are together, the stronger the electric field in that region
  • E = constant between 2 closely spaced // plates
  • The electric field inside a good conductor is 0 in the static condition. Any net charge distributes itself on the surface on a good conductor. The E is always perpendicular to the surface outside of a conductor

Physics 16: Electric Charge and Electric Field - Christopher Chui

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