Applied Natural Sciences

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1 Het basisvak Toegepaste Natuurwetenschappen Applied Natural Sciences Leo Pel e mail: 3nab0@tue.nl

2 Bijles

3 Content of the course 3NAB0 (see study guide) 17 0 November diagnostic test! Week 1 : 13 November Week : 0 November Introduction, units (Ch1), Circuits (Ch5,6) Heat (Ch17), Kinematics (Ch 3) Week 3: 7 November Newton, Energy (Ch4 6) Week 4: 4 December Energy, Momentum (Ch7 8) 7 December Intermediate assessment Week 5: 11 December Week 6: 18 December Week 7: 8 January (016) otation, Elasticity, Fluid mechanics (Ch9 1) Harmonic oscillator and Waves (Ch14 15) Sound (Ch16) Light (Ch33) 4 January Final assessment

4 Chapter 5 Current, esistance, and Electromotive Force PowerPoint Lectures for University Physics, Thirteenth Edition Hugh D. Young and oger A. Freedman Lectures by Wayne Anderson Copyright 01 Pearson Education Inc.

5 Introduction Electric currents flow through light bulbs. Electric circuits contain charges in motion. Circuits are at the heart of modern devices such as computers, televisions, and industrial power systems. 5

6 LEANING GOALS The meaning of electric current, and how charges move in a conductor. What is meant by the resistivity and conductivity of a substance. How to calculate the resistance of a conductor from its dimensions and its resistivity. How an electromotive force (emf) makes it possible for current to flow in a circuit. How to do calculations involving energy and power in circuits. 6

7 esistance Ohm s law V= I V = E x L Georg Simon Ohm I= V/ 10

8 The current flow 10 6 m/s electron motion velocity I = dq/dt 10-4 m/s Drift velocity Electrical current (I) in amperes is defined as the rate of electric charge flow in coulombs per second. 1 ampere (A) of current is a rate of charge flow of 1 coulomb/second 11

9 Mechanical analogue 1

10 esistance Ohm s law V= I esistance length L inverse surface A resistivity L A 13

11 esistivity is intrinsic to a metal sample (like density is) L A 14

12 Nerves Myelin Axon 15

13 Ohm law There is a blank wire: voltage 1 V current 100 A If I touch it? 1. I die.. No problem, I will not feel anything. Answer:. V=I. 1V, body 1 M Ω, hence current to small to feel 16

14 Water A bucket filled with pure water is in contact with 0V. If I put my hand in the water? 1. I die.. No problem, I will not feel anything. Answer: No problem, conductivity of pure water is very low 17

15 18

16 esistivity and Temperature 19

17 esistivity and Temperature 0

18 1

19 esistor Color Codes Example: Colors (left to right) red, yellow, green, and gold Using our table, we can see that the resistance is =.4 M with a tolerance of 5%

20 Ohm s law an idealized model Linear Nonlinear I 1 V Slope 1 Ohm s law applies V I Logic elements: computer chips 3

Electromotive force and circuits If an electric field is produced in a conductor without a complete circuit, current flows for only a very short

21 Electromotive force and circuits If an electric field is produced in a conductor without a complete circuit, current flows for only a very short time. An external source is needed to produce a net electric field in a conductor. This source is an electromotive force, emf, ee em eff, (1V = 1 J/C) 4

22 Electromotive force and circuits An electromotive force (emf) makes current flow. An emf is not a force. (elektromotorische kracht EMK) 5

Electromotive force and circuits Any device which increases the potential energy of charges which flow through it is called source of emf, SI unit for emf : Volt (V) The emf may originate from a

23 Electromotive force and circuits Any device which increases the potential energy of charges which flow through it is called source of emf, SI unit for emf : Volt (V) The emf may originate from a chemical reaction as in a battery or from mechanical motion such as in a generator. A battery is a device that uses chemical reactions to produce a potential difference between its two terminals. 6

