chapter18.1-18.4_updated - Direct Current Circuits I I I I...

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2/4/2009 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 Potential energy of charges circulating in circuits are increased by sources of emf Examples: batteries, generators, adapters, solar cells, etc SI units are Volts; symbol : Meaning: Work done by the emf per unit charge Same as electrical potential Q W
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2/4/2009 2 Batteries: store chemical potential energy When connected to a circuit, chemical energy is converted to electrical energy The electrical energy is finally converted into other forms of energy (eg., into heat by resistors, light/heat by light bulb, etc) V All sources of emf have certain internal resistance r The terminal voltage is the voltage measured from the two terminals of the battery Terminal voltage in general is not the same as the emf of the battery Demo: Voltmeter readings (a) Before connecting the resistor (b) After connecting the resistor V V
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2/4/2009 3 ε is equal to the terminal voltage When the current is zero Also called the open-circuit voltage R is called the load resistance The current depends on both the resistance external to the battery and the internal resistance V V Why the difference in terminal voltage? internal resistance 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
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2/4/2009 4 Calculate the internal resistance of a battery (1) Current (2) (3) V 4 . 1 2 V 5 . 1 R V R R V I R R r R r V V V V I V r Ir V r r R V Battery A emf Internal resistance V 5 . 1 5 . 0 r Battery B emf Internal resistance V 2 . 1 1 . 0 r Which batter delivers a greater current when connected to a 1 external resistor? (1) Battery A (2) Battery B ) ( ); ( r R I r R I
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2/4/2009 5 Battery A emf Internal resistance V 5 . 1 5 . 0 r Battery B emf Internal resistance V 2 . 1 1 . 0 r Which batter delivers a greater current when connected to a 12 external resistor? (1) Battery A (2) Battery B ) ( ); ( r R I r R I When R >> r, r can be ignored Generally assumed in problems From , we have the power relationship When R >> r, most of the power delivered by the battery is transferred to the load resistor IR Ir R I r I IR Ir I I 2 2 ) (
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