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Chapter 29 Emf and Cir cuits 第第第第第 第第第第第第

Chapter 29 Emf and Circuits

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Chapter 29 Emf and Circuits. 第二十九章 電動勢與電路. Emf devices. An emf device is a charge pump that can maintain a potential difference between a pair of terminals. - PowerPoint PPT Presentation

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Page 1: Chapter 29 Emf and Circuits

Chapter 29 Emf and Circuits

第二十九章 電動勢與電路

Page 2: Chapter 29 Emf and Circuits

Emf devices

An emf device is a charge pump that can maintain a potential difference between a pair of terminals.

Emf devices include battery, electric generator, solar cell, fuel cell, and thermopile. Physiological emf devices include electric eel, human being, and some plants.

Page 3: Chapter 29 Emf and Circuits

Work, energy, and emf

The emf of an emf device is defined to be the work per unit charge that the device does in moving charge from its low-potential terminal (-) to its high-potential terminal (+).

dW

dqE

Page 4: Chapter 29 Emf and Circuits

An ideal emf device versus a real emf device

Page 5: Chapter 29 Emf and Circuits

Resistance in series

1 2 1 2( )V IR IR I R R

Page 6: Chapter 29 Emf and Circuits

PowerPower of the emf device is:

emfP i E

Power transferred out is:

( ) emf rP I Ir P P E

Ir R

E

2rP I r

2( )

rP

r R r R

2 2E E

Page 7: Chapter 29 Emf and Circuits

Sample problem 1The emfs and resistances in the circuit have the following values: 1 = 4.4 V, 2 = 2.1 V, r1 = 2.3 , r

2 = 1.8 , R = 5.5 . (a) What is the current i in the circuit? (b) What is the potential difference between the terminals of battery 1?

Ans: (a) i = 240 mA; (b) 3.8 V

Page 8: Chapter 29 Emf and Circuits

Multiloop circuits

Page 9: Chapter 29 Emf and Circuits

Kirchhoff rulesJunction rule: The sum of the currents entering any junction in a circuit must equal to the sum of currents leaving that junction.

Loop rule: The sum of the potential differences across all elements around any closed loop must be zero.

Page 10: Chapter 29 Emf and Circuits

Resistance in parallel

Page 11: Chapter 29 Emf and Circuits

Sample problem 2The elements in the circuit have the following values: = 12 V, R1 = 20 , R2 = 20 , R3 = 30 , R4 = 8.0 . (a) What is the current through the battery? (b) What is the current through R2? (c) What is the current through R3?

Ans: (a) 0.30 A; (b) 0.18 A; (c) 0.12 A.

Page 12: Chapter 29 Emf and Circuits

Sample problem 3

The elements in the circuit have the following values: 1 = 3.0 V, 2 = 6.0 V, R1 = 2.0 , R2 = 4.0 . The three batteries are ideal batteries. Find the magnitude and direction of the current in each of the three batteries.

Ans: i1 = 0.50 A; i2 = -0.25 A; i3 = 0.25 A.

Page 13: Chapter 29 Emf and Circuits

Sample problem 4Electric fish are able to generate current with biological cells called electroplaques, which are physiological emf devices. The electroplaques in the South American eel shown in the photograph are arranged in 140 rows, each row stretching horizontally along the body and each containing 5000 electroplaques shown in following page. Each electroplaque has an emf = 0.15 V and an internal resistance r = 0.25 . The water surrounding the eel completes a circuit between the two ends of electroplaque arrays, one end at the animal’s head and the other near its tail.

Page 14: Chapter 29 Emf and Circuits

Sample problem 4 continue(a) If the water surrounding the eel has resistance Rw = 800 , how much current can the eel produce in the water? (b) How much current travels through each row of the eletroplaques?

Page 15: Chapter 29 Emf and Circuits

Galvanometer

Page 16: Chapter 29 Emf and Circuits

Ammeter

Page 17: Chapter 29 Emf and Circuits

Voltmeter

Page 18: Chapter 29 Emf and Circuits

The ammeter and the voltmeter

An ideal ammeter has zero resistance, and an ideal voltmeter has an infinite resistance.

Page 19: Chapter 29 Emf and Circuits

RC circuits

Page 20: Chapter 29 Emf and Circuits

Discharging a capacitor

Page 21: Chapter 29 Emf and Circuits

The RC time constant

0dq q

Rdt C

dqi

dt

/t RCq Qe

/t RCdq Qi edt RC

i

Page 22: Chapter 29 Emf and Circuits

Charging a capacitor

Page 23: Chapter 29 Emf and Circuits

Charging a capacitor

dq qRdt C

E

/(1 )t RCq C e E

Page 24: Chapter 29 Emf and Circuits

Charging a capacitor/(1 )t RCq C e E /t RCdq

i edt R

E

Page 25: Chapter 29 Emf and Circuits

Sample problem 5A capacitor of capacitance C is discharging through a resistor of resistance R. (a) In terms of the time constant = RC, when will the charge on the capacitor be half its initial value? (b) When will the energy stored in the capacitor be half its initial value?

Ans: (a) RCln2 = 0.69; (b) ½RCln2 = 0.35

Page 26: Chapter 29 Emf and Circuits

Sample problem 6

Page 27: Chapter 29 Emf and Circuits

Sample problem 7

Page 28: Chapter 29 Emf and Circuits

Sample problem 8

Page 29: Chapter 29 Emf and Circuits

Sample problem 9

Page 30: Chapter 29 Emf and Circuits

Sample problem 10

Page 31: Chapter 29 Emf and Circuits

Sample problem 11

Infinite resistor network

Page 32: Chapter 29 Emf and Circuits

Home work

Question ( 問題 ): 16, 19, 21

Exercise ( 練習題 ): 17, 20

Problem ( 習題 ): 18, 38, 42, 43, 44