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Chapter 18: Q5Q (page 755)
Since there is an electric field inside a wire in a circuit, why don’t the mobile electrons in the wire accelerate continuously?
Electrons in a wire don't accelerate continuously because the backward force due to resistance cancels out the forward force from the external electric field.
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The circuit shown in Figure 18.107 consists of a single battery, whose emf is , and three wires made of the same material but having different cross-sectional areas. Each thick wire has a cross-sectional area and is long. The thin wire has a cross-sectional area and is long. In this metal, the electron mobility is , and there are mobile electrons/m3.
(a) Which of the following statements about the circuit in the steady state are true? (1) At location B, the electric field points toward the top of the page. (2) The magnitude of the electric field at locations F and C is the same. (3) The magnitude of the electric field at locations D and F is the same. (4) The electron current at location D is the same as the electron current at location F . (b) Write a correct energy conservation (loop) equation for this circuit, following a path that starts at the negative end of the battery and goes counterclockwise. (c) Write this circuit's correct charge conservation (node) equation. (d) Use the appropriate equation(s), plus the equation relating electron current to electric field, to solve for the magnitudes EDand EF of the electric field at locations D and F . (e) Use the appropriate equation(s) to calculate the electron current at location D in the steady state.
Suppose that a wire leads into another, thinner wire of the same material that has only a third the cross-sectional area. In the steady state, the number of electrons per second flowing through the thick wire must be equal to the number of electrons per second flowing through the thin wire. If the drift speedin the thick wire is , what is the drift speed in the thinner wire?
During the initial transient leading to the steady state, the electron current going into a bulb may be greater than the electron current leaving the bulb. Explain why and how these two currents come to be equal in the steady state.
When a single thick-filament bulb of a particular kind and two batteries are connected in series, 18 electrons pass through the bulb every second. When two batteries in series are connected to a single thin-filament bulb, with a filament made of the same material and length as the thick-filament bulb but a smaller cross-section, only 18 electrons pass through the bulb every second. (a) In the circuit shown in Figure 18.109, how many electrons per second flow through the thin-filament bulb? (b) What approximations or simplifying assumptions did you make? (c) Show approximately the surface charge on a diagram of the circuit.
What is the most important general difference between a system in steady state and a system in equilibrium?
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