Physics Study Notes 1 2 3 4 Lesson 19 Electric Circuits A Battery and a Bulb a. In order to light up a bulb, you need a voltage source (dry cell or battery), a load (light bulb), and wires to form a complete path (circuit). b. The flow of charge (electrons) flow from the negative part of the battery through the wire to the side (or bottom) of the bulb, through the filament inside the bulb, and out of the bottom (or side) and through another piece of wire to the positive part of the battery. The current then pass through the interior of the battery to complete the circuit. Electric Circuits a. Electric circuit—Any path along which electrons can flow is a circuit. There must be a complete circuit with no gaps. b. A gap is usually provided by an electric switch which can either cut off/allow electron flow. A gap/break in electric circuit results in a complete stop in the flow of electricity. c. Most circuits have more than one device that receives electrical energy. These devices are commonly connected in one of the two ways, series or parallel. d. When connected in series, they form a single pathway for electron flow between the terminals of the power source (such as battery, generator, or wall socket). e. When connected in parallel, they form branches, each of which is a separate path for the flow of electrons. Series Circuits a. Electric current has but a single pathway through the circuit. This implies that that the current passing through each electronic device is the same. b. The total resistance is the sum of individual resistances along the circuit path. c. The current in the circuit is numerically equal to the voltages supplied by the source divided by the total resistance of the circuit. (Ohm’s law) d. Ohm’s law also applies separately to each device. The voltage drop, or potential difference, across each device depends directly on its resistance. A device with higher resistance will consume more power than a device with lower resistance. e. The total voltage impressed across a series circuit divides among the individual devices so that the sum of the voltage drops across the individual devices is equal to the total voltage supplied by the source. So, the amount of power consumed by the entire circuit is equal to the sum of the power consumed by each device. f. The main disadvantage of a series circuit: if one device fails, current in whole circuit ceases and none of the devices will work. Parallel Circuits a. Electric devices connected in parallel are connected to the same two points of an electric circuit. Each device has its own path connecting to the two terminals of the battery. Therefore, the voltage is the same across each device. b. The total current in the circuit divides among the parallel branches. The current in one device does not pass through the other devices. Each device operates independently of the other devices. Current passes more readily into devices with lower resistance, so the amount of current in each branch is inversely proportional to the resistance of the branch. c. The total current in the circuit equals the sum of currents in its parallel branches. As the number of parallel branches increase, the overall resistance of the circuit is decreased. The overall resistance of the circuit is lowered than any resistance of the added branches. Mr. Lin 1 Physics Study Notes 5 Lesson 19 Electric Circuits Schematic Diagram a. Electric circuits are frequently described by simple diagrams called schematic diagrams. Different symbols are used to represent different circuit elements. The followings are some frequently used symbols: - + + Diode + - - Battery Light Emitting Diode (LED) Switches + Resistor Capacitor Wires b. 6 Circuit elements are connected through wires (nets) to form a diagram. A simple circuit example is shown in the following schematic diagram. Combining Resistors in a Compound Circuit a. Equivalent resistance: The equivalent resistance is the value of a single resistor that would comprise the same load of several resistors in its network to the battery or power source. b. The equivalent resistance can be found by the rule of adding resistance in series and parallel. i) Series resistors: Requivalence = R1 + R2 + R3 + + Rn R2 R1 Rn R1 + R2+ ……+ R2 => € ii) Parallel resistors: 1 Requivalence = 1 1 1 1 + + ++ R1 R2 R3 Rn 1 1 1 1 1 + + ++ R1 R2 R3 Rn R1 € R2 => € Rn c. Compound circuit: R1 R2 R1 + R2 R1 R3 R2 R1 R2 + R3 R1 + R2 R3 => => € € Mr. Lin 2 Physics Study Notes Lesson 19 Electric Circuits 2Ω 1Ω 2Ω 3Ω 2Ω 1.5Ω 2Ω => 3Ω d. => 3Ω Circuit analysis: i) V = V1 + V3 = V2 + V3, and V1 = V2 R1 R2 V R1 + R2 viii) I1 = 1 = V RR R1 R1 ( 1 2 + R3 ) R1 + R2 ii) I = I1 + I2 = I3 iii) V = IR, V1 = I1R1, V2 = I2R2, and V3 = I3 R 3 iv) R = R1 R2 + R3 R1 + R2 v) € € € I = I3 = € V R1 R2 V R1 + R2 ix) I2 = 2 = V RR R2 R2 ( 1 2 + R3 ) R1 + R2 R1 + V1 - R1 R2 + R3 R1 + R2 R1 R2 RR R1 + R2 vi) V1 = V2 = I 1 2 = V R1 R2 R1 + R2 + R3 R1 + R2 vii) 7 3Ω => V3 = I3 R3 = IR3 = V R3 R1 R2 + R3 R1 + R2 € I1 R3 + V3 - I2 I3 R2 + V2 + V - I Equivalent Resistance: R Parallel circuits and Overloading € a. All home outlets are arranged in parallel. The more appliances are connected to the lines, the more pathways are provided to the current, and the lower the combined resistance. b. Overload: When a greater amount of current occurs in the lines and is over its safe amount is said to be overloaded. c. To prevent overloading in circuits, fuses are connected in series along the supply lines. A short circuit draws a dangerously large amount of current and melts the fuses. d. Circuit breaker: Circuit breaker use magnets and bimetallic strips to open the switch. Mr. Lin 3