1. Learning objectives Resistance and Resistors Electronic Symbols
Ohm’s law

2. Resistance (R) is Opposition to Current
Juice = Charge
Air pressure using mouth = Voltage
Flow of juice = Current
Straw = Wire
Pressing straw by hand = Resistance
(a)
Fig. 1-10: (a) photograph of resistors. (b) Schematic symbol for any type of fixed resistor.
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3. Resistance Is Opposition to Current
Resistance is the opposition to the flow of current.
A component manufactured to have a specific value of resistance is called a resistor.
Conductors, like copper or silver, have very low resistance.
Insulators, like glass and rubber, have very high resistance.
The letter used for resistance is R.
The unit of resistance is the ohm (Ω).

4. 3-5: Multiple and Submultiple Units
Units of Resistance
The basic unit of resistance is the Ohm (Ω).
Multiple units of resistance are:
kilohm (kΩ) 1 thousand ohms or 103 Ω
Megohm (MΩ) 1 million ohms or 106 Ω

5. Variable Resistors
A variable resistor is a resistor whose resistance value can be changed.
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6. Learning objectives Resistance and Resistors Electronic Symbols
Ohm’s law

7. Electronic Symbols American National Standards Institute (ANSI)
also known as IEEE Std or CSA Z
IEC 60617 (also known as British Standard BS 3939).

8. Trace junction and crossing

9. Battery

10. Resistor

11. Switches

12. Lamps

13. Learning objectives Resistance and Resistors Electronic Symbols
Ohm’s law

14. Ohm’s Law Formulas There are three forms of Ohm’s Law: where: I = V/R
V = IR
R = V/I
where:
I = Current
V = Voltage
R = Resistance
Fig. A circle diagram to help in memorizing the Ohm’s Law formulas V = IR, I = V/R, and R= V/I. The V is always at the top.
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15. Practical Units Applying Ohm’s Law V I R ? 20 V 4 W I = = 5 A 1 A ?
V = 1A × 12 W = 12 V
3 A
6 V ?
R =
3 A
= 2 W

16. The Linear Proportion between V and I
The Ohm’s Law formula I = V/R states that V and I are directly proportional for any one value of R.
Fig. Experiment to show that I increases in direct proportion to V with the same R. (a) Circuit with variable V but constant R. (b) Table of increasing I for higher V. (c) Graph of V and I values. This is a linear volt-ampere characteristic. It shows a direct proportion between V and I.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.