# Physics 16: Electric Charge and Electric Field - PowerPoint PPT Presentation

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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

Christopher Chui

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

• 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

• 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 (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

• 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

• 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