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Calculating Power and Energy in a Battery Circuit

Learn how to calculate the rate of energy conversion, power dissipated internally, and power output in a battery circuit with resistors.

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Calculating Power and Energy in a Battery Circuit

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  1. Example: A 12 V battery with 2  internal resistance is connected to a 4  resistor. Calculate (a) the rate at which chemical energy is converted to electrical energy in the battery, (b) the power dissipated internally in the battery, and (c) the power output of the battery. R=4 - +  = 12 V I r=2

  2. Calculate (a) the rate at which chemical energy is converted to electrical energy in the battery. R=4 - +  = 12 V I r=2 Start at negative terminal of battery…  - I R2 - I R4 = 0 I =  / (R2 + R4) =12 V / 6  = 2 A Energy is converted at the rate Pconverted=I=(2 A)(12 V)=24W.

  3. Calculate (b) the power dissipated internally in the battery. R=4 - +  = 12 V r=2 I=2A Pdissipated = I2r = (2 A)2 (2 ) = 8 W. Calculate (c) the power output of the battery. Poutput = Pconverted - Pdissipated = 24 W - 8 W = 16W.

  4. Calculate (c) the power output of the battery (double-check). R=4 - +  = 12 V r=2 I=2A The output power is delivered to (and dissipated by) the resistor: Poutput = Presistor = I2 R = (2 A)2 (4 ) = 16W.

  5. Example: a 3 volt and 6 volt battery are connected in series, along with a 6 ohm resistor. The batteries* are connected the “wrong” way (+ to + or - to -). What is the power dissipated in the resistor? In the 3 volt battery? 6  - - + + a I 6 V 3 V Starting at point a... + 6 – 3 – 6 I = 0 I = (6 – 3) / 6 = 0.5 A *Assume zero internal resistance unless the problem suggests otherwise.

  6. What is the power dissipated in the resistor? 6  - - + + a PR = I2R = (0.5)2 (6) = 1.5 W I = 0.5 A 6 V 3 V What is the power dissipated in the 3 volt battery? P3V = IV = (0.5) (3) = 1.5 W P6V = IV = (0.5) (6) = 3 W = PR + P3V Note:

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