Transcript Lecture 11
Lecture Eleven Physics 100 Fall 2012 Energy and Power Applying V=iR to circuits 1. Calculate the total resistance in the circuit using: Resistors in Series R = R1 + R 2 Resistors in Parallel 1 1 1 R R1 R2 2. Then apply V=iR where V is the voltage of the battery. V=iR We can apply this relationship to individual resistors OR To the circuit as a whole. • Once you know the total current in the circuit, you can use what you know about how current divides in a circuit to figure out the current through each resistor. • Once you know that, you can use V=iR to calculate the voltage across each resistor. Bulbs Don’t Use Up Current • Why do batteries go “dead”? • Chemical Energy stored in the battery gets “used up”. • Energy is not electrons in motion (current). It is not the strength of the battery’s push. Energy • Energy is not matter. You can not touch it. • Energy is the quantity necessary to make something happen….to do work, make a sound, generate light, heat something up. • Energy is only indirectly observable. Energy is detected by observing its effect….see a motion, hear a sound, see light, feel heat and so on. Energy • Symbol is E • Unit is the Joule • Abbreviation for the unit Joule is J • So, E= 6 J Energy • Energy is conserved. • It can be transferred from one object to another and from one “form” to another….for example from chemical energy in a battery to light. • But, the total amount of energy you start with is the amount you end with…Any “missing” energy just went to another object and/or changed form. Bulbs Don’t Use Up Current • Why do batteries go “dead”? • Chemical Energy stored in the battery gets “used up”….transformed into heat and light. • Energy is not matter. It is not electrons in motion (current). It is not the strength of the battery’s push (voltage). • We will be wrapping up electricity and circuits and then will study energy in more depth. Power Amount of energy transfered Power P how long it took for the transfer Conceptually, power is energy used or provided per unit time. Therefore, it is the (time) rate of energy transfer into or out of a system. The standard unit then is J/s which is called a WATT (W) Power defined for electric circuit components Amount of energy transfered Power P how long it took for the transfer P iV General Expression: Always true Specific Equation for electric circuit elements Power defined for electric circuit components P iV Power issues associated with electric circuits components are critical for understanding things like: Energy loss in power transmission, Heating in circuit elements, Electric bills. Power Bill or Energy Bill? Amount of energy transfered Power P how long it took for the transfer ( Power ) x(how long you use it) Amount of energy transfered Watt x hour or a kilowatt-hour is a unit of energy which can be converted into a Joule.