Physics B Chapter 17 Sections 1 and 4

History of the Battery Luigi Galvani – played with frog legs on garden trellis and noticed them twitch - said electricity was coming from frog itself

History of the Battery Alessandro Volta – electricity came from the 2 metals brought in contact and the frog was a meter - built the 1st voltaic pile (stack of small disks of zinc, brine-soaked cardboard, and silver) - built the 1st voltaic wet cell (glass of dilute acid into which a metal strip of zinc and copper was immersed and connected in series)

Battery Two or more cells electrically attached to one another How does it work? chemical energy stored in the inter-atomic bonds is converted into electrical PE as the solution and one or both of the conducting plates become involved in the chemical reaction

Voltage Sources Dry cells and wet cells (battery) - both release energy during a chemical reaction that is converted to electrical energy Generator - converts mechanical energy to electrical energy

Electromotive Force Also called emf Not a force Maximum potential difference a device can provide Є ex – car battery has Є = 12 V and provides 12 J of energy per Coulomb of charge

Internal Resistance Once a voltage source is put into use it doesn’t deliver its rated Є due to internal resistance In a battery internal resistance is due to resistance encountered by current due to chemicals with the battery In a generator the internal resistance is resistance of wires and components

Terminal Voltage Actual voltage delivered by a source Vab = Є – Ir where Vab – terminal voltage Є – emf I – current r – internal resistance

Sample Problem A car battery has an emf of 12 V and an internal resistance of 0.01 ohms. This resistance is relatively large because the battery is old and the terminals are corroded. What is the terminal voltage when the current drawn from the battery is a) 10 A and b) 100 A? 11.9 V and 11 V

Sample Problem A battery has an emf of 12 V and an internal resistance of 0.15 ohms. A) What is the terminal voltage when the battery is connected to a 1.5 ohm resistor? B) What is the power dissipated in the 1.5 ohm resistor? 10.9 V and 79.3 W

Sample Problem A 2 ohm resistor is connected across a 6 V battery. The voltage between the terminals of the battery is observed to be only 4.9 V. Find the internal resistance of the battery. .45 ohms

Electric Power Consumption Rate at which electric potential energy is used power = energy used/time P = U/t or since U = qV P = IV = V2/R = I2R [P] = Watts, W = J/s

Sample Problem The heating element in the rear window defroster of a Mazda RX-7 has a resistance of 3 ohms. The element is connected directly across the car’s 12 V battery. How much heat is produced in the element in 10 min? 28800 J

Kilowatt - hour Continuous use for 1 hour at the rate of 1000 J/s Equal to 3.6 x 106 J

Sample Problem What would you be billed for running a 1600 W hairdryer for 15 min a day for 30 days if the utility company charges $0.08 kwh? $0.96