Chapter 21: Q28CQ (page 773)
Why can a null measurement be more accurate than one using standard voltmeters and ammeters? What factors limit the accuracy of null measurements?
Standard devices alter the circuit during the measurement whereas the null measurements do not alter it. Thus, null measurements are more accurate.
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Electric fish generate current with biological cells called electro-plaques, which are physiological emf devices. The electro-plaques in the South American eel are arranged in \(140\) rows, each row stretching horizontally along the body and each containing \(5000\) electro-plaques. Each electro-plaque has an emf of \(0.15{\rm{ }}V\) and internal resistance of \(0.25{\rm{ }}\Omega \). If the water surrounding the fish has resistance of \(800{\rm{ }}\Omega \), how much current can the eel produce in water from near its head to near its tail?
A \(240{\rm{ }}kV\) power transmission line carrying \(5.00 \times {10^2}\;A\)is hung from grounded metal towers by ceramic insulators, each having a \(1.00 \times {10^9}{\rm{ }}\Omega \) resistance. Figure \(21.51\).
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(b) Calculate the power dissipated by \(100\) of them.
(c) What fraction of the power carried by the line is this? Explicitly show how you follow the steps in the Problem-Solving Strategies for Series and Parallel Resistors.
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The \({\rm{RC}}\) time constant in heart defibrillation is crucial to limiting the time the current flows. If the capacitance in the defibrillation unit is fixed, how would you manipulate resistance in the circuit to adjust the \({\rm{RC}}\) constant \(\tau \)? Would an adjustment of the applied voltage also beaded to ensure that the current delivered has an appropriate value?
Your car's 30.0-W headlight and starter are ordinarily connected in parallel in asystem. What power would one headlight and the starter consume if connected in series to a 12.0-V battery? (Neglect any other resistance in the circuit and any change in resistance in the two devices.)
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