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zhang18-1

# zhang18-1 - Direct Current Circuits I I I Direct Current(DC...

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1/31/2010 1 Direct Current Circuits Direct Current (DC): Current goes only in one direction. I I I I I I The source that maintains the current in a closed circuit is called a source of emf Examples: batteries, generators, adapters, solar cells, etc SI units are Volts; symbol : Work done by the emf per unit charge: Q W

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1/31/2010 2 The terminal voltage: the voltage measured from the two terminals of the battery Terminal voltage in general is not the same as the emf of the battery ε is equal to the terminal voltage when the current is zero, also called the open-circuit voltage V Demo: Voltmeter readings (a) Before connecting the resistor (b) After connecting the resistor Why the difference in terminal voltage? internal resistance All sources of emf have certain internal resistance r V V
1/31/2010 3 A battery with an internal resistance r can be represented by an “ideal” battery in serial with a resistor r From the above diagram, R r V V R V r V ) ( R r I IR Ir drop voltage Internal : Ir V r Calculate the internal resistance of a battery (1) Current I in the circuit (Ohm’s law): (2) Internal voltage drop: (3) Internal resistance: V 4 . 1 : voltage terminal 2 V 5 . 1 : emf R V R R V I R R r V V I V r r R V A 7 . 0 2 4 . 1 R r V V V 1 . 0 4 . 1 5 . 1 Ir V r 14 . 0 7 . 0 1 . 0

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1/31/2010 4 When R >> r, internal resistance r can be ignored Usually assumed in problems From , we have As a consequence of internal resistance, a battery can only deliver a maximum power to an external load. FYI: The maximum power delivered to the load resistor is achieved when R=r. IR Ir R I r I IR Ir I I 2 2 ) ( When batteries are worn out, their internal resistance increases The emf of the battery, however, remains the same “Ideal” batteries: Internal resistance r=0 Unless otherwise stated, we will assume that batteries in circuits are “ ideal “Ideal” conducting wire: We will assume all conducting wires
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zhang18-1 - Direct Current Circuits I I I Direct Current(DC...

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