lecture13

# lecture13 - 1 2 3(a) All circuits have the same...

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Unformatted text preview: 1 2 3 (a) All circuits have the same equivalent resistance: In all cases, R1 and R2 are in parallel, and the equivalent resistor R12=1/(1/R1+1/R2)=R1R2/(R1+R2) is in series with R3, so R123=R3+R1R2/(R1+R2) (b) If all resistors are the same, we use P=i^2R, so the one with largest current will dissipate more power: that is R3. The current through R1 and R2 is only half, so each of them will dissipate ¼ the power dissipated by tr3. (c) If the current through the baJery and R3 is I, the voltage across R3 is V3=iR. Since only half the current goes through each of R1 and R2, then V1=V2=iR/2. Thus, R1 and R2 have the smallest potenMal diﬀerence across them. 4 (a) The current will ﬂow from A to B through the baJeries (counterclockwise). (b) E1 is doing posiMve work, E2 is doing negaMve work (as in a car baJery geSng charged) (c) Voltage diﬀerence between A and B is the voltage diﬀerence of the baJeries, +3V (B is at a higher potenMal). (d) Current is 3Volts/3Ohms=1Amp, so power dissipated by resistor is P=i^2 R = 3 WaJs. (e) E1 delivers P=iV=12 WaJs, while E2 absorbs P=iV= 9 WaJs. 5 (a) There are three paths for current, or three resistors in parallel;; top and boJom paths have two resistors in series. The three parallel paths have: 10 Ohms on top path, 5 Ohms in middle path, 10 Ohms in boJom path; equivalent resistance is then 1/ (1/10+1/5+1/10)=1/(4/10)=10/4=2.5 Ohms. (b) We ﬁrst replace top resistors in series with the 10 Ohms equivalent; this 10Ohms is in parallel with 5 Ohms, resulMng in 3/10 Ohms. That one is now in series with 5 Ohms between H and G, resulMng in 5.3 Ohms, now in parallel with 5 Ohms between F and G. The total equivalent resistance is then 1/(1/5.3+1/5)=2.6 Ohms. 6 Power through a light bulb is P=i^2R. In X, with i=V/R. In (i), i=E/2R: dimmer. In (ii), i=2E/2R=E/R: same as in X. In (iii), i=2E/3R: dimmer. In (iv), current through the baJery is i=E/(R/2)=2E/R, but current through each light bulb is E/R; same as in X. In (v), i=E/(2R): dimmer. So, (ii) and (iv) are as bright as X. 7 8 9 (a)  Time constants are 2RC, RC/2, RC, so Capacitor in (1) takes the longest. (b)  Q=CV, and in all cases, V=E at the ﬁnal equilibrium. (c)  BaJeries in (1) and (2) do not deliver any current when the capacitor is charged; BaJery in (3) will deliver a current i=E/R. (d)  Capacitor will stay charged in (1) and (2); charge in capacitor (3) will start a current, and will get discharged with a Mme constant 2RC. 10 (a)  At t=0, there is no voltage across the capacitor, so R2 and R3 are in parallel, with a total equivalent resistance equal to 1.5 MOhm, and the baJery delivers a total current i=1kV/1.5MOhm= 0.67 mA.: we have 0.67 mA through R1, and half that (0.33 mA) through each of R2 and R3. (b)  Aeer a long Mme, the capacitor is charged, no current ﬂows through R3, and R2 and R3 are in series, with a total equivalent resistance equal to 2 MOhm, and a baJery delivered by the baJery of 1kV/2MOhm=0.5 mA: this current ﬂows through R1 and R2. (c)  The Mme constant for the capacitor geSng charged is 1MOhm x 10 uF = 10 sec : we need to wait tens of seconds. 11 ...
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