Single resistor Circuit. Chapter 21: Electric Circuits. Circuit with more than one Resistors. Think, Answer; Talk to your neighbors, Answer

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1 Chapter 21: Electric Circuits A simple reflex circuit Stimulation of peripheral sensors (a muscle stretch receptor in this case) initiates receptor potentials that trigger action potentials that generate current in the neural circuit. Single resistor Circuit Ground pot = 0 Connecting wire. Same voltage everywhere. Voltage drops linearly from upper to lower part of the resistor. V Vt Vb Same as battery voltage. Once the charge reaches the low potential, battery does work to increase its potential. Ohm s law relates current, voltage and resistance as, V V emf For ideal battery, Circuit with more than one esistors V 1 V 2 Take the case where 1 = 5 Ω, 2 = 10 Ω, and 3 = 20 Ω f i 1 = 5.0 A, what are the other currents? i 1 i 2 i 3 Series: Current is same. eq eq Parallel: Potential difference is same eq eq A) i2 = 10 A, i3 = 20 A B) i2 = 2.5 A, i3 = 1.25 A C) i2 = 1.25 A, i3 = 0.30 A D) i2 = 0.0 A, i3 = 0.0 A E) None of the above answers. What is equivalent resistor? What is the potential difference across 1 and 2? A 1 = 6.0 f 8.0 attery is connected between A and B, what is the. V AB B 9.0V 2 = 12 What is current through each resistors. 1

2 What is the current flowing through 3? The switch was open, but then we closed the switch. What happened to the current through the fuse? Fuse A: 1.5 A B: 2 A C: 3 A D: 4 A E: 6 A A: t doubled B: t halved C: t stayed the same D: Something else! 120 V This switch was open, but is now closed! (Comparing AFTE to BEFOE the switch was closed) A 276 V emf is connected to the terminals A and B. What is the current in 12? What is the equivalent resistance? A 1 = 15 2 = 12 3 = 24 B A second look at circuit with different look a b c 1 3 = a d c 3 d 2 f 1 = 1, 2 = 4, 3 = 4 = 2 ohms, what is the equivalent resistor? 4 b Kirchhoff s ules 2. Loop ule: 1. Junction rule: Conservation of charge gives Conservation of energy gives V 0 n general, V1 1 V1 a Vab Vbc in out Vc2 V2 d Vd 1 in out 0 V

3 Sign of V s in Loop rule Current is flowing through wires which are part of a circuit, as shown. What is the current in the wire at the bottom right? A) 1 A to the left B) 1 A to the right C) 3 A to the left D) 3 A to the right E) none of these/not sure V ve V ve Solve for using loop rule. mportant: Both loop and current has arrow. Tail of the current arrow means high and tip means low potentials. As you walk past the two circuit elements shown, moving FOM a TO b, what is ΔV in each situation? What is the correct voltage equation for loop 1? 1 Situation #1 #2 A) +V, + B) -V, + C) +V, - D) -V, - V Loop 1. A) V =0 B) V =0 C) V =0 D) V =0 V2 Find each currents. A real battery r 1 = 5 3 V1 = 10 V = 50 V2 = 25 V 2r nternal resistance, r. 3

OFF, B is brighter D: Equal Capacitors in a circuit Measuring Currents and Voltages V 1 2 3 Ceq C1 C2 C3 A 1 1 1 A 2 2 2 A 3 1 1 1 1 C C C C eq 2 Current flows through the wires, ammeters are placed

4 A flashlight requires 2 AA (1.5V) batteries, and is arranged as shown. The bulb Which bulb is brighter? Both bulbs A and B are similar. A: has 1.5 V across it, & glows B: has 3 V across it, & glows C: has 3 V across it, & is dark D: has 0 V across it, & is dark E: has 0 V across it, &glows A: A is brighter B: A is ON, B is brighter C: A is OFF, B is brighter D: Equal Capacitors in a circuit Measuring Currents and Voltages V Ceq C1 C2 C3 A A A C C C C eq 2 Current flows through the wires, ammeters are placed in series and has low resistance. Potential difference is between two points so voltmeter stays parallel and has very high. Discharging Capacitors Charging and discharging a Capacitor Charging Discharging (briefly) C C Pay attention on time it takes to go from say 4 o 3V vs. 3o 2V. Second case: Three bulbs are connected in parallel to each other. Current decreases but voltage increases and becomes V. t 0 e. Both Current and voltage decrease and becomes 0. t where C, is called time constant. t VC t 1 e 0 maximum current. 4

5 Current and Voltage: exponential function Problem: What is time constant of this C circuit? (a) f it stores 2 µc charge when it is fully charged, what is the voltage across it? (b)what is the voltage across it after 2 ms? (C) At what time voltage becomes a quarter of maximum voltage? C = 10 nf =2 k When, t, ( t) where, e or, t e 0 When t =, V (1 1/ e) c V c 5

= (0.400 A) (4.80 V) = 1.92 W = (0.400 A) (7.20 V) = 2.88 W

= (0.400 A) (4.80 V) = 1.92 W = (0.400 A) (7.20 V) = 2.88 W Physics 2220 Module 06 Homework 0. What are the magnitude and direction of the current in the 8 Ω resister in the figure? Assume the current is moving clockwise. Then use Kirchhoff's second rule: 3.00