2001 普通物理第28章摘要 | Series And Parallel Circuits | Capacitor

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  Chapter 28 Direct Current Circuits 28.1 Electromotive Force 28.2 Resistors in Series and in Parallel 28.3 Kirchhoff s Rules 28.! RC Circuits 28. #$ptional% Electrical &nstruments 1 Chapter 28 Direct Current Circuits 28.1 Electromotive Force A variety of devices do the opposite of the discussion in the previous chapter; that is, they take one form of energy and convert it into a potential difference 'et(een t(o terminals. Examples are batteries, photovoltaic ('solar' cells, !an der raaff gen
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  Chapter 28 Direct Current Circuits 28.1   Electromotive Force28.2   Resistors in Series and in Parallel28.3   Kirchhoffs Rules28.!   RC Circuits28.   #$ptional% Electrical &nstruments  1  Chapter 28 Direct Current Circuits 28.1   Electromotive Force A variety of devices do the opposite  of the discussion in the previous chapter; that is, they take one form of energy and convert it into a potential difference   'et(een t(o terminals . Examples are batteries,  photovoltaic ('solar' cells, !an der raaff generators, a 'thermocouple' that converts temperature differences into a potential difference, and mechanical generators in po#er plants. $here also are biochemical reactions that maintain a  potential drop across your cell membranes % you die #hen this gets too lo#. $hese systems are called sources of ' emf  ', or electromotive force . $his is some#hat of a misnomer since it has little to do #ith force. $he common symbol for emf is a script ε  . &  n the simple circuit sho#n belo#, a battery is connected to a light bulb. $he battery provides the energy to move charge through the light bulb, #hich emits light and heat % the light bulb is essentially a resistor $he battery lifts  the hypothetical positive charges to a hi)her potential , and they then run 'do#nhill' to#ard lo#er potential of the negative battery terminal. )e of course kno# better and #ould state this differently in terms of the real ne)ative char)es , but the ideas is the same. *  A battery is a source of emf that generates a current in a particular direction . $his is called direct current  (+. An electric generator or po#er plant normally generates an emf that generates currents that vary in magnitude and direction at - cycles per second. $his is an example of alternatin) current  (A. /ou #ill learn more about A vs. + currents later. 0hysically the emf   measures the (or* per unit char)e delivered '+ the source,  and the .. unit is the !olt. 2or example, a 1.3! flashlight battery that delivers * mA for one minute does an amount of #ork e4ual to ) 5 (1.3!(.*A(- sec 5 &6 7. A perfect battery #ill deliver all this #ork to some (external resistor, e.g., to the light or to some something like the follo#ing heatin) element . $he circuit diagram #ould look 8
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