24 Electromotive Force and Circuits Ideal Source Voltage rise in current direction Ideal source of electrical energy EMF + I + V Complete path needed for current (I) to flow Voltage drop in current direction V = EMF = I V I EMF eal Source rs I a Internal source resistance eal source of electrical energy + EMF + V ab b V External resistance ab EMF Ir s I 7

25 Internal resistance eal sources of emf actually contain some internal resistance r. The terminal voltage of an emf source is V ab = Ir. The terminal voltage of the 1 V battery shown at the right is less than 1 V when it is connected to the light bulb. 8

26 Symbols for circuit diagrams 9

27 A source in an open circuit What is the open source voltage? 1. 1 V.. 1V- V=10V 3. depends on current Answer: V ab EMF Ir 1V 0r 1V 30

28 A Source with a Short Circuit V ab 1V Ir I I 0 0 I 6 A r Ir 0 Ir V ab 0 31

29 A source in a complete circuit V ab Ir I I Ir I( r) I 1 A r 4 V ab Ir 1 () 8V Vab Va' b' I (4) 8V 3

30 Using voltmeters and ammeters What do Ammeters and Voltmeters do to circuits? Ammeters measure flow of current PAST a point. Ideally, they should NOT influence the current Ideally, (ammeter) = 0! Put them IN SEIES with circuit legs 33

31 Using voltmeters and ammeters What do Ammeters and Voltmeters do to circuits? Voltmeters measure potential difference across (or between) points in the circuit. Ideally, they should NOT influence the current Ideally, (voltmeter) =! Put them in parallel! 34

32 A source in a complete circuit 35

33 Potential ises and Drops in a Circuit V ab Ir I Ir I 0 The net change in potential must be zero for a round trip in a circuit. 36

34 Energy and power in electric circuits The rate at which energy is delivered to (or extracted from) a circuit element is P = V ab I. The power delivered to a pure resistor is P = I = V ab /. (V ab =I ) 38

35 Power Output of an EMF Source EMF + rs + I a + V ab b V P ab ab V EMF ab I Ir s I ( EMF Ir ) I ( EMF) I I r s s I ( EMF) I I r s I Power output of battery Power dissipated in Power dissipated in battery resistance Power supplied by the battery 39

36 Power matching 40

37 Power matching 41

38 Power matching amplifier r V loudspeaker Power loudspeaker P i V i r P V ( r ) Question: For which is P max? Take derivative 4

39 ( ) r V P 0 ) ( r V d d d dp Max if derivative is zero 0 ) ( ) ( ) ( 3 r V r V 0 ) ( ) ( ) ( 3 3 r V r r V 0 ) ( ) ( 3 r r V r Match amplifier and loudspeaker 43 Power matching

40 Power transmission ~ 1500 km 580 km 44

41 Power transmission Assume power needed 700 MW Average household KWh 700 MW > 700 x 4 x 365 = KWh households 45

42 Power transmission Actual cable = 580 Km and 760 mm Diameter = 30 mm Be aware: total kg is at 5.17 /kg = 40 M 46

43 Power transmission What is the efficiency = P loss / P supply? L A ρ= Ωm at 50 o C Cable ~ 9 Ω Power PVI P loss I P I V P V 47

44 Power transmission P loss I P loss 9 P V Take 0V P loss = MW P loss / P input = >100% V High voltage 48

45 Power transmission High voltage 10 kv, 100 kv, 350 kv, 900 kv kv Modern 900 kv 49

46 Power transmission P loss I P loss 9 P V 0V P loss = MW P loss / P input = >100% 10 kv P loss = MW P loss / P input = >100% 100 kv P loss = MW P loss / P input = >100% 350 kv P loss = 116 MW P loss / P input = 16 % 900 kv P loss = 17.5 MW P loss / P input = % 1500 kv P loss = 6 MW P loss / P input = 0.8 % V Iceland in reach 50

47 Summary 51

Chapter 25 Current, Resistance, and Electromotive Force

Chapter 25 Current, Resistance, and Electromotive Force Lecture by Dr. Hebin Li Goals for Chapter 25 To understand current and how charges move in a conductor To understand resistivity and conductivity