Physics 30 - Lesson 1: Electrostatics PDF

# Physics 30 - Lesson 1: Electrostatics ## Key - The study of Electric Charges at rest ## Modern Theory of Electrostatics - All matter is composed of atoms - Protons are positive, electrons are negative, neutral objects have equal amounts of protons and electrons - Electron-Proton transf...

# Physics 30 - Lesson 1: Electrostatics ## Key - The study of Electric Charges at rest ## Modern Theory of Electrostatics - All matter is composed of atoms - Protons are positive, electrons are negative, neutral objects have equal amounts of protons and electrons - Electron-Proton transfer model states: Electrons move, protons do not - Excess *e-* = negative charge - Deficit of *e-* = positive charge - Law of Charges: - Like charges repel and unlike attract - Neutral objects are attracted to charged objects - Conservation of charge - In a closed system the net charge stays the same (is conserved) ## Measuring Charge - The unit for charge is the Coulomb (C) and its symbol is *q* - A Coulomb of positive charge is equal to the combined charge of 6.25 x 10¹⁸ protons or an equal number of electrons. - The number 1.6 x 10^-19 is called the elementary charge (*e*) - Charge on: - one *e- is -1.6 x 10^-19 C - one *p+ is +1.6 x 10^-19 C ## Distribution of Charges - Conductivity describes how easily electrons can move through a material - Conductors allow *e-* to flow easily (metals, ionic solutions, etc.) - Insulators do not allow *e-* to flow easily (wood, plastics, glass, etc.) - Increasing conductivity - Insulators - Semiconductors - Conductors - Superconductors - Negatively Charged Metallic Sphere: - On a conductor, the electrons are free to move as far away from one another as possible - Negatively Charged Plastic Rod: - On an insulator, the electrons are forced to stay at one end of the rod since they are not free to flow to the uncharged end. ## Distribution of Charges (continued) - Solid conducting sphere: excess charges distribute evenly on surface to balance repulsion - Irregularly shaped solid conductor: charge density is greatest where surface curves most sharply (ie. Where the surface area is greatest) ## Electroscopes - An instrument that will detect the presence of charge and indicate type of charge. - The metal leaf electroscope: - When neutral, the leaves hang straight down. - When charged, the leaves repel one another as they have the same charge. - Tin Foil Straw Electroscope: - When neutral, the straw hangs vertically. - When charged, the support column repels the straw. - For both electroscopes, the greater the charge, the greater the deflection. - A positively and negatively charged electroscope look the same. However, they will respond differently when a charged object is brought near. ## Methods of Charging Objects - Objects become charged through a transfer of electrons. - Occurs by: - Friction - Conduction - Induction - **1. Charging by Friction** - Objects may gain or lose *e-* when rubbed together depending on how tightly they hold onto *e-*. - Materials on... - Top: have a strong hold on their *e-* and do not lose them easily - Bottom: have a weak hold on their *e-* and will lose them easily - Net charge is conserved. - **2. Charging by Contact (Conduction)** - Charging a neutral object by touching it with a charged object. - Quantity of charge transferred depends on size and shape of two objects - **Charging an Electroscope by Conduction** - A positively charged rod is brought near to a neutral electroscope. Electrons are attracted to the rod and move to the top of the electroscope. The leaves are left positively charged and repel. - Contact. Electrons move from the electroscope onto the rod. - Rod is removed. The electroscope is positively charged. - **Charging by Contact (Conduction)** - Charging by conduction results in the electroscope receiving the same charge as the charging device and the charging device loses some of its charge in the process. - Charge will either be distributed over surface of object (conductor) or remain on surface at point of contact (insulator). - Charge is permanent until another interaction. - Net charge of system remains constant. - **3. Induced Charge Separation (Induction)** - An induced charge separation (polarization) is temporary. When the charged rod is removed from the vicinity of the pith ball or electroscope, the pith ball/electroscope return to normal charge distribution. Hence, induced charge separation is not a method of charging. The pith ball/electroscope remains neutral throughout the process. - Charging by Induction (Influence) - To give a permanent charge through induction, a ground is required. - Could be a wire, or a person! - How would you charge an electroscope positively by induction? - Bring a negatively charged rod near the electroscope. - Touch the electroscope with your finger (e's will leave the electroscope through your finger) - Remove your finger (electroscope is left with a deficit of e's) - Then remove the rod - Charging by induction causes the object to become opposite in charge to the charging device - Grounding allows charge to be permanent - Charging device does not lose any charge in the process. ## Permanent Charge by... - Conduction - Objects touch - Object becomes same charge - Share the same excess or deficit of e’s - Induction - Ground required and objects do not touch. - Object becomes oppositely charged - Charging device does not lose charge ## Homework: - Part 1: Introduction to Electrostatics Inquiry Activity - Part 2: Introduction to Electrostatics Problems