Transcript Slide 1
Unit 3 Day 1: Voltage, Current, Resistance & Ohm’s Law • • • • • • Batteries Electric Current Conventional Current Resistance Resistors Energy Dissipated in a Resistor Batteries • Discovered by Luigi Galvani (1780) And improved upon by Alessandro Volta • A battery produces electricity by transforming chemical energy into electrical energy, using various metal electrodes immersed in an electrolytic solution Batteries • Exterior to the battery, the electrodes are connected to terminals • 2 or more batteries connected together in series ( + to – terminals) adds the voltage • Batteries are the source of potential difference ΔV =VA-B & measured in SI units of volts (V) Electric Current • Electric current is the flow of electric charge in an electric circuit • An electric circuit consists of a source of ΔV (battery), connecting wires, and a load (lamp) Electric Current • When a circuit forms a closed loop, conventional current flows out of the (+) terminal of the battery, into the load, and back into the (-) terminal of the battery Electric Current Q I t • Where ΔQ is the amount of charge that passes through a conductor at any location, during some interval of time. • SI: Amperes (A) 1A = 1 Coulomb / sec • Charge carriers are the electrons which flow out of the (-) terminal of the battery and into the (+) terminal Electric Current • Conventional current flows in the opposite direction of electron flow ! Electric Current + E-Field - - • Current flows because the potential difference produced by the battery sets up an electric field parallel to the wires. Free electrons in the wire are attracted to the (+) end of the electric field • There will be a continuous flow of electrons as long as ΔV is available Resistance • In 1850 Georg Simon Ohm determined that in a circuit, I V . • The current is the circuit is limited or impeded by the amount of voltage applied to the circuit. • This impediment to current flow is due to electrons colliding with metal atoms in the wire. This impediment to current flow is called resistance. Resistance V V R or I or V I R I R where R is the resistance, SI: Ohms (Ω) 1 Ω = 1 volt / ampere • This is called Ohm’s Law Resistors • All electronic devices which represent a load in an electronic circuit can be modeled as a resistor • A resistor is a device which contains a defined amount of resistance and is used to control current in an electronic circuit Resistors Load Resistances I + + VL - • In an electronic circuit, current flows into the load resistor establishing a potential difference across it in the polarity shown Energy Dissipated into a Load Resistor + - E-Field I e A B • The energy dissipated into a load resistor is the energy delivered by the battery per unit time: U e U A U B qVA VB q V U U e qi V Q V U Q V