Physics 202: Lecture 5, Pg 1

Transcription

1 Resistance Resistors Series Parallel Ohm s law Electric Circuits Current Physics 132: Lecture e 15 Elements of Physics II Kirchhoff s laws Agenda for Today Physics 202: Lecture 5, Pg 1

2 Electric Current How does a capacitor get discharged? d? Figure (a) shows a charged capacitor in equilibrium. Figure (b) shows a wire discharging the capacitor. As the capacitor is discharging, there is a current in the wire. Physics 202: Lecture 5, Pg 2

3 Charge Carriers The outer electrons of metal atoms are only weakly bound to the nuclei. In a metal, the outer electrons become detached from their parent nuclei to form a fluid-like sea of electrons that can move through the solid. Electrons are the charge carriers in metals. Physics 202: Lecture 5, Pg 3

4 Within a conductor in electrostatic equilibrium, there is no electric field. In this case, an electron bounces back and forth between collisions, but its average velocity is zero. A Model of Conduction Physics 202: Lecture 5, Pg 4

5 In the presence of an electric field, the electric force causes electrons to move along parabolic trajectories between collisions. Because of the curvature of the trajectories, t there is a slow net motion in the downhill direction. A Model of Conduction Physics 202: Lecture 5, Pg 5

6 Disks = electrons Plinko Disk bounces through atoms on it s way down the incline Angle of incline = emf (potential difference) Youtube demo Physics 202: Lecture 5, Pg 6

7 Discharging a Capacitor How long should it take to discharge this capacitor? A typical drift speed of electron current through h a wire is v d 10 4 m/s. At this rate, it would take an electron about 2000 s (over half an hour) to travel 20 cm. Physics 202: Lecture 5, Pg 7

8 Discharging a Capacitor The wire is already full of electrons! We don t have to wait for electrons to move all the way through the wire from one plate to another. We just need to slightly rearrange the charges on the plates and in the wire. Physics 202: Lecture 5, Pg 8

9 Creating a Current A book on a table will slow down and stop unless you continue pushing. Analogously, the sea of electrons will slow down and stop unless you continue pushing with an electric field. Physics 202: Lecture 5, Pg 9

10 Electromotive force Electromotive force (emf) is the maximum potential difference a battery or power source can give a circuit. A car battery has an emf of 12 V Sometimes shown as a script (E) Symbol for emf - + Physics 202: Lecture 5, Pg 10

11 Electrical Current When we connect the two terminals of an emf to a circuit ( a continuous conducting path), charge will move continuously through the circuit. Since there is a potential difference electrons in circuit will feel a force emf Electrons will flow in direction of positive terminal Battery pushes electrons Physics 202: Lecture 5, Pg 11

12 Electrical Current This flow is called electric current Net amount of charge through a point in the circuit per unit time. Units Ampere = Coulomb/second I Q Q t Physics 202: Lecture 5, Pg 12

13 Electric Current The direction of current flow from the positive terminal to the negative one was decided before it was realized that electrons are negatively charged. Therefore, current flows around a circuit in the direction a positive charge would move; electrons move the other way. However, this does not matter in most circuits. Physics 202: Lecture 5, Pg 13

14 Establishing the Electric Field in a Wire The figure shows two metal wires attached to the plates of a charged capacitor. This is an electrostatic situation. What will happen if we connect the bottom ends of the wires together? Physics 202: Lecture 5, Pg 14

15 Establishing the Electric Field in a Wire Establishing the Electric Field in a Wire Within a very brief interval of time ( 10 9 s) of connecting the wires, the sea of electrons shifts slightly. The surface charge is rearranged into a nonuniform distribution, as shown in the figure Physics 202: Lecture 5, Pg 15

16 Establishing the Electric Field in a Wire The nonuniform distribution of surface charges along a wire creates a net electric field inside the wire that points from the more positive end toward the more negative end of the wire. This is the internal electric field that pushes the electron current through the wire. Physics 202: Lecture 5, Pg 16

17 Give it a try: Surface charge is distributed on a wire as shown. Electrons in the wire A. Drift to the right. B. Drift to the left. C. Move upward. D. Move downward. E. On average, remain at rest. Physics 202: Lecture 5, Pg 17

18 Resistance Resistance: Traveling through a conductor, electrons bump into things which h slows them down. : Resistivity: Density of bumps L: Length of conductor A: Cross sectional area of conductor R ρ L A All electrical devices will resist the flow of electrical current L A Physics 202: Lecture 5, Pg 18

19 R ρ L A Plinko Physics 202: Lecture 5, Pg 19

20 Ohm s law Rule for current flow in most conductors: Higher resistance leads to lower current Higher potential difference leads to higher current Ohm s law V IR Higher resistance lower current Units are Ohms ( ) I V R Physics 202: Lecture 5, Pg 20

21 Ohm s law Ohm s law V I R V IR Physics 202: Lecture 5, Pg 21

22 Resistor A device that resists current Used to control o current flow in a circuitcu Resistor in a circuit: Generally in a circuit the resistance of the connecting wires are very small and we can neglect them. Wires will resist current, but we will assume they have zero resistance!! Physics 202: Lecture 5, Pg 22

23 Battery-Wire-Resistor-Wire Circuit The figure shows a resistor connected to a battery with current- carrying wires. Current must be conserved; hence the current I through the resistor is the same as the current in each wire. Physics 202: Lecture 5, Pg 23

24 Kirchhoff s Junction Law For a junction, the law of conservation of current requires that where the symbol means summation. This basic conservation statement is called Kirchhoff s junction law. Physics 202: Lecture 5, Pg 24

25 Give it a try: The current in the fourth wire is A. 16 A to the right. B. 4 A to the left. C. 2 A to the right. D. 2 A to the left. E. Not enough information to tell. Physics 202: Lecture 5, Pg 25