Physics 106 Lesson #10 Circuits Dr. Andrew Tomasch 2405 Randall Lab

Cells may be connected together to form a battery: Review: Electrochemical Cells A basic electrochemical cell is constructed from two dissimilar metals (we used Copper and Zinc) and a chemical solution called the electrolyte (we used soft drink) The electrolyte is typically a strong acid or base, but sometimes a concentrated solution of chemical salts can also be used + + Battery Cell

Review: Electrochemical Cells 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:

Review: 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 In series voltages add In parallel current and capacity add

Review: Current 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” 1 A = lots of moving charge (1 C = 6.25 x electrons)! Typical house: 200 A Direct current (DC): flows in one direction Alternating current (AC): flows back and forth Ampere

Review: 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

Review: 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) Electron flow Conventional Current

Good and Bad Wiring Good:Bad: Ugly: = Broken! Short! No device!

Power has a precise definition in physics: Power is the rate at which work is done or how much work is done per unit of time The Mechanical Definition of Power Units: J/s  Watt ( W ) James Watt invented the steam engine In an electric circuit power is the product of current and voltage:

Resistors When an extension cord is used with a space heater, the cord must have a resistance that is sufficiently small to prevent overheating of the cord Resistors are used in appliances to convert electrical energy into thermal energy (heat) or light Stove Heating Element Toaster Space Heater Light Bulb

Series Resistors For resistors R 1 & R 2 connected in series (sequentially), the current i passing through each resistor must be the same The voltages across R 1 & R 2 must add up to V V +

Parallel Resistors Resistors R 1 & R 2 connected in parallel have the same potential difference (voltage) V across them Charge must be conserved, so the currents I 1 & I 2 flowing through the two resistors must add up to the total current I leaving the battery (continuity for electric current!) + V