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Transcript

Direct Current Circuits CHAPTER 18 Sources of emf The source that maintains the current in a closed circuit is called a source of emf. Any devices that increase the potential energy of charges circulating in circuits are sources of emf. Examples include batteries and generators. SI units are Volts The emf is the work done per unit charge. emf and Internal Resistance A real battery has some internal resistance. Therefore, the terminal voltage is not equal to the emf. The terminal voltage is ΔV = Vb-Va ΔV = ε – Ir For the entire circuit, ε = IR + Ir Resistors in Series When two or more resistors are connected end-to-end, they are said to be in series. The current is the same in all resistors because any charge that flows through one resistor flows through the other. Ieq = I1 = I2 = … The sum of the potential differences across the resistors is equal to the total potential difference across the combination. Resistors in Series Potentials add: The equivalent resistance has the effect on the circuit as the original combination of resistors. ΔV = IR1 + IR2 = I (R1+R2) Consequence of Conservation of Energy Req = R1 + R2 + R3 + … The equivalent resistance of a series combination of resistors is the algebraic sum of the individual resistances and is always greater than any of the individual resistors. Sample Problem Four resistors are arranged as shown in the figure. Find (a) the equivalent resistance and (b) the current in the circuit if the emf of the battery is 6.0 V. Resistors in Parallel The potential difference across each resistor is the same because each is connected directly across the battery terminals. ∆Veq = ∆V1 + ∆V2 + … The current, I, that enters a point must be equal to the total current leaving that point. I = I 1 + I2 The currents are generally not the same. Consequence of Conservation of Charge. Resistors in Parallel Equivalent resistance can replace the original resistances: 1 1 1 1 R eq R1 R 2 R 3 The inverse of the equivalent resistance of two or more resistors connected in parallel is the algebraic sum of the inverses of the individual resistance. The equivalent is always less than the smallest resistor in the group. Sample Problem Three resistors are connected in parallel as in the figure. A potential difference of 18 V is maintained between points a and b. (a) Find the current in each resistor. (b) Calculate the power delivered to each resistor and the total power delivered to the three resistors. Sample Problem Four resistors are connected as shown in the figure. (a) Find the equivalent resistance between points a and c. (b) What is the current in each resistor if the terminals of a 42 V battery are connected between a and c? Sample Problem (a) Find the equivalent resistance between points a and b in the figure. (b) Calculate the current in each resistor if a potential difference of 34.0 V is applied between points a and b. Sample Problem Find the current in the 12 Ω resistor. Household Circuits The utility company distributes electric power to individual houses with a pair of wires. Electrical devices in the house are connected in parallel with those wires. Each appliance has a Power rating. (e.g. hairdryer is 1500W) The potential difference between the wires is about 120V. Electrical Safety Electric shock can result in fatal burns. Electric shock can cause the muscles of vital organs (such as the heart) to malfunction.. The degree of damage depends on: The magnitude of the current. The length of time it acts. The part of the body through which it passes. Effects of various currents: 5 mA or less 10 mA Can cause a sensation of shock Generally little or no damage Hand muscles contract May be unable to let go of a live wire 100 mA If passes through the body for just a few seconds, can be fatal Ground Wire Electrical equipment manufacturers use electrical cords that have a third wire, called a case ground. Prevents shocks. 240-V Connections Heavy-duty appliances may require 240 V to operate. The power company provides another wire at 120 V below ground potential. High voltage appliances are safer if they have three prong plugs going into a three hole outlet. The 3rd prong is the Earth (Ground) wire which draws any stray electricity or built up static away. Direct Current Circuits THE END CHAPTER 18