1. Lesson 18 Electric Current
Eleanor Roosevelt High School
Chin-Sung Lin

2. Flow of Charge and Electric Circuits

3. Water Flow Model High Potential Energy Water Water Pipe Height
Water Doing Work
Pump Doing Work
Low Potential Energy Water

4. Water Flow vs. Electric Current
High Potential Energy Water
Water
Pipe
Height
Water Doing Work
Pump Doing Work
Low Potential Energy Water

5. Electric Current Model
High Voltage
Electric
Circuits
Voltage
Electric Current
Doing Work
Battery
Doing Work
Low Voltage

6. Water / Electric Current Model

7. Electric Current Model
High Voltage
Electric
Circuits
Voltage
Electric Current
Doing Work
Battery
Doing Work
Low Voltage

8. Electric Current Model
High Voltage
Electric
Circuits
Electric Current
Doing Work
Battery
Doing Work
Voltage
Low Voltage

9. Light Bulb Structure

10. Electric Circuit Symbols
Wire
Resistor
Battery
Voltage

11. Electric Circuits
Wire
Battery
Resistor

12. Electric Circuits

13. Ohm’s Law

14. Electric Current
Current (I)
Voltage (V)
Resistance (R)

15. Electric Current
I (amperes , A)
V (volts, V)
R (ohms, Ω)

16. Ohm’s Law V I R

17. Ohm’s Law
V: Voltage (V)
I : Current (A)
R: Resistance (Ω)

18. Voltmeter – Measuring Voltage

19. Ammeter – Measuring Current

20. Multimeter – Measuring All
V: Voltage (V)
I : Current (A)
R: Resistance (Ω)

21. Digital Multimeter – Measuring All
V: Voltage (V)
I : Current (A)
R: Resistance (Ω)

22. Digital Multimeter – Measuring All

23. Digital Multimeter – Measuring All
Voltage /
Resistance
Current
Ground

24. Digital Multimeter – Measuring All

25. Digital Multimeter – Measuring All
Resistance
DC Voltage
AC Voltage
DC Current
AC Current
Transistor

26. Ohm’s Law A
V (V)
I (A)
R (Ω) V

27. Ohm’s Law

28. voltage = constant x current V = R x I
Ohm’s Law
voltage = constant x current
V = R x I
V: Voltage (V)
I : Current (A)
R: Constant or Resistance (Ω)

29. Ohm’s Law R = or V = I R or I = V R I V: Voltage (V) I : Current (A)
R: Resistance (Ω)

30. Ohm’s Law Example
What is the current through a 4-ohm resistor connected to a 12-V power supply?

31. Ohm’s Law Example
What is the current through a 4-ohm resistor connected to a 12-V power supply?
I = V / R
I = 12 V / 4 Ω = 3 A

32. Ohm’s Law Example
A 5-ohm resistor has a 8-A current in it. What is the voltage across the resistor?

33. Ohm’s Law Example
A 5-ohm resistor has a 8-A current in it. What is the voltage across the resistor?
V = I R
V = 8 A x 5 Ω = 40 V

34. Ohm’s Law Example
What is the resistance of a device that uses 2.5 A of current when connected to a 15-volt power source?

35. Ohm’s Law Example
What is the resistance of a device that uses 2.5 A of current when connected to a 15-volt power source?
R = V / I
R = 15 V / 2.5 A = 6 Ω

36. Ohm’s Law Exercises
What is the current through a 6-KΩ resistor connected to a 9-V power supply?
What is the resistance of a device that uses 0.5 mA of current when connected to a 5-volt power source?
A 2.5-MΩ resistor has a 0.02-mA current in it. What is the voltage across the resistor?

37. Resistors

38. Resistors
A resistor is a two-terminal electronic component which implements electrical resistance
Resistors are used to control the amount of current flowing in a circuit
Resistor are usually measured by the units of Ω, kΩ, and MΩ

39. Resistors

40. Resistors on PCB

41. Resistor Color Coding

42. Surface Mount Resistors

43. SMT Resistors on PCB

44. Resistance and Resistivity

45. Resistance
Electric current in a circuit is decided by voltage (V) and resistance (R): I = V / R
What are the factors to determine the resistance?

