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Unit 12 Electricity and RC Circuits

Unit 12 Electricity and RC Circuits. 12-1. Conduction Electrons. In organic compounds, electrons are bound to specific atoms. In metallic compounds, some of the electrons are not bound to a specific atom. They are free to move throughout the metal.

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Unit 12 Electricity and RC Circuits

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  1. Unit 12Electricity and RC Circuits

  2. 12-1 Conduction Electrons • In organic compounds, electrons are bound to specific atoms. • In metallic compounds, some of the electrons are not bound to a specific atom. • They are free to move throughout the metal. • These electrons are called conduction electrons. • If a potential difference is placed across the wire (like when you connect the wire to a battery), then the electrons will move. • As they move, the electrons collide with the metallic atoms. • Depending upon the number of collisions an electron has, it may move faster or slower through the metallic structure. • Remember, moving electrons in a wire are known as current.

  3. Durable 12-1A Durable Durable Durable Sure Start Batteries • Batteries come in many shapes and sizes that have a variety of voltage and currents. • Identify the voltages of the four common battery types shown below.

  4. 12-2 Durable Batteries • This slide will explain how a battery provides electricity for use in your small electrical appliances. • Free electrons and “holes,” which are the absences of electrons, are produced within the battery due to electrochemical reactions.

  5. 500 F 12-3 1500 pF 2300 F 2300 F Capacitors • Like batteries, capacitors come in a wide assortment of shapes. • A capacitor acts as a buffer to temporally store excess charges. • At other times a capacitor acts as a short lived battery in order to provide additional current (electrons) when the need is larger than a battery can provide. • Basically, a capacitor consists of two parallel plates. • One of these has a net positive charge while the other has a net negative charge. • In the next slide we will take a closer look at the operation of a capacitor.

  6. Durable 12-4 1500 pF Capacitors • Observe the capacitor acting in the capacity of a short lived battery.

  7. Durable 12-5 Resistors • Resistors impede the flow of electrons (current). • They impede this flow because certain electrical items have maximum limitations on the current they can handle. • Consider the Light Emitting Diode (LED) in the figure to the right. • The battery supplies too high a current to the LED. • As a result, the LED is damaged by the current. • When a resistor is placed into the circuit, the current is reduced to a level appropriate for use with the LED, and the LED is not damaged.

  8. Durable 12-6 Batteries and Current • A complete circuit is one that connects a battery to an electrical component back to a battery. • Remember, current flows from the negative end of a battery through the light bulb (resistor) and back into the positive end of the battery. • The symbol “I” is used to denote current. • Current is the number of electrons passing a certain point in a circuit per unit of time. • We draw an arrow in the direction that the current would flow through the circuit. I

  9. Durable Durable Durable 12-7 Multiple Batteries and Current • Depending on how they are placed, multiple batteries in a circuit can enhance or impede the current in a circuit. • Consider the circuit below. The current flowing through both batteries travels in the same direction. • As a result, the net current is enhanced in the circuit. • Now, let us flip one of the batteries. • When we flip the battery, its current now acts in the opposite direction. • As a result, it impedes the current. • Since both batteries are the same, no net current flows, and the bulb goes out.

  10. Durable Durable 12-8 Durable Multiple Batteries and Current • In this figure the batteries are of different voltages • As in the previous figure, when the batteries are connected in the same way, the net current is enhanced. • When one battery is flipped, the net current is impeded. • Determine the net current direction and the net voltage of the circuit below. • The key to determining the net current direction lies in considering the voltages of the batteries. • The net current will flow in the direction of the current belonging to the battery with the highest voltage.

  11. Durable Durable 12-9 + + - - + + - - Durable Durable 6 Volt 6 Volt Durable Durable Durable 6 Volt 6 Volt WS ???? #2 • Determine the voltage and the net current direction in the circuits below.

  12. DBHS Big Burger 12-10 Circuit Configurations: Series Resistors • Look at the drawing animation below. • The skateboarders leaving the school only have one place to go. • They also only have one place to return to. • Their round trip occurs in a series circuit.

  13. Durable DBHS Big Burger 12-11 Circuit Configurations: Series Resistors • Now lets replace the school building with a battery. • Turn the house and restaurant into a light bulb (resistor). • Change the roads into wires. • Now let some electrons flow through the circuit. • They have only one destination: through the resistor and back the battery.

