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Physics 106 Lesson #9 Electrochemical Cells and Batteries Dr. Andrew Tomasch 2405 Randall Lab [email protected] Review: Properties of Electric Charges Franklin • Two types of charge: positive and negative (Ben Franklin in early 1700’s) • Like charges repel; unlike charges attract • Charge is conserved • Charge is quantized (comes in discreet units) • Objects usually have as much negative charge in them as they do positive charge → the total charge is zero (electrically neutral) Review: Charging a Conductor by Conduction Conduction = charging by contact Rub a Teflon rod with fur to separate charge The process of separating charge by friction is called triboelectricity. Review: Conductors and Insulators We can classify materials according to their ability to conduct electrical charge: – Conductors: charges (free electrons) move freely (metal) – Insulators: charge is not readily transported (glass) – Semiconductors: electrical properties in between Gold is the best conducting metal http://www.physicspost.com Review: Electric Forces In 1785 Charles Coulomb established the fundamental law of electric force between two stationary charged particles: – Force directed along the line joining the particles – Force inversely proportional to the square of separation distance between particles – Force proportional to the product of the two charges – Force attractive if particles have charges of opposite sign and repulsive if charges have same sign Review: Comparing the Electrostatic Force to Gravity • Gravitational • Coulomb Force: Force (Newton): GMm FG 2 r FC k q1 q2 r However, the gravitational force can only be attractive! 2 The Electrochemical Cell • Used to establish electric current in a circuit • Transformation of chemical energy stored in the cell to kinetic energy of the charge carriers (electrons) • Two oppositely charged electrodes (terminals) • An electrical potential difference (voltage) exists between the terminals • Electric potential is potential energy per unit charge. • Multiple cells connected together comprise a battery of cells or battery A 50’s-style Carbon-Zinc Cell Cells in Series and Parallel • A battery pack consists of four 1.2-volt cells in series (end-to-end).. • The nominal voltage of the battery is the sum of the individual cell voltages = V × 4 = 4.8 V • With parallel cells, the voltage stays the same = 1.2 V • The amount of available charge increases → larger current Current: Charge in Motion • Definition: Current is the amount of charge moving past a point per unit time • Charge flows in a confined channel (wire) like a river • Current is caused by potential differences (voltages) • Charge is measured in Coulombs (C) • The unit of current is the Ampere (A): 1 A = 1 C/s “One Coulomb per second” Ampere • 1 A = lots of moving charge (1 C = 6.25 x 1018 electrons)! • Typical house: 200 A • Direct current (DC): flows in one direction • Alternating current (AC): flows back and forth Cells, Batteries & Current • When connected to a circuit, the potential difference between the cell or battery terminals creates an electric force on the charges in the conductor causing them to move and establishing an electric current • The conventional current is from regions of higher potential to regions of lower potential, positive to negative Conventional current + Battery + Cell Conventional Current and Electron Flow • Truth: the particles which carry charge through wires in a circuit are mobile electrons • Ben Franklin: positive charges move (oops!) • The direction of current is by convention the direction in which a positive charge would move (so current flows from high potential to low potential) Conventional Current Electron flow Natural Currents: Lightning and the Northern Lights Resistance • Life is tough for free electrons • Resistance: Repulsion from other electrons Vibration of atoms Impurities • Energy is dissipated • Worse at high temperatures • The symbol for a resistor: http://regentsprep.org/Regents/physics/phys03/bresist/default.htm