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Electric Current and Ohm's

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Electric Current and Ohm's ELECTRIC CURRENT AND OHM’S LAW Electric Current • The continuous flow of electric charge is an electric current. • The two types of current are: • direct current • alternating current • Charge flows only in one direction in direct current (DC). • A flashlight and most other battery-operated devices use direct current. • Alternating current (AC) is a flow of electric charge that regularly reverses its direction. • Electric current in your home and school is mostly alternating current. • The SI unit of electric current is the ampere (A), or amp, which equals 1 coulomb per second. • Even though electrons flow in an electric current, scientists define current as the direction in which positive charges would flow. Conductors and Insulators • An electrical conductor is a material through which charge can flow easily. Electrons are free to move • An electrical insulator is a material through which charge cannot flow easily. Electrons are not free to move • Metals such as copper and silver are good electrical conductors. Wood, plastic, rubber, and air are good electrical insulators. • A metal wire is usually coated with plastic or rubber. • The metal wire is an electrical conductor. • The rubber and plastic are electrical insulators. • The coating around a wire helps to control the current and keep it where it is needed. Resistance • Resistance is opposition to the flow of charges in a material. • As electrons move through a conducting wire, they collide with electrons and ions. These collisions convert some kinetic energy into thermal energy, and the current is reduced. • The SI unit of resistance is the ohm. • A material’s thickness, length, and temperature affect its resistance. • Using a thick straw to drink a milkshake is easier than using a thin straw. Similarly, electrons flow more easily through a thick wire than they flow through a thin wire of the same material. • Resistance is greater in a longer wire because the charges travel farther. • As temperature increases, a metal’s resistance increases because electrons collide more often. • A superconductor is a material that has almost zero resistance when it is cooled to low temperatures. Voltage - Potential Difference Water falls spontaneously from a higher to a lower height. Likewise, electric charges flow from a higher to a lower potential energy. • Potential difference is the difference in electrical potential energy between two places in an electric field. • Potential difference is measured in joules per coulomb, or volts. Because it is measured in volts, potential difference is also called voltage. • In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy. Voltage Sources • A source of voltage does work to increase the potential energy of electric charges. • Three common voltage sources are: • Batteries • Solar cells • Generators Ohm’s Law • According to Ohm’s law: The voltage (V) in a circuit equals the product of the current (I) and the resistance (R). • When the current is in amperes, and the resistance is in ohms, the voltage is in volts. • Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current. Electric Circuits Circuit Diagrams Circuit diagrams use symbols to represent parts of a circuit, including a source of electrical energy and devices that are run by the electrical energy. An electric circuit is a complete path through which charge can flow. • A circuit diagram shows one or more complete paths in which charge can flow. • Arrows show the direction of current, from positive to negative. The direction of current is defined as the direction in which positive charges would flow, but electrons flow in the opposite direction. Switches show places where the circuit can be opened. • If a switch is open, the circuit is not a complete loop, and current stops. This is called an open circuit. • If the switch is closed, the circuit is complete, and charge can flow. This is called a closed circuit. Series Circuits • In a series circuit, charge has only one path through which it can flow. • If one bulb burns out in a series circuit, it becomes an open circuit. • The bulbs in a circuit are a source of resistance. Adding bulbs to a series circuit increases the resistance. The current decreases, and each bulb shines less brightly. Parallel Circuits • A parallel circuit is an electric circuit with two or more paths through which charges can flow. • If one bulb in a parallel circuit burns out, charge still flows along the other path, and the other bulb stays lit. • In a home, electric circuits are wired in parallel so they can operate independently. • Most circuits in a house are parallel. Even if one device stops working, the others will still work. • Less charge will flow through a series circuit than through a parallel circuit. Power and Energy Calculations • The rate at which electrical energy is converted to another form of energy is electric power. Recall that power is the rate of doing work. • The unit of electric power is the joule per second, or watt (W). Power often is measured in thousands of watts, or kilowatts (kW). • An appliance's power rating lets you know how much power it uses under normal conditions. An electric stove uses about 6000 watts, and a microwave oven uses about 1000 watts. • Find the electrical energy used by an appliance by multiplying power by time. Example: The power rating of a typical clothes dryer is 5400 watts, or 5.4 kilowatts. If you use the clothes dryer for 2 hours, the energy use is 5.4 kilowatts multiplied by 2 hours, or 10.8 kilowatt-hours. A kilowatt-hour equals 3,600,000 joules. Electrical Safety • Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy safe to use. • Correct wiring is not enough to prevent electrical accidents. • Most household circuits usually have an average voltage of 120 volts. Each device that is turned on increases the current. If the current exceeds the circuit’s safety limit, the wire may overheat. • Home Safety o A fuse prevents current overload in a circuit. A wire in the center of the fuse melts if too much current passes through it. o A circuit breaker is a switch that opens when current in a circuit is too high. The circuit breaker must be reset before the circuit can be used again. • Personal Safety o Electrical wiring in a home is insulated to protect people. If the insulation is damaged, you may accidentally touch the bare wire and get a shock. o Avoid touching electrical devices with wet hands, because your hands conduct current more readily when they are wet. • Insulation also prevents short circuits. • A three-prong plug can prevent shocks caused by short circuits. If a short circuit develops, the current takes an easier path to ground through the grounding wire. • The transfer of excess charge through a conductor to Earth is called grounding. .
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