  14. 12-12 Durable Circuit Configurations: Multiple Series Resistors • The lights below are in series with each other because the same current flows through all three of them. • Notice how the first bulb is brighter than each consecutive bulb thereafter. • This is because the electric potential (voltage) drops as it passes through the resistor. • As a result, there is less voltage available for the next resistor.

  15. DBHS Big Burger 12-13 Circuit Configurations: Parallel Resistors • In a parallel circuit, the skateboarders have a choice of going home or to the Big Burger for lunch. • They make this choice at the intersection. • When there is an intersection resulting in a choice between two or more directions, the circuit is a parallel circuit.

  16. Durable DBHS Big Burger 12-14 Circuit Configurations: Parallel Resistors • Now let us make the appropriate replacements in this circuit and turn it into an electrical circuit. • Now allow the current to flow. • Notice how more electrons flow through the small bulb. • This action happens because the small bulb has a lower resistance than the large bulb. • We will look at resistance more in the next slide.

  17. Durable DBHS 12-15 Resistance • Lets consider the circuit to now have two ramps. • Most unskilled skateboarders would choose the lower ramp. • As a result, we could say that the lower ramp has a lower resistance. • Now let us make the appropriate replacements. • In this replacement, we replaced the ramps with actual resistors instead of with light bulbs. • As you can see, more electrons flow through the orange resistor because it has a lower resistance.

  18. 12-16 + - Durable Circuit Configurations: Multiple Parallel Resistors • The circuit to the right shows three parallel resistors (bulbs) that are in series with a fourth resistor. • When we close the top switch, the two lighted bulbs are in series, and the top bulb is slightly brighter than the bottom bulb. • They share equal currents. • What do you think will happen when we close the middle switch? • Notice that the top two bulbs were equally dimmed while the bottom bulb remained the same. • The currents through the top bulbs are equal but half that through the bottom bulb. • A similar result is observed when the bottom switch is closed.

  19. 12-17 + - Durable A Closer Look – Series Resistors • When the switch to the right is closed, which bulb will be brighter? • Why? • The voltage drops across R1 making R2 less bright than R1. • If we remove R2, then what will happen to R1? • Why? • The same current flows through both light bulbs (resistors). • If you remove one of these series resistors, then the current can not flow, and the bulbs go out. R1 R2

  20. 12-18 + - Durable 6 Volt A Closer Look – Parallel Resistors • Note: R2 = 2.0  and R1 = 1.0  • When the switch to the right is closed, which bulb will be brighter? • Why? • R1 will be brighter because more current will flow through it due to the fact that it has less resistance. • If we remove R1, then what will happen to R2? • Why? • R2 will get brighter because all of the current now passes through R2 instead of being split between R1 and R2. R1 R2

  21. 12-19 + - Durable A Closer Look – Combination Resistors • Note: R4 = 4.0 , R3 = 3.0 , R2 = 2.0 , and R1= 1.0 . • When the switch to the right is closed, which bulb will be brighter? • Why? • All of the current flows through R1 before it splits after R1. • What will happen to R4 if we remove R1? • Why? • All of the bulbs go out because the electrons can not flow through R1. • What would happen to R1, R3, and R4 if we removed R2? • Why? • What would happen to R2 and R1 if we removed R4? • Why? R4 R1 R2 R3

  22. Durable 12-20 V A Sure Start Schematic Symbols • When engineers design electrical circuits, they replace actual pictures of electrical components with schematic symbols. • Here are five electrical components we will use frequently. • The schematic symbol for each of these electrical components is as follows. • Here are some other symbols you must be familiar with. Volt Meter Capacitor Switch Ammeter Multicell Battery Diode AC Load Battery Ground Resistor

  23. 12-21 Parallel and Series Electrical Configurations • There are two basic electrical configurations: series and parallel. • In a series connection, all electrical components share the same current. • In a parallel connection, the current through each component varies depending upon the components resistance. • Let us take a look at the schematic diagrams for the circuits below. • In the lower left picture, the two resistors are series. • Note that we can move one of the resistors any where in the circuit while maintaining the same current through each. • In the lower right picture the resistors are parallel. • Again, we may move one resistor and still have a parallel circuit. • The resistors do not need to be geometrically parallel in order to be electrically parallel.

