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HW4solutions-10 - Physics 2213 HW#4 Solutions Spring 2010...

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Physics 2213 HW #4 – Solutions Spring 2010 Q25.8 [Batteries] Labeling batteries by emf makes sense because the potential difference across the battery is constant in the ideal case, and nearly constant for real batteries. Labeling batteries by current would not be appropriate, because the current depends on the resistance in the circuit. Q25.9 [Current & Charge] Placing a charge on an object means an additional charge, which can lead to very high electric fields. However, when current flows in a wire, the wire is still electrically neutral, since the moving charges are matched by the background opposite charge of the conductor. Q25.15 [Circuits] a) The current must be the same everywhere in a series circuit, so the readings must be the same. b) The brightness of the bulb is the same in both configurations, since the brightness depends on the current through the bulb and that doesn’t change. Q25.16 [Circuits] In (a), the bulb glows because an ideal ammeter has zero resistance and therefore doesn’t affect the current in the circuit. In (b), the bulb will not glow because an ideal voltmeter has infinite resistance and therefore the current in the circuit will be zero. 25.37. [Internal Resistance] The voltmeter reads the potential difference ab V between the terminals of the battery. Open circuit 0. I = The circuit is sketched to the right ( ε stands for battery emf.) 3.08 V ab V ε = = Switch closed 2.97 V ab V Ir ε = = The circuit is sketched to the right. 2.97 V r I ε = 3.08 V 2.97 V 0.067 1.65 A r = = And 2.97 V so 1.80 . 1.65 A ab ab V V IR R I = = =
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