Key concepts for electrical circuits Electrical circuits oElectron oConductor oInsulator oCurrent oBattery and Voltage oSwitch oResistance oEnergy Electrical Chemical HeatLight Mechanical

Electric current and conductor Motion of electrons in a conductor I Atom _ _ _ electrons nucleus protons + neutrons Some electrons are free to move Typical conductors: metals like copper, gold, silver, aluminum, …. In a metal, there are about free electrons / cm 3 Can apply a potential difference across the metal to move electrons electric current Some electrons form bonds, and are not free to move + + _ _ - + -

Insulator I=0 All the electrons are used in forming chemical bonds. No electron is “free to move” Typical insulators: plastics, ceramics, wood, ……..

Mechanical analogy of an electric circuit with battery and motor elevator = battery potential energy wheel = motor water flow = electron flow Potential energy given to electrons by battery is used to do work with the motor, but could also produce light in light-bulb, produce heat in radiator, etc..

Electric circuit Key point: circuit must be closed; all parts must conduct electricity switch battery radiator heat connecting wires light bulb light + heat electromagnet motor current

Electrochemical Battery H 2 SO 4 H+H+ H+H+ SO 4 -- Zinc Carbon electric wire Acid electrolyte molecules break up in three ions: two H + and one SO 4 -- Zn atoms at surface of the anode lose two electrons to become Zn ++ Zn ++ ions combine with SO 4 – to form ZnSO The two electrons recombine with two H + ions at the graphite cathode Two H atoms form a molecule, and hydrogen bubbles off H2H2 + - The process is very similar in a regular Duracell or Energizer battery Electrodes are zinc and manganese-oxide, with an alkaline electrolyte This chemical reaction “wants” to happen. It is “exothermic”. The strength of the reaction determines how strongly electrons can be “pushed” through the circuit, and what the voltage, or potential, across the battery is.

Resistance Element of a circuit made to oppose resistance to the flow of electrons omade of a “poor” conductor oV = I x R, where V is the voltage across the resistance (in volts, V), I is the current (in amperes, A) and R is the value of the resistance (in ohms, Ώ) Symbol: Analogy: Diode Liquid flow Symbol: Element of a circuit that lets current flow only in one direction Analogy: I V I V 0 0