  24. 12-22 Equivalent Circuits • It is often desirable to reduce numerous electrical resistors in a circuit to an equivalent circuit with fewer resistors. • In a series circuit, the series resistors may be replaced with a single resistor with the equivalent resistance to that of the ones it replaced. • On paper, simply redraw the circuit with only one resistor in the place of the two (or more) you are replacing. • The same concept holds true for parallel resistors. • The exact same procedure is followed when doing equivalent circuits with capacitors instead of resistors.

  25. 12-23 C1 STOP STOP R3 Series Test • Series Component Test – must be able to go from only one side of a component to only one side of an equivalent component without passing an intersection or a nonequivalent component. • An intersection kills a series possibility. • Are R4 and R2 Series? • Are R5 and R3 Series? • Are R4 and R5 Series? • Are C2 and C1 Series? • Are C3 and C4 Series? R5 C4 C3 C2 R4 R2 R1 Not Not Series Series

  26. R2 C3 12-24 C2 STOP R4 R1 R3 C1 Parallel Test • Parallel Component Test – must be able to go from both sides of a component to both sides of an equivalent component without passing through a nonequivalent component. • An intersection has no impact on a parallel circuit possibility. • Are R3 and R1 Parallel? • Are R4 and R2 Parallel? • Are C2 and C1 Parallel? Not Parallel

  27. 12-25 Series & Parallel Test • The circuit to the right is a very complicated RC (Resistor-Capacitor) circuit. • Let us apply the tests for series and parallel circuits in order to reduce the circuit to its simplest form. • Always replace series components with their equivalent series component before attempting to replace parallel components. • Do you see any series components? • Once all series components are replaced, proceed to replace any parallel components that may remain. • Are the remaining resistors series? • Why or why not? • Once there are no longer any series or parallel components, the circuit is reduced as far as possible. ?

  28. 12-26 Equivalent Resistance for Series Circuits • When you replace series resistors with an equivalent resistance, you must calculate the value of the new resistor. • When replacing two series resistors with an equivalent resistance, use the following formula in calculating the equivalent resistance. • If you are replacing many resistors, use the following formula in order to calculate the equivalent resistance.

  29. 12-27 Equivalent Resistance for Parallel Circuits • When you replace parallel resistors with an equivalent resistance, you must calculate the value of the new resistor. • When replacing two parallel resistors with an equivalent resistance, use the following formula in calculating the equivalent resistance. • If you are replacing many resistors, use the following formula in order to calculate the equivalent resistance.

  30. 12-28 2300 F 2300 F Equivalent Capacitance for Series Circuits • When you replace series capacitors with an equivalent capacitance, you must calculate the value of the new capacitor. • When replacing two series capacitors with an equivalent capacitance, use the following formula in calculating the equivalent capacitance. • If you are replacing many capacitors, use the following formula in order to calculate the equivalent capacitance.

  31. 12-29 2300 F 2300 F Equivalent Capacitance for Parallel Circuits • When you replace parallel capacitors with an equivalent capacitance, you must calculate the value of the new capacitor. • When replacing two parallel capacitors with an equivalent capacitance, use the following formula in calculating the equivalent capacitance. • If you are replacing many capacitors, use the following formula in order to calculate the equivalent capacitance.

  32. R1 R3 12-30 R2 C1 C2 C3 WS ??? # 1 and WS ??? #1 • Find the equivalent capacitance or resistance for the circuits in the following problems. WS ??? # 1. If C1 = 5.0 F, C2 = 25.0 F, and C3 = 9.5 F, then Ceq = ____? WS ??? # 1. If R1 = 5.0 , R2 = 25.0 , and R3 = 9.5 , then Req = ____?

  33. 12-31 C1 R1 R2 R2 C3 C2 WS ??? # 1 and WS ??? #1 • Find the equivalent capacitance or resistance for the circuits in the following problems. WS ??? # 1. If C1 = 5.0 F, C2 = 25.0 F, and C3 = 9.5 F, then Ceq = ____? WS ??? # 1. If R1 = 5.0 , R2 = 25.0 , and R3 = 9.5 , then Req = ____?

