# Physics 22: Homework 4

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1 Physics 22: Homework 4 The following exercises encompass problems dealing with capacitor circuits, resistance, current, and resistor circuits. 1. As in Figure 1, consider three identical capacitors each of capacitance, C arranged linearly between points a and b. Whereas Figure 1a consider these same capacitors with perfect conductors connecting different parts of the circuit, as shown in Figures 1b and 1c. Determine the equivalent capacitance in each of the arrangements shown in Figure 1. (a) (b) (c) Figure 1: Identical capacitors in a linear arrangement across points a and b. Variations are made in (b) and (c) with ideal conducting wires connecting different parts of the original arrangement shown in (a). 2. Consider the capacitor combination circuits shown in Figures 2, 3, and 4, which serve as individual parts. In these figures, C n = nc for n = 1, 2, 3, 4, 6, and 8, a battery, of potential V ab, is placed between the points a and b. (a) Figure 2: Two capacitors, C 4, are arranged with a single capacitor, C 8, in between electrodes a and b. (b) v. If C 8 is rated for a voltage V, while the C 4 capacitors are rated at 5V each, what is the maximum voltage that can be applied between points a and b without generating a breakdown in any of the capacitors? Figure 3: Capacitors C 1, C 2, C 3, and C 6 are arranged in between electrodes a and b. v. Suppose C 1 is disconnected. A. Does the energy in C 6 increase or decrease? Apart from your quantitative result, explain the result using a qualitative argument as well. 1

2 (c) B. Does the energy of the entire circuit increase or decrease? Apart from your quantitative result, explain the result using a qualitative argument as well. Figure 4: Capacitors C 2, C 3, C 4, and C 6 are arranged in between electrodes a and b. v. If only C 6 may be replaced with another capacitor, what is the largest possible amount of energy that the circuit can store with the potential, V ab, provided? 3. A 5.0-µF capacitor is charged to 50 V, and a 2.0-µF capacitor is charged to 100 V. The two are disconnected from charging batteries and connected in parallel, with the positive plate of one attached to the positive plate of the other. (a) What is the common voltage across each capacitor after they are connected in this way? (b) Compare the total electrostatic energy before and after the capacitors are connected. Speculate on the discrepancy. (c) Repeat Parts (a) and (b) with the charged capacitors being connected with the positive plate of one attached to the negative plate of the other. 4. When a switch is open, there can be a potential difference across it. (Indeed, an open switch effectively acts like a capacitor; although, the capacitance is quite small, particularly because of the relatively small area of the electrodes.) When a switch is closed, charges flow until the two ends of the switch are at the same potential (with the conducting system reaching electrostatic equilibrium). In the circuit shown in Figure 5, point a is grounded and point b is at a voltage of V b = +6V. Also, C 1 = C and C 2 = 2C, and the switch is in between points c and d. Figure 5: Capacitors C 1 and C 2 are arranged in between two electrodes: a that is grounded, and b that is held at a higher potential. A switch exists between points c and d which is initially open, but then is closed. (a) Determine the potentials, with respect to ground, of points c and d when the switch is open. (b) Determine the potentials, with respect to ground, of points c and d when the switch is closed. (c) How much charge actually flowed through the switch after it was closed? 5. Consider the following questions, which are to be explained qualitatively. (a) Although the electric field inside a conductor in electrostatic equilibrium is zero, why is this not the case inside a conductor that is carrying a current? (b) When a light switch is turned on, the light comes on right away by, for example, having electrons pass through the resistive filament causing the filament to glow. However, the electrons in such conductors typically move with a drift speed that is commonly in the range of 10 4 m/s = 0.1 mm/s. Explain this apparent disparity. 2

3 (c) A constant electric field inside conducting wires produces a constant average velocity of the charge carriers which we call the drift velocity. How is this consistent with Newton s 2 nd Law of Motion? 6. Consider a marching band consisting of a uniform formation of identical band members (each of mass m) separated from nearest neighbors in some square lattice arrangement with some separation l and moving at a uniform velocity, v. As viewed from a helicopter, the band consists of a very large number of columns of band members (with a column, for example, being denoted as a line of band members perpendicular to the band s movement), but consists of η rows (with a row, for example, being denoted as a line of band members parallel to the band s movement). Consider a reference line, perpendicular to the band s velocity, through which the band members will cross. (a) Considering an arbitrary time over which the counting of band members is administered as they cross the reference line, obtain a formula for the rate at which band members cross this reference line. (b) Again, considering an arbitrary time over which the counting of band members is administered as they cross the reference line, obtain a formula for the rate of mass flow across this reference line. (c) For the value of m = 60 kg, l = 4 m, v = 2 m/s, and η = 5, verify the results of Parts (a) and (b) by brute-force counting. (d) How can this exercise be likened to the flow of charge in a conducting wire? Discuss. (e) Why can the concept of current be thought as a concept of flux, and of what is the current the flux? 7. Copper is the most common metal used in conducting wires (e.g., to run the electricity in residential and commercial buildings). It second only to silver in its conductivity, it is quite malleable, it is relatively cheap, and, if kept away from moisture, it would tend not to rust. The concentration of conduction electrons in copper is m 3. (a) Show that the value for this concentration is quite consistent with the facts that copper has a mass density of about 8.9 g/cm 3 and that its atomic mass is g/mol. (b) How many conduction electrons does each atom of copper contain? 8. As in Figure 6, consider two cylindrical conductors A and B connected end-to-end, thus effectively forming one cylindrical conductor. The other ends are connected to a battery, of voltage V. The conductors have the same cross-section, but A is twice as long and has three times the resistivity of B. Figure 6: Two cylindrical conductors, A and B, fused end-to-end, with a current running through the cylinders supplied by a battery, of voltage V, to which the cylinders are connected on either of the non-fused ends. (a) Determine the following. i. The ratio of the current running through A to the current running through B: I A /I B. ii. The ratio of the resistance of A to the resistance of B: R A /R B. iii. The ratio of the potential difference across A to the potential difference across B: V A / V B. iv. The ratio of the electric field in A to the electric field in B: E A /E B. v. The ratio of the power dissipated through A to the power dissipated through B: P A /P B. (b) Determine V A and V B in terms of the battery voltage, V. (c) Determine the potential (relative to ground) at the junction between the two conductors. (d) Use Gauss s Law to determine the net charge density that accumulates at the boundary between the two conductors. (e) From the result in Part (d), how does the charge accumulation at the junction between the conductors explain the difference in electric field inside each conductor? 9. Electric devices are rated for the power they consume, assuming that they will be plugged into a standard American outlet delivering a Root-Mean-Square (or RMS) voltage of about 115 V. (The RMS value arises because the voltage being delivered to these outlets is actually alternating.) (a) How much current flows through a typical 60-W incandescent bulb when it is on? 3

