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Understanding Electrical Circuits and Ohm's Law

This chapter provides an overview of electrical circuits and explains Ohm's law, including how to calculate voltage, current, and resistance. It also covers open circuits, short circuits, and different types of circuit protection.

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Understanding Electrical Circuits and Ohm's Law

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  1. OBJECTIVES After studying Chapter 4, the reader should be able to: • Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “A” (General Electrical/Electronic Systems Diagnosis). • State Ohm’s law. • Identify the parts of a complete circuit. • State Watt’s law. • Describe the characteristics of an open, a short-to-ground, and a short-to-voltage.

  2. CIRCUITS • A circuit is a path that electrons travel from a power source (such as a battery) through a load such as a light bulb and back to the power source. FIGURE 4-1 All complete circuits must have a power source, a power path, protection (fuse), an electrical load (light bulb in this case), and a return path back to the power source.

  3. CIRCUITSParts of a Complete Circuit • Every complete circuit contains the following parts. • A power source. • Protection from harmful overloads. • An insulated path for the current to flow through from the power source to the resistance. • The electrical load or resistance which converts electrical energy into heat, light, or motion. • A ground (return) path for the electrical current from the load back to the power source so that there is a complete circuit. • Switches and controls that turn the circuit on and off

  4. CIRCUITSParts of a Complete Circuit FIGURE 4-3 An electrical switch opens the circuit and no current flows. The switch could also be on the return (ground) path wire. FIGURE 4-2 The return path back to the battery can be any electrical conductor, such as the metal frame or body of the vehicle.

  5. CIRCUITSOpen Circuits • An open circuit is any circuit that is not complete, or that lacks continuity. FIGURE 4-4 Examples of common causes of open circuits. Some of these causes are often difficult to find.

  6. CIRCUITSShort to Voltage • If a wire (conductor) or component is shorted to voltage, it is commonly called shorted. • A short-to-voltage is where the power side of one circuit is electrically connected to the power side of another circuit

  7. CIRCUITSShort to Voltage • A short circuit: • Is a complete circuit in which the current bypasses some or all of the resistance in the circuit. • Involves the power side of the circuit. • Involves a copper-to-copper connection. • Is also called a short-to-voltage. • Usually affects more than one circuit. • May or may not blow a fuse.

  8. CIRCUITSShort to Voltage FIGURE 4-5 A short circuit permits electrical current to bypass some or all of the resistance in the circuit.

  9. CIRCUITSShort to Voltage FIGURE 4-6 A fuse or circuit breaker opens the circuit to prevent possible overheating damage in the event of a short circuit.

  10. CIRCUITSShort to Ground • A short-to-ground is a type of short circuit wherein the current bypasses part of the normal circuit and flows directly to ground. FIGURE 4-7 A short-to-ground affects the power side of the circuit. Current flows directly to the ground return, bypassing some or all of the electrical loads in the circuit. There is no current in the circuit pass the short.

  11. CIRCUITSShort to Ground FIGURE 4-8 Electrical flow through a circuit is similar to water flowing over a waterwheel. The more the water (amperes in electricity), the greater the amount of work (waterwheel). The amount of water remains constant, yet the pressure (voltage in electricity) drops as the current flows through the circuit.

  12. OHM’S LAW • Ohm’s law states: • It requires 1 volt to push 1 ampere through 1 ohm of resistance.

  13. OHM’S LAW FIGURE 4-9 To calculate one unit of electricity when the other two are known, simply use your finger and cover the unit you do not know. For example, if both voltage (E) and resistance (R) are known, cover the letter/(amperes). Notice that the letter E is above the letter R, so divide the resistor’s value into the voltage to determine the current in the circuit.

  14. OHM’S LAW • Ohm’s law can determine the resistance if the volts and amperes are known: R =E/I. • Ohm’s law can determine the voltage if the resistance (ohms) and amperes are known: E =I xR. 3. Ohm’s law can determine the amperes if the resistance and voltage are known: I =E/R.