46. Resistance & Length
Which one has larger resistance?

47. Resistance & Length
Which one has larger resistance?

48. Resistance & Length
Which one has larger resistance?
R ~ L

49. Resistance & Area
Which one has larger resistance?

50. Resistance & Area
Which one has larger resistance?

51. Resistance & Area
Which one has larger resistance?
R ~ 1 A

52. Resistance & Resistivity
Which one has larger resistance?
Copper
Silver

53. Resistance & Resistivity
Which one has larger resistance?
Copper
Silver

54. Resistance & Resistivity
of Materials

55. Resistance & Resistivity
Which one has larger resistance?
Copper
Silver
R ~ ρ

56. Resistance ρ L R = A Resistance is determined by ρ resistivity [Ω m]
L length [m]
A cross-section area [m2]
R =
ρ L A

57. Resistance Example
A 0.5-m long Nichrome wire has m2 cross-section. What is the resistance of the wire?

58. Resistance Example
A 0.5-m long Nichrome wire has m2 cross-section. What is the resistance of the wire?
R = ρ L / A
= (150 x Ωm) (0.5 m) / m2
= 7.5 x 10-4 Ω

59. Resistance Exercise
A 1.2 x 10-5 Ω Copper wire has m2 cross- section. What is the length of the wire?

60. Resistance Exercise
A 1.2 x 10-5 Ω Copper wire has m2 cross- section. What is the length of the wire?
R = ρ L / A
1.2 x 10-5 Ω = (1.72 x Ωm) L / m2
L = 2.79 m

61. Electric Current

62. Electric Current Electric current is the flow of charge
In solid conductors: the electrons
In fluids: positive and negative ions as well as electrons
Electric current is usually measured in units of A and mA

63. Electric Current
When electrons flow in a wire, the number entering one end is the same as the number leaving the other
The net charge of the wire is normally zero

64. Electric Current
Electric (conventional) current is from positive to negative, while electron flow is from negative to positive

65. Electric Current
Current is the charge flow rate past a given cross-section
(current) = (charge) / (time) or I = Q / t

66. Electric Current
1 A is 1 C of charge going past a given cross-section per second
1 C is 6.24 x 1018 electrons

67. Electric Current Example
How much current must there be in a circuit if 100 coulombs flow past a point in the circuit in 4 seconds?

68. Electric Current Example
How much current must there be in a circuit if 100 coulombs flow past a point in the circuit in 4 seconds?
I = Q / t
= 100 C / 4 s
= 25 A

69. Electric Current Exercise
If there is a current of 20 amperes in a circuit for 10 minutes, what quantity of electric charge flows in through the circuit? 


70. Electric Current Exercise
If there is a current of 20 amperes in a circuit for 10 minutes, what quantity of electric charge flows in through the circuit?
Q = I t
= 20 A x 600 s
= C 


71. Electric Current Exercise
How much time is required for 10 coulombs of charge to flow past a point if the rate of flow (current) is 2 amperes?

72. Electric Current Exercise
How much time is required for 10 coulombs of charge to flow past a point if the rate of flow (current) is 2 amperes?
t = Q / I
= 10 C / 2 A
= 5 s

73. Voltage Sources

74. Voltage Source
A voltage source is a two-terminal circuit element which supplies a constant DC or AC potential difference for any current flow through it
Voltage sources: dry cell batteries, wet cell batteries, solar panel, or generators
Voltages are usually measured by the units of V, kV, and MV

75. Voltage Source
The potential energy per coulomb of charge available to electrons moving between terminals is the voltage
V = W / q
Voltage is sometimes called the electromotive force, or emf

76. Voltage Sources: Dry Cell

77. Voltage Sources: Wet Cell

78. Voltage Sources: Solar Panel

79. Voltage Sources: Generator

80. Batteries in Parallel / Series

81. Batteries in Parallel / Series

82. Batteries in Parallel / Series Exercise
Calculate the current in each case
1 KΩ

83. Electric Shock

84. Electric Shock
The damaging effects are the result of current passing through the body
The current depends on the voltage applied and also on the resistance of the human body (I = V / R)
The resistance of the body range from ~100 Ω to ~500 KΩ

85. Electric Shock Current (A) Effect 0.001 can be felt 0.005 painful
0.010
involuntary muscle contractions (spasms)
0.015
loss of muscle control
0.070
if through the heart, serious disruption; probably fatal if current lasts for more than 1 second

86. Electric Shock
Why do the birds perch on the high- voltage wires not getting electric shock?

87. Electric Shock
Why do the birds perch on the high- voltage wires not getting electric shock?
Because every part of their bodies is at the same high potential as the wire

88. DC & AC

89. Direct Current (DC)
A flow of charge that flows in one direction, even if the current moves in unsteady pulses
A battery produces direct current
Electrons always move through the circuit in the same direction from the negative terminal and toward the positive terminal

90. Direct Current (DC)

91. Direct Current (DC) Waveform
Time
Voltage
Voltage
Time

92. Alternating Current (AC)
A flow of charge is alternating its directions
This is accomplished by alternating the polarity of voltage at voltage source

