Transcript ch13
13 Electric Circuits Chapter Outline 1 Electric Circuits and Electric Current 2 Ohm’s Law and Resistance 3 Series and Parallel Circuits 4 Electric Energy and Power 5 Alternating currents and Household Current Everyday Phenomenon: The Hidden Switch in Your Toaster How to Get the Bulb to Light? How to Get the Bulb to Light? Electric Current The electric current is the amount of charge per unit time that passes through a surface that is perpendicular to the motion of the charges. q I . t The SI unit of electric current is the ampere (A), after the French mathematician André Ampére (1775-1836). 1 A = 1 C/s. Ampere is a large unit for current. In practice milliampere (mA) and microampere (μA) are used. Direction of Current Flow Electric current is a flow of electrons. In a circuit, electrons actually flow through the metal wires. Conventional electric current is defined using the flow of positive charges. It is customary to use a conventional current I in the opposite direction to the electron flow. Direction of Current Flow What Limits the Flow of Current? What Limits the Flow of Current? A: Resistance Electric Current Is Analogous to Water Flow Ohm’s Law Georg Simon Ohm (1787-1854), a German physicist, discovered Ohm’s law in 1826. This is an experimental law, valid for both alternating current (ac) and direct current (dc) circuits. When you pass an electric current (I) through a resistance (R) there will be a potential difference or voltage (V) created across the resistance. Ohm’s law gives a relationship between the voltage (V), current (I), and resistance (R) as follows: V=IR What Is the Current? Electromotive Force (emf) The energy needed to run electrical devices comes from batteries. Within a battery, a chemical reaction occurs that transfers electrons from one terminal (leaving it positively charged) to another terminal (leaving it negatively charged). Because of the positive and negative charges on the battery terminals, an electric potential difference exists between them. The maximum potential difference is called the electromotive force* (emf) of the battery. The electric potential difference is also known as the voltage, V. The SI unit for voltage is the volt, after Alessandro Volta (17451827) who invented the electric battery. 1 volt = 1 J/C. Circuits Series Circuit Parallel Circuit Try This Box 13.2 Use of Voltmeter Voltmeter is connected across the resistance. Use of Ammeter Ammeter is inserted into the circuit to measure current. Electrical Energy Electrical Energy and Power Our daily life depends on electrical energy. We use many electrical devices that transform electrical energy into other forms of energy. For example, a light bulb transforms electrical energy into light and heat. Electrical devices have various power requirements. Electrical power, P is defined as the electrical energy transfer per unit time, Energy P . time Electric Power: Energy P . time Since the electrical energy is charge times voltage (QV), the above equation becomes, QV P . t Since the current is charge flow per unit time (Q/t), the above equation becomes, QV Q P V I V . t t Since V = IR, the above equation can also be written as, V2 P IV I R . R 2 Killowatt-hour (kWh) The SI unit of power is watt, after James Watt (17361819), who developed steam engines. joule J watt W . sec ond s Utility companies use the unit kilowatt-hour to measure the electrical energy used by customers. One kilowatthour, kWh is the energy consumed for one hour at a power rate of 1 kW. Exercises 1. State Ohm’s law in an equation form in terms of voltage and current. 2. Define power in an equation form in terms of voltage and current. 3. When an appliance is plugged in a 120-volt outlet, it draws a current of 8 amperes. Calculate the power of the appliance. 4. If the above appliance is used 10 hours a day for 28 days per month, and if the cost of electricity is 12 cents per kilowatt-hour, how much does it cost to operate the appliance for a year? Electrical Power Transmission AC adapter INPUT: AC 120 V, 60 Hz, 15 W OUTPUT: DC 9V, 1A Alternating Current Alternating Voltage Effective voltage = 115 V Household Circuits Power and Current Ratings of some common Appliances Appliance Stove Clothes dryer Water heater Clothes washer Dishwasher Iron Coffeemaker TV Power (W) 6000 (220V) 5400 (220V) 4500 (220V) 1200 1200 1100 1000 100 Current (A) 27 25 20 10 10 9 8 0.8 Everyday Phenomenon Bimetallic Thermostat CP5