Presentation on theme: "Chapter 20 Static Electricity. Objectives Charged Objects Conductors and Insulators Forces on Charged Bodies Coulomb’s Law The unit of charge."— Presentation transcript:
Chapter 20 Static Electricity
Objectives Charged Objects Conductors and Insulators Forces on Charged Bodies Coulomb’s Law The unit of charge “The Coulomb” (coo – lome)
Electrical Charges Electrical comes from the Greek word for Amber: “ Elektron ” Amber, when rubbed, attracts things. Balloons, when rubbed, stick to walls. Benjamin Franklin called the two states of charge “Positive” and “Negative”. Static electricity is produced when one object takes negative charges from another object Like charges repel, opposites attract.
Charging Amber Amber before rubbing, neutral chargeWool before rubbing, neutral charge Amber after rubbing, more negatives than positives = negatively charged. Wool after rubbing, more positives than negatives = positively charged.
Charge; Microscopic Level Electrons have a charge of “-e”. Protons have a charge of “+e” Neutron has no charge. e = 1.60 x 10 – 19 C The total electric charge in the universe is Constant
Charge: Microscopic Mass of an electron: m e = 9.11 x kg Mass of a proton: m p =1.673 x 10 – 27 kg Mass of a neutron: m n = x kg The atom that loses an electron is a positive “ion” The atom that gains an electron is a negative “ion”
Conductors and Insulators Charges in insulators are not free to move about. Charges on conductors are free to move. Conductors give up an electron easily. Most insulators are non-metallic. Most metals are good conductors. Some materials exhibit qualities of both and are called “Semiconductors”.
Charged Bodies There are two charges: + and – Like charges repel, opposites attract. When a charged object touches a neutral object charges are transferred. This is termed “Charging by Conduction”. When a charged object is “near” a neutral object, the charges on the neutral object will separate as opposite charges gather near one another and like charges repel one another. This is termed “Charging by Induction”.
Charging by Conduction If the charged object is negative, electrons can flow into the neutral object resulting the object acquiring negative charge. Although the originally negatively charged object loses some of its negative charge, it still is negative. The originally charged object and the newly charged object will repel one another because they have the same charge.
Charging by Induction first induce a charge separation. “-” charged object is brought close to a neutral conductor “-” electrons on the neutral object will be repelled by the “-” charged object. This results in a charge separation or “polarization” In order to charge the object, we need to provide a passage out for “-” charges. We ground the object. Since the electrons repel one another, they will freely flow into the earth where they can spread out. As the electrons flow out of the neutral object, the object becomes positively charged.
Electrical Force: F e F e is measured in Newtons. Unlike gravity, F e can attract OR repel. Gravity is a weak force compared to F e F e will vary inversely with the square of the distance between two charges; the farther apart, the less F e Coulomb put forth his ideas as “Coulomb’s Law” Formula to use to determine Force of gravity on two charges Coulomb’s Law
The Unit of Charge In Coulomb’s Law k = 9 x 10 9 N(m 2 /C 2 ) One Coulomb “C” is the charge of 6.25x10 18 electrons. When talking of F e, if two like charges (q 1 and q 2 ) repulse one another, the sign of the force is positive. If the two charges are attracted, then the force has a negative sign.
Recap There are two types of charges Positive + Negative- Insulators prohibit electron movement. Conductors allow electron movement. The unit of charge is the Coulomb. Electrical Force is in Newtons (N).