Download presentation

Presentation is loading. Please wait.

Published byBrice Sam Modified over 3 years ago

1
Short Version : 25. Electric Circuits

2
Electric Circuit = collection of electrical components connected by conductors. Examples: Man-made circuits: flashlight, …, computers. Circuits in nature: nervous systems, …, atmospheric circuit (lightning).

3
25.1. Circuits, Symbols, & Electromotive Force Common circuit symbols All wires ~ perfect conductors V = const on wire Electromotive force (emf) = device that maintains fixed V across its terminals. E.g., batteries (chemical), generators (mechanical), photovoltaic cells (light), cell membranes (ions).

4
Ohm’s law: Energy gained by charge transversing battery = q E ( To be dissipated as heat in external R. ) g ~ E m ~ q Lifting ~ emf Collisions ~ resistance Ideal emf : no internal energy loss.

5
25.2. Series & Parallel Resistors Series resistors : I = same in every component For n resistors in series: Voltage divider Same q must go every element. n = 2 :

6
Real Batteries Model of real battery = ideal emf E in series with internal resistance R int. I means V drop I R int V terminal < E

7
Example 25.2. Starting a Car Your car has a 12-V battery with internal resistance 0.020 . When the starter motor is cranking, it draws 125 A. What’s the voltage across the battery terminals while starting? Voltage across battery terminals = Typical value for a good battery is 9 – 11 V. Battery terminals

8
Parallel Resistors Parallel resistors : V = same in every component For n resistors in parallel :

9
Analyzing Circuits Tactics: Replace each series & parallel part by their single component equivalence. Repeat.

10
Example 25.3. Series & Parallel Components Find the current through the 2- resistor in the circuit. Equivalent of parallel 2.0- & 4.0- resistors: Total current is Equivalent of series 1.0- , 1.33- & 3.0- resistors: Voltage across of parallel 2.0- & 4.0- resistors: Current through the 2- resistor:

11
25.3. Kirchhoff’s Laws & Multiloop Circuits Kirchhoff’s loop law: V = 0around any closed loop. ( energy is conserved ) This circuit can’t be analyzed using series and parallel combinations. Kirchhoff’s node law: I = 0at any node. ( charge is conserved )

12
Multiloop Circuits INTERPRET ■ Identify circuit loops and nodes. ■ Label the currents at each node, assigning a direction to each. Problem Solving Strategy: DEVELOP ■ Apply Kirchhoff ‘s node law to all but one nodes. ( I in > 0, I out < 0 ) ■ Apply Kirchhoff ‘s loop law all independent loops: Batteries: V > 0 going from to + terminal inside the battery. Resistors: V = I R going along +I. Some of the equations may be redundant.

13
Example 25.4. Multiloop Circuit Find the current in R 3 in the figure below. Node A: Loop 1: Loop 2:

14
Application: Cell Membrane Hodgkin-Huxley (1952) circuit model of cell membrane (Nobel prize, 1963): Electrochemical effects Resistance of cell membranes Membrane potential Time dependent effects

15
25.4. Electrical Measurements A voltmeter measures potential difference between its two terminals. Ideal voltmeter: no current drawn from circuit R m =

16
Example 25.5. Two Voltmeters You want to measure the voltage across the 40- resistor. What readings would an ideal voltmeter give? What readings would a voltmeter with a resistance of 1000 give? (b) (a)

17
Ammeters An ammeter measures the current flowing through itself. Ideal voltmeter: no voltage drop across it R m = 0

18
Ohmmeters & Multimeters An ohmmeter measures the resistance of a component. ( Done by an ammeter in series with a known voltage. ) Multimeter: combined volt-, am-, ohm- meter.

19
25.5. Capacitors in Circuits Voltage across a capacitor cannot change instantaneously.

20
The RC Circuit: Charging C initially uncharged V C = 0 Switch closes at t = 0. V R (t = 0) = E I (t = 0) = E / R C charging: V C V R I Charging stops when I = 0. V R but rate I but rate V C but rate

21
Time constant = RC V C ~ 2/3 E I ~ 1/3 E/R

22
The RC Circuit: Discharging C initially charged to V C = V 0 Switch closes at t = 0. V R = V C = V I 0 = V 0 / R C discharging: V C V R I Disharging stops when I = V = 0.

23
Example 25.6. Camera Flash A camera flash gets its energy from a 150- F capacitor & requires 170 V to fire. If the capacitor is charged by a 200-V source through an 18-k resistor, how long must the photographer wait between flashes? Assume the capacitor is fully charged at each flash.

24
RC Circuits: Long- & Short- Term Behavior For t << RC: V C const, C replaced by short circuit if uncharged. C replaced by battery if charged. For t >> RC: I C 0, C replaced by open circuit.

25
Example 25.7. Long & Short Times The capacitor in figure is initially uncharged. Find the current through R 1 (a) the instant the switch is closed and (b) a long time after the switch is closed. (a) (b)

Similar presentations

Presentation is loading. Please wait....

OK

Fundamentals of Circuits: Direct Current (DC)

Fundamentals of Circuits: Direct Current (DC)

© 2019 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google