4 (b) If we assume that the cord delivering the current to the bulb is about 3 m long, with a 1-mm diameter, and made of copper (ρ = Ω m), determine the power being consumed by the cord. (c) Based on the result to Part (b), is it a good approximation to ignore the connecting wires in a circuit, or is it quite relevant to consider the resistive effects of such wires? Explain. (d) Compare a 40-W bulb to a 60-W one. Which has more resistance? (e) If one was to connect a 40-W bulb in series with a 60-W bulb, and then made the free ends of those bulbs connect to the opposite terminals of an American wall outlet, which one would be brighter? Explain your reasoning. 10. Consider the following questions, which are to be explained qualitatively. (a) Consider decorative lights that one may put in the front yard of one s house to celebrate some holiday. Would it be wise to use wiring such that the lights are arranged in parallel strands or is it better to have all of the lights arranged in series? Explain. (b) Conceptually, discuss why resistors in series add while resistors in parallel add inversely. (c) From what conservation laws do Kirchhoff s Rules derive and why? 11. Four identical light bulbs A, B, C, and D are connected to an ideal battery, as shown in Figure 7. Figure 7: An arrangement of identical (incandescent) light bulbs placed in between the terminals of an ideal battery. (a) First of all, what is meant by an ideal battery? (b) Determine the relative brightness of each bulb. (c) Suppose A is unscrewed from its socket. i. How would the brightness of the remaining bulbs compare with their brightness when A was not unscrewed? ii. What would be the relative brightness of these remaining bulbs? 12. The circuits in Figure 8 consist of combinations of resistors, some of which also contain one battery, or multiple batteries. The circuits for which no battery is shown contain electrodes at points a and b, across which a battery may be connected. All resistors are the same, and so are all the batteries; the subscripts are merely shown to be able to distinguish one element from the other. (a) (b) (c) (d) (e) (f) Figure 8: Resistor circuits with varying combinations. All resistors are identical (R j = R) and all batteries are identical (V j = V ). 4

5 (a) For Figures 8a - 8f, determine (i) which resistors are in series, (ii) which resistors are in parallel, (iii) the equivalent resistance of the circuit, and (iv) the most powerful resistor, if any. (b) Reconsider the answers to Part (a) specifically for Figures 8d, 8e, and 8f if one of the resistors (R 4 for instance) was different (i.e., R j = R for j 4, and R 4 = R ). 13. A non-ideal battery, of voltage V and internal resistance r, is connected to a potentiometer, which has a variable resistance, R. (a) Derive an expression for the power dissipated by each resistor in terms of the given quantities. (b) Determine the value of the potentiometer, R max, that maximizes the power output to the external resistor. (c) When R = R max, how much power is being dissipated by the battery? Explain why it would not be a practical endeavor to put a battery through such a situation. (Even though this is not a good predicament in which to put a battery, such a technique is often used to match loads in many electronic devices (e.g., amplifiers and speakers).) (d) Discuss how the power generated by the battery s voltage output is divided between its internal resistance and the external resistance for the cases where R > r and also for R < r. 14. The circuit shown in Figure 9 is designed in such a way so that it makes absolutely no difference if the switch is closed or open in terms of, for example, the current running through each of the resistors. In this circuit, V 1 = V, V 2 = 5V, R 1 = R, and R 2 = 2R. Figure 9: A resistor circuit with three separate branches, with each branch containing a battery. The voltage of battery V 3 is unknown, but it is known that leaving the switch open or closed has no bearing on the current running through the branches with resistors. (a) What must be the voltage across V 3 in order to satisfy the conditions described in this circuit? (b) Is there a potential difference across the switch when it is open? If there is, then what is it? 15. Consider the circuit in Figure 10, which consists of three batteries V 1 = 6.0 V, V 2 = 1.5 V, and V 3 = 4.5 V and four resistors R 1 = 270 Ω, R 2 = 560 Ω, R 3 = 820 Ω, and R 4 = 150 Ω. Figure 10: A resistor circuit with three separate branches, with each branch containing a battery. (a) Find the current in R 2 and give its direction. (b) Suppose that V 2 is replaced with another battery that makes the current in R 2 run upwards. Determine the minimum value of the replacement battery that would satisfy this requirement. 5

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