  15. OHM’S LAW RELATIONSHIP

  16. OHM’S LAW RELATIONSHIPOhm’s Law Applied to Simple Circuits • Using Ohm’s law, we can calculate the number of amperes that will flow through the wires and the resistor. • Remember, if two factors are known (volts and ohms in this example), the remaining factor (amperes) can be calculated using Ohm’s law.

  17. OHM’S LAW RELATIONSHIPOhm’s Law Applied to Simple Circuits I =E/R = 12 V/4 Ω The values for the voltage (12) and the resistance (4) were substituted for the variables E and R, and I is thus 3 amperes (12/4 = 3). FIGURE 4-10 This closed circuit includes a power source, power-side wire, circuit protection (fuse), resistance (bulb), and return path wire.

  18. WATT’S LAW • Electricity can also be expressed in a unit of power called a watt and the relationship is known as Watt’s law, which states: • A watt is a unit of electrical power represented by a current of 1 ampere through a circuit with a potential difference of 1 volt. W = I xE

  19. WATT’S LAW FIGURE 4-11 To calculate one unit when the other two are known, simply cover the unknown unit to see what unit needs to be divided or multiplied to arrive at the solution.

  20. WATT’S LAWMagic Circle FIGURE 4-12 “Magic circle” of most of the formulas for problems involving Ohm’s law. Each quarter of the “pie” has formulas used to solve for a particular unknown value: current (amperes), in the upper right segment; resistance (ohms), in the lower right; voltage (E), in the lower left; and power (watts), in the upper left.

  21. SUMMARY • All complete electrical circuits have a power source (such as a battery), a circuit protection device (such as a fuse), a power-side wire or path, an electrical load, a ground return path, and a switch or a control device. • A short-to-voltage involves a copper-to-copper connection and usually affects more than one circuit. • A short-to-ground involves a copper-to-steel connection and usually causes the fuse to blow. • An open is a break in the circuit resulting in absolutely no current flow through the circuit.

  22. REVIEW QUESTIONS • List the parts of a complete electrical circuit. • Describe the difference between a short-to-voltage and a short-to-ground. • Describe the difference between an open and a short. • State Ohm’s law. • Explain what occurs to current flow (amperes) and wattage if the resistance of a circuit is increased because of a corroded connection.

  23. CHAPTER QUIZ • If an insulated wire rubbed through a part of the insulation and the wire conductor touched the steel body of a vehicle, the type of failure would be called a(n) _____. • Short-to-voltage • Short-to-ground • Open • Chassis ground

  24. CHAPTER QUIZ 2. If two insulated wires were to melt together where the copper conductors touched each other, the type of failure would be called a(n) _____. • Short-to-voltage • Short-to-ground • Open • Floating ground

  25. CHAPTER QUIZ 3. If 12 volts are being applied to a resistance of 3 ohms, _____ amperes will flow. • 12 • 3 • 4 • 36

  26. CHAPTER QUIZ 4. How many watts are consumed by a light bulb if 1.2 amperes are measured when 12 volts are applied? • 14.4 watts • 144 watts • 10 watts • 0.10 watt

  27. CHAPTER QUIZ 5. How many watts are consumed by a starter motor if it draws 150 amperes at 10 volts? • 15 watts • 150 watts • 1,500 watts • 15,000 watts

  28. CHAPTER QUIZ 6. If the temperature of a conductor increases, what will happen to its resistance? • Stays the same • Increases • Decreases • Decreases slightly, then increases

  29. CHAPTER QUIZ 7. A wire was replaced with the same type and size but is twice as long as the original. Technician A says that the resistance will be the same as the original. Technician B says that the resistance will be less than the resistance of the original. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  30. CHAPTER QUIZ 8. If 200 amperes flow from the positive terminal of a battery and operate the starter motor, how many amperes will flow back to the negative terminal of the battery? • Cannot be determined • Zero • About one-half (about 100 amperes) • 200 amperes

  31. CHAPTER QUIZ 9. What is the symbol for voltage used in calculations? • V • E • EMF • I

  32. CHAPTER QUIZ 10. Which circuit failure is most likely to cause the fuse to blow? • Open • Short-to-ground • Short-to voltage • High resistance

  33. END

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