93. Alternating Current (AC)

94. Alternating Current (AC) Waveform
Voltage
Time
Voltage
Time

95. Alternating Current (AC)
Nearly all of the commercial AC circuits in North America involves 120 V and 60 Hz
Europe adopted 220 V as their standard

96. Alternating Current (AC)
The 120 V refers to the “root-mean-square” (RMS) average of the voltage The actual voltage in a 120 V AC circuit varies between V and – 170 V peaks. It delivers the same power as a 120 V DC circuit

97. Alternating Current (AC)
Because most electric service in the United States is three-wire: one wire at V, one wire at 0 V (neutral), and the other wire at -120 V
Most of the appliance use +120V/-120 V and the neutral wires, producing 120 V. When use both +120V and -120 V wires, a 240 V is produced

98. AC-to-DC Conversion

99. Speed of Electrons

100. Thermal Speed vs. Drift Speed

101. Thermal Speed vs. Drift Speed
Thermal motion (random motion) speed inside a metal wire is about 1/200 the speed of light
Under electric field, the Drift Speed (net speed) is only about 0.01 cm/s

102. Speed of Electrons
The electrons will collide with the metallic ions in their path and transfer some kinetic energy to them
The extremely high speed of electricity is not due to the electrons but due to the signal. The signal is traveling at near high speed

103. Speed of Electrons
The electrons inside the conductor will shift forward (DC) or forward and backward (AC)
Why does the electric power company charge you money when they provide you AC electricity which no net electrons enter your home?

104. Speed of Electrons
The AC outlets in your home do not supply you electrons but supply you energy
The source of the electrons is the conducting circuit material itself

105. Electric Power

106. Electric Power
The rate at which electrical energy is converted into another form (mechanical energy, heat, or light) is called electric power
(Electric Power) = (Electric Energy) / (Time)
Unit: Watts (W)
P = W t

107. Electric Power P = I V P = = V I = I V t = q W
Electric Power = Energy / Time
= (Charge/Time) x (Energy/Charge)
= Current x Voltage
Unit: 1 watt = (1 ampere) x (1 volt)
P =
= V I = I V W t = q
P = I V

108. Electric Power
Derive the formulas of
P, I, R,
P, V, R

109. Electric Power P = I V = I (I R) = I 2 R P = I V = ( ) V =
P = I V = I 2 R =
V 2 R

110. Electric Power W = P t Since Energy / Time = Power,
Energy = Power x Time
Derive the formulas of
W, I, V, and t
W, I, R, and t
W, V, R and t
W = P t

111. Electric Power W = P t = I V t = I 2 R t W = P t = I V t = ( ) t = t
Since Energy/Time = Power, so
Energy = Power x Time
W = P t = I V t = I 2 R t
W = P t = I V t = ( ) t = t
V 2 R
W = Pt = IVt = I2Rt = t
V 2 R

112. Electric Power
Energy can be represented in units of kilowatt-hours (kW·h)
1 kW·h = 3.6 x 106 J
A kilowatt is 1000 watt, and a kilowatt- hour is the energy consumed in 1 hour at the rate of 1 kilowatt

113. Electric Power Example
A light bulb is plugged into a 120-volt outlet and has a 0.7 A current in it. What is the power rating of the light bulb?

114. Electric Power Example
A light bulb is plugged into a 120-volt outlet and has a 0.7 A current in it. What is the power rating of the light bulb?
P = I V
= (0.7 A)(120 V)
= 84 W

115. Electric Power Example
A heater uses 21 A when connected to a 110-V line. If electric power costs 10 cents per kilowatt-hour in this location, what is the cost of running the heater for 13 hours?

116. Electric Power Example
A heater uses 21 A when connected to a 110-V line. If electric power costs 10 cents per kilowatt-hour in this location, what is the cost of running the heater for 13 hours?
W = I V t
= (21 A)(110 V)(13 hr)
= W-hr
= kW-hr
Cost = ($ 0.1 /kW-hr)(30.03 kW-hr) = $3.00

117. Electric Power Exercise
A 120 V outlet in Tony’s house is wired with a circuit breaker on an 8 A line. a) If Tony tries use his newly-bought 1200-Watt hair dryer, will he trip the circuit breaker? b) What is the resistance of the hair dryer?

118. Electric Power Exercise
Alice likes to keep her 40-Watt front porch light on at night time from 10 p.m. to 6 a.m., and Alice pays 8.00¢ per kWh, how much does it cost to run the light for this amount of time each week?

119. The End