Physics 16: Electric Charge and Electric Field

1. Physics 16: Electric Charge and Electric Field Christopher Chui Physics 16: Electric Charge and Electric Field - Christopher Chui

2. 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

3. 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

4. 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

5. 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

6. 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