  34. R1 12-32 R2 R2 R1 R3 WS ??? # 4 • Find the equivalent capacitance or resistance for the circuits in the following problems. • If R1 = 5.0 , R2 = 25.0 , and R3 = 9.5 , then Req = ____?

  35. 12-33 C1 C2 C3 C4 C4 WS ??? # 4 • Find the equivalent capacitance or resistance for the circuits in the following problems. • If C1 = 12.0 F, C2 = 25.0 F, C3 = 5.0 F, and C4 = 1.5 F, then Ceq = ____?

  36. R1 C1 R2 R3 C3 12-34 C2 Resistor/Capacitor Circuits WS ??? # 2 • Reduce the circuit below as far as possible by finding the equivalent capacitance and resistance. Draw the final circuit for each problem. C1 = 22.8 F, C2 = 2.3 F, C3 = 5.9 F, C4 = 5.0 F, R1 = 2.2 , R2 = 14.8 , R3 = 9.5 , and R4 = 12.0 .

  37. 12-35 Ohm’s Law • Ohm’s Law gives us a mathematical expression relating the voltage (V), Current (I), and Equivalent Resistance (R) of a circuit. • Previously, we reduced the circuit below to its equivalent resistance. • If we do not know the voltage but we do know the current and the resistance, then we can use the equation to find the voltage. • If we know the voltage of the battery and the resistance, then we can find the current flowing through the resistor.

  38. 12-36 Ohm’s Law • You can use Ohm’s Law to calculate the current through a resistor if you know the voltage across the resistor and the resistance of the resistor. • Consider the parallel circuit below. • Suppose the voltage (V) of the battery in both circuits below is 10.0 V. • Since both sides of the battery are connected to both sides of both resistors, the voltage across both resistors would be 10 volts. • However, they would not have the same current because the current splits before it reaches the resistors. • You can use Ohm’s law in order to find the current through these resistors.

  39. 12-37 Ohm’s Law • Both sides of the resistors below are not connected to both sides of the battery. • As a result, they do not have the same voltage across them. • However, as these resistors are in series, they share the same current. • The voltage drop across R1 (from A to B) is given by the equation below right (Ohm’s Law). • The voltage would also drop across R2 and can be calculated with the equation below left.

  40. 12-38 Durable Ohm’s Law Example 1 • R1 = 10.0 , R2 = 20.0 , and R3 = 30 . • What is the current through the circuit below? • What are the voltage drops through R1, R2, and R3? R1 R2 R3

  41. 12-39 + - Durable Ohm’s Law Example 2 • R1 = 10.0 , R2 = 20.0 , and R3 = 30 . • What is the current through each resistor in the circuit below? • What are the voltage drops through R1, R2, and R3? R1 R2 R3

  42. 12-40 Power • Once we know both the current and the voltage across a resistor, we can determine the power consumed by that resistor. • The power consumed may be determined using the following equation. • The power consumed in R1 in both circuits below may be determined as follows.

  43. 12-41 Durable 1.5 Volts Batteries and emf • So far we have considered batteries as perfect sources of electrons meaning that all the electrons produced by the electrochemical reactions inside the battery are delivered to the electrical circuit. • However, this statement is not true because of the fact that the materials the battery is made of resist the flow of electrons produced by the battery itself. • We call this resistance the internal resistance (r) of the battery. • This internal resistance, when multiplied by the current flowing through the battery, reduces the electric potential (voltage) the battery can deliver to the circuit. • Consider the D-Cell battery below that provides a voltage (V) of 1.5 V to a circuit. • The actual electrical potential produced by the battery known as the electromotive force (emf or ) is larger than the voltage V delivered. • The relationship between the emf and the battery voltage is given in the below equation where I is the current produced by the battery. • The schematic symbols for a battery with and without internal resistance is given below.

  44. R R 12-42 V  r A A B B Electro Motive Force (emf) • The V below is the ideal value of the voltage across the battery (i.e. 9 Volts). • Although the potential difference across the terminals of a battery is V when no current is flowing, the actual potential difference is reduced when a current is flowing through the battery due to internal resistance (r)in the battery. • The actual potential difference is known as the electromotive force, . • When we consider the internal battery resistance, the figure below would change to look as follows. • The electromotive force may be found by using the following equation. • The internal battery resistance is treated like a regular resistor, R, when doing calculations.

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