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Chapter 18: Q3Q (page 755)
How can there be a nonzero electric field inside a wire in a circuit? Isn’t the electric field inside a metal always zero?
The field inside a wire in a circuit is non zero due to the absence of an opposing electric field because there is no accumulation of charges anywhere
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In the circuit shown in Figure 18.87, bulbs 1 and 2 are identical in mechanical construction (the filaments have the same length and the same cross-sectional area), but the filaments are made of different metals. The electron mobility in the metal used in bulb 2 is three times as large as the electron mobility in the metal used in bulb 1, but both metals have the same number of mobile electrons per cubic meter. The two bulbs are connected in series to two batteries with thick copper wires (like your connecting wires).
(a)In bulb 1, the electron current is and the electric field is . In terms of these quantities, determine the corresponding quantities and for bulb 2, and explain your reasoning.
(b)When bulb 2 is replaced by a wire, the electron current through bulb 1 is and the electric field in bulb 1 is . How big is in terms of ? Explain your answer, including explicit mention of any approximations you must make. Do not use ohms or series-resistance equations in your explanation, unless you can show in detail how these concepts follow from the microscopic analysis introduced in this chapter.
(c)Explain why the electric field inside the thick copper wires is very small. Also explain why this very small electric field is the same in all of the copper wires, if they all have the same cross-sectional area.
(d)Figure 18.88 is a graph of the magnitude of the electric field at each location around the circuit when bulb 2 is replaced by a wire. Copy this graph and add to it, on the same scale, a graph of the magnitude of the electric field at each location around the circuit when both bulbs are in the circuit. The very small field in the copper wires has been shown much larger than it really is in order to give you room to show how that small field differs in the two circuits.
In the circuit shown in Figure 18.91, all of the wire is made of Nichrome, but one segment has a much smaller cross-sectional area. On a copy of this diagram, using the same scale for magnitude that you used in the previous question for Figure 18.90, show the steady-state electric field at the locations indicated, including in the thinner segment. Before attempting to answer these questions, draw a copy of this diagram. All of the locations indicated by letters are inside the wire.
(a)On your diagram, show the electric field at the locations indicated, paying attention to relative magnitude. Use the same scale for magnitude as you did in the previous question.
(b)Carefully draw pluses and minuses on your diagram to show the approximate surface charge distribution that produces the electric field you drew. Make your drawing show clearly the differences between regions of high surface charge density and regions of low surface-charge density. Use your diagram to determine which of the following statements about this circuit are true.
(1) There is a large gradient of surface charge on the wire between locations Cand E. (2) The electron current is the same at every location in this circuit.
(3) Fewer electrons per second pass location Ethan location C.
(4) The magnitude of the electric field at location Gis smaller in this circuit than it
was in the previous circuit (Figure 18.90).
(5) The magnitude of the electric field is the same at every location in this circuit.
(6) The magnitude of the electric field at location D is larger than the magnitude of the electric field at location G.
(7) There is no surface charge at all on the wire near location G.
(8) The electron current in this circuit is less than the electron current in the previous circuit (Figure 18.90).
There are very roughly the same number of iron atoms per m3 as there are copper atoms per m3 , but copper is a much better conductor than iron. How does uiron compare with ucopper?
In a table like the one shown, write an inequality comparing each quantity in the steady state for a narrow resistor and thick connecting wires, which are made of the same material as the resistor.
Electron current in resistor | <,=, or > | Electron current in Thick Wires |
nR | nw | |
AR | Aw | |
uR | uw | |
ER | Ew | |
vR | vw |
Question: The following questions refer to the circuit shown in Figure 18.114, consisting of two flashlight batteries and two Nichrome wires of different lengths and different thicknesses as shown (corresponding roughly to your own thick and thin Nichrome wires).
The thin wire is 50 cm long, and its diameter is 0.25 mm. The thick wire is 15 cm long, and its diameter is 0.35 mm. (a) The emf of each flashlight battery is 1.5 V. Determine the steady-state electric field inside each Nichrome wire. Remember that in the steady state you must satisfy both the current node rule and energy conservation. These two principles give you two equations for the two unknown fields. (b) The electron mobility
in room-temperature Nichrome is about . Show that it takes an electron 36 min to drift through the two Nichrome wires from location B to location A. (c) On the other hand, about how long did it take to establish the steady state when the circuit was first assembled? Give a very approximate numerical answer, not a precise one. (d) There are about
mobile electrons per cubic meter in Nichrome. How many electrons cross the junction between the two wires every second?
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