EE301 Lesson 2 : Resistance (Chapter 3) and Ohms Law (Chapter 4)

1. EE301 Lesson 2:Resistance (Chapter 3) and Ohms Law (Chapter 4)

2. Learning Objectives • Describe the concept of resistance. • Use Ohm’s law to calculate current, voltage, and resistance values in a circuit. • Discuss the difference between an open circuit and a short circuit. • Given a color code table, determine the value and tolerance of fixed resistors using their color codes. • Demonstrate how to measure current, voltage, and resistance in a circuit.

3. Resistance is the opposition to charge movement. As electrons move through a material, they constantly collide with atoms in the material and with other electrons. These collisions cause some of the energy carried by the charge to be released as heat. So resistance is dependent on several factors: type of material length of the conductor cross-sectional area temperature Resistance of Conductors

4. Type of material: Atomic differences of materials cause variations in how electron collisions affect resistance. Differences produce resistivity. Resistance is directly proportional to the resistivity. The higher the resistivity, the greater is the resistance of the conductor. Resistivity is represented by the symbol  (Greek letter rho). Units of :Circular Mils*Ohms/ft (CM∙Ω/ft). Resistance of Conductors

5. Length of conductor: Resistance is directly proportional to the conductors length. The longer the conductor, the greater the resistance. If you double the length of the wire, the resistance will double. = length Measured in feet Resistance of Conductors • Cross-sectional area: • Resistance is inversely proportional to cross-sectional area of the conductor. The greater the area of the conductor, the less resistance. • If cross-sectional area is doubled, resistance will be one half as much. • A =Cross-sectional area, Circular Mils (or centimeters2in Metric calculations).

6. Temperature: For most conductors, a temperature increase causes an increase in resistance. Increase is relatively linear. However, not that for semiconductors and insulators, an increase in temperature results in decrease in resistance. Resistance of Conductors

7. Resistance of a conductor at a given temperature can be expressed mathematically: Resistance of Conductors A(CM) resistivity, Circular Mils-Ohms/foot (CM-Ω/ft) length, in feet (ft) cross-sectional area, in Circular Mils (CM) 1x10-3 inches = 1 mil & ACM = (diametermils)2

8. Example Problem 1 • You need to measure length of this copper coil but don’t want to unroll it. Resistance measured is 103.7 Ohms. Diameter is 0.01 inches. Find the length. 1x10-3 inches = 1 mil = 1/1000 inch & ACM = (diametermils)2

9. Fixed Resistors • Used to provide control of electrical circuits. • Produced in various shapes and sizes.

10. Variable resistors (VAR) have an adjustable value of resistance and have two principle functions: Potentiometers are used to adjust voltage. Rheostats are used to adjust current. VARs are used to adjust volume, set level of lighting, adjust temperature,… a a Rab Rab b b Rbc Rbc c c Variable Resistors

11. Resistor color coding Resistors usually have 4, 5 or 6 color bands. 4 color bands: Band 1: First Digit Band 2: Second Digit Band 3: Multiplier (10X) Band 4: Tolerance Sometimes a 5th or 6th band is added (Reliability) to meet MILSPEC requirements. 5 color bands: Band 1: First Digit Band 2: Second Digit Band 3: Third Digit Band 4: Multiplier (10X) Band 5: Tolerance gray NOTE: That for a 4-band resistor, if the 4th band is not present, it is assumed that the resistor has a tolerance of ± 20%

12. Resistor color coding Yellow = 4Purple = 7Orange = 3=>103 Red = 2% 47*103 = 47kΩ±2% Read left To right MULTIPLIER (i.e)4 = 1045 = 105 IF THIRD BAND gray Green = 5Blue = 6Black = 0 Red = 2 =>102 Brown = 1% 560*102 = 56kΩ±1% 2nd Digit 3rd Digit Multiplier 1st Digit Tolerance

13. Brown = 1Black= 0Brown = 101 Gold = ± 5% tolerance => ± 5 Ω R = 100 Ω±5% 10 * 101 = 100 Ω Green = 5Blue = 6Yellow = 4 Green = 105 Silver = ± 10% tolerance => ± 5.64MΩ R = 56.4 MΩ ± 10% 564 x 105= 56.4 MΩ Example Problem 2 Determine the resistance of a carbon resistors having the color codes shown in the figure below. Make sure you identify that this is a 4-band resistor Make sure you identify that this is a 5-band resistor gray

14. Ohm’s Law • Every conversion of energy from one form to another can be related to this equation. • In electric circuits, the effect we are trying to establish is the flow of charge, or current. • The potential difference, or voltage, between two points is the cause (“pressure”), and the opposition is the resistance encountered.

15. Georg Ohm discovered experimentally that voltage and current in a wire were linearly related to each other by a constant (Resistance) Current in a resistive circuit: Directly proportional to its applied voltage. Inversely proportional to its resistance. Ohm’s Law Plotting Ohm’s Law

16. Two different symbols are commonly used to represent voltage. E for sources. V for loads (such as the voltage drop across a resistor). Ohm’s Law • When using Ohm’s Law be careful that you use the voltage across the load (in the above that would be the voltage across the resistor (R).

17. Ohm’s Law • In summary, therefore, the absence of an applied “pressure” such as voltage in an electric circuit will result in no reaction in the system and no current in the electric circuit. • Current is a reaction to the applied voltage and not the factor that gets the system in motion. • EXAMPLE: The greater the pressure in a hose, the greater is the rate of water flow through the hose, just as applying a higher voltage to the same circuit results in a higher current.

18. Example Problem 3 Determine the current provided by the source in the system below. Rearrange the equation =>

19. Current can only exist where there is a conductive path. When the current is zero, the circuit will have an infinite resistance (remember Ohm’s Law). This is called an open circuit. Open Circuits

20. Short circuits occur when a very low (or near zero) resistance is placed across a circuit. Since resistance is near zero, all current will bypass the rest of circuit and go through the short (again, recall Ohm’s Law). With zero resistance, current draw will approach the limits of the power source (infinite current), frequently causing damage to wiring or the power source. Short Circuits

21. Measuring Voltage Voltage and current is measured using voltmeters and ammeters, or a Digital Multi-Meter (DMM). Measure voltage by placing the voltmeter leads across the component whose voltage you wish to measure. Polarity can be determined by probe placement. “Auto-polarity”

22. Meter Symbol: Voltmeter

23. The current depicted in the following circuits is identical. Current Representation

24. Measuring Current The current that you wish to measure must pass through the meter. Be careful that the meter’s maximum current rating is not exceeded!

25. Meter Symbol: Ammeter

26. For the voltage across a resistor, always place the plus sign at the tail of the current reference arrow. To ensure the correct voltage sign on the DMM, place the red lead where you think the “+” sign should be. For correct current polarity, the current should enter the red lead of the DMM. Voltage Polarity and Current Direction

27. An ohmmeter is a device used to measure the resistance of a component. Resistance cannot be measured when voltage is supplied to the circuit. Measuring Resistance

28. You must isolate the component that you are measuring from the circuit! Measuring Component Resistance

29. You must remove the power source prior to measuring total resistance! Measuring Total Resistance

30. QUESTIONS?