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Chapter 19: Q26Q (page 796)
Why can birds perch on the bare wire of a power line without being electrocuted?
A bird doesn't get electrocuted while sitting on a live wire since the point where its feet touch the wire are at a same potential and hence no current passes through its body.
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When glowing, a thin-filament bulb has a resistance of about and a thick filament bulb has a resistance of about . If they are in parallel, what is their equivalent resistance? How much current goes through two flashlight batteries in series if a thin-filament bulb and a thick filament bulb are connected in parallel to batteries?
A circuit consists of a battery, whose emf is K, and five Nichrome wires, three thick and two thin as shown in Figure 19.78. The thicknesses of the wires have been exaggerated in order to give you room to draw inside the wires. The internal resistance of the battery is negligible compared to the resistance of the wires. The voltmeter is not attached until part (e) of the problem. (a) Draw and label appropriately the electric field at the locations marked × inside the wires, paying attention to appropriate relative magnitudes of the vectors that you draw. (b) Show the approximate distribution of charges for this circuit. Make the important aspects of the charge distribution very clear in your drawing, supplementing your diagram if necessary with very brief written descriptions on the diagram. Make sure that parts (a) and (b) of this problem are consistent with each other. (c) Assume that you know the mobile-electron density n and the electron mobility u at room temperature for Nichrome. The lengths and diameters of the wires are given on the diagram. Calculate accurately the number of electrons that leave the negative end of the battery every second. Assume that no part of the circuit gets very hot. Express your result in terms of the given quantities . Explain your work and identify the principles you are using. (d) In the case that , what is the approximate number of electrons that leave the negative end of every second? (e) A voltmeter is attached to the circuit with its + lead connected to location B (halfway along the leftmost thick wire) and its - lead connected to location C (halfway along the leftmost thin wire). In the case that , what is the approximate voltage shown on the voltmeter, including sign? Express your result in terms of the given quantities .
A circuit consists of two batteries (with negligible internal resistance), five ohmic resistors (Figure 19.88). The connecting wires that have negligible resistance. The letters A through are shown to make it possible to refer to specific parts of the circuit.
(a) Write all the equations necessary to solve for the unknown currents , , , and , whose directions are indicated on the circuit diagram. Do not solve the equations but do explain very clearly what your equations are based on and to what they refer.
Assume that a computer program has solved your equations in terms of known battery voltages and known resistances so that the currents , , ,and are are known. (b) In terms of known quantities calculate and check that your sign makes sense. (c) In terms of known quantities, calculate the power produced in battery number 2.
A particular capacitor is initially charged. Then a high-resistance Nichrome wire is connected between the plates of the capacitor, as shown in Figure 19.69. The needle of a compass placed under the wire deflects to the east as soon as the connection is made. After the compass needle no longer deflects.
(a)Which of the diagrams in Figure 19.69 best indicates the electron current at three locations in this circuit? (1) after the circuit is connected, (2) after the circuit is connected, (3) after the circuit is connected.
(b)Which of the diagrams in Figure 19.70 best indicates the net electric field inside the wire at three locations in this circuit? (1) after the circuit is connected, (2) after the circuit is connected, (3) after the circuit is connected.
A long Iron slab of width w and height h emerges from a furnace, as shown in Figure 19.79. Because the end of the slab near the furnace is hot and the other end Is cold, the electron mobility increases significantly with the distance x. The electron mobility is whereis the mobility of the iron at the hot end of the slab. There are n iron atoms per cubic meter, and each atom contributes one electron to the sea of the mobile electron (we can neglect the small thermal expansion of the iron). A steady state conventional current runs through the slab from the hot end towards cold end, and an ammeter (not shown) measures the current to have a magnitude I in amperes. A voltmeter is connected to two locations a distance d apart, as shown. (a) Show the electric field inside the slab at two locations marked with. Pay attention to the relative magnitudes of the two vectors that you draw. (b) Explain why the magnitude of the electric field is different at these two locations. (c) At a distance x from the left voltmeter connection, what is the magnitude of the electric field in terms x and the given quantities w,h,d,u0,k,l, and n ( and fundamental constants)? (d) What is the sign of potential difference displayed on the voltmeter? Explain briefly. (e) In terms of the given quantities w,h,d,u0,k,l, and n and ( and fundamental constants), what is the magnitude of the voltmeter reading? Check your work. (f) What is the resistance of this length of the iron slab?
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