Electricity The Electric Charge The Electron Insulators and Conductors The Gold-Leaf Electroscope Redistribution of Charge Charging by Induction Coulomb’s Law of Electric Force

Electricity Charge

Objectives Define the nature of electric charge, and describe how objects can obtain a negative or positive charge Distinguish between insulators, conductors, and semi-conductors Explain how the gold-leaf electroscope can be used to determine a charge State and apply Coulomb’s Law

Electrostatics Static electricity Unit of charge Stationary electricity in the form of a electric charge “at rest” Symbol for charge is “q” or “Q” Unit of charge Coulomb Symbol for coulomb is “C”

Elementary Charge Smallest unit of charge Charge is quantized Atoms contain elementary charges Electron (-e) = -1.6 x 10-19 C Proton (+e) = 1.6 x 10-19 C Neutron (0) = no charge

The Electric Charge Neutral objects Charged objects Number of negative charges equals the number of positive charges Charge is balanced (equilibrium) Charged objects Charge is transferred from one object to another Charge is separated within an object

The Electric Charge A force of repulsion exists between two substances that are electrified in the same way. First Law of Electrostatics: Like charges repel and unlike charges attract.

The Electron An object that has an excess of electrons is negatively charged. An object that has a deficiency of electrons is positively charged.

Insulators and Conductors A conductor is a material through which charges can be easily transferred. Such as metals, that allow charges to move about freely Charges spread out over the surface An insulator is a material that resists the transfer of charge. Materials through which charges will not move easily Charges stay where they are placed A semiconductor is a material that transfers charges better than an insulator but not as well as a conductor.

Charging by Conduction Charging a neutral object by touching it with a charged body Charges are transferred to the object Charges spread over all metal surfaces

Charging by Induction A charged body is brought near another object without touching it Charges are separated in the object Like charges within the object are forced to move away from a charged body to another part of the object OR Opposite charges within the object are forced to move toward a charged body leaving like charges in another part of the object

Charging by Induction H O Dipole moment of water molecule Water bends toward positive charge H O

Charging by Induction H O Dipole moment of water molecule Water bends toward negative charge H O

The Gold-Leaf Electroscope

Redistribution of Charge When a charged rod is brought close to an uncharged pith ball there is an initial attraction. No charge is gained or lost during this process--the charge on the neutral body is simply redistributed.

Charging by Induction Charging by induction: The redistribution of charge due to the presence of a nearby charged object. Charging by induction can be accomplished without any loss of charge from the charging body.

Coulomb’s Law Coulomb’s Law The force of attraction or repulsion between two point charges is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them. k = 8.99 x 109 N·m2/C2 One coulomb is the charge transferred through any cross-section of a conductor in one second by a constant current of one ampere. 1 C = 6.25 x 1018 electrons

Summary of New Terms electrostatics charging electron negative charge positive charge ion induced charge conductor insulator semiconductor Coulomb’s law coulomb microcoulomb electroscope

What is the electrical force on A? Example 3.0 cm q = 6 μC q = 3 μC A B What is the electrical force on A?

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