Physics 16 electric charge and electric field
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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

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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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