20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Objectives • Describe conduction and classify materials • Use Ohm’s Law 20.2 Electric Current & Ohm’s Law Agenda • Bellwork • Ch20.2 Classnotes • Ch20 packet • Exit
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law
An electric current can Electric Current only flow when there is a closed path in which electrons can move. What are the two types of current?
The two types of current are direct current and alternating current.
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Electric Current Electric Current The continuous flow of electric charge is an The SI unit of electric current is the ampere electric current. (A), or amp, which equals 1 coulomb per • Charge flows only in one direction in direct second. current (DC). A flashlight and most other Even though electrons flow in an electric battery-operated devices use direct current. current, scientists define current as the • Alterna ting current (AC) i s a fl ow of el ect ri c direction in which positive charges would charge that regularly reverses its direction. Electric current in your home and school is flow. mostly alternating current.
1 20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Electric Current Conductors and Insulators A complete path is required for charge to flow in a flashlight. Batteries must be placed so that charge can flow from negative to What are some examples of conductors and positive, passing through the bulb. insulators? Flow of current Switch Negative terminals Metals such as copper and silver are good electrical conductors. Wood, plastic, rubber, and air are good electrical insulators.
Spring Positive terminals
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Conductors and Insulators Conductors and Insulators An electrical conductor is a material A metal wire is usually coated with plastic or through which charge can flow easily. rubber. An electrical insulator is a material through The metal wire is an electrical conductor. which charge cannot flow easily. The rubber and plastic are electrical insulators. The coating around a wire helps to control the current and keep it where it is needed.
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Conductors and Insulators Resistance A metal is made up of ions in a lattice. The ions are not free to move. • Each ion has one or more electrons that are not tightly bound to it. What factors affect electrical resistance? • These free electrons can conduct charge. • Most materials do not easily conduct charge A material’s thickness, length, and because they don’t have free electrons. temperature affect its resistance.
2 20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Resistance Resistance Resistance is opposition to the flow of Using a thick straw to drink a milkshake is charges in a material. easier than using a thin straw. Similarly, • As electrons move through a conducting wire, electrons flow more easily through a thick they collide with electrons and ions. These wire than they flow through a thin wire of the collisions convert some kinetic energy into same material. thermal energy, and the current is reduced. • The SI unit of resistance is the ohm.
metal ions
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Resistance Voltage Resistance is greater in a longer wire because the charges travel farther. As temperature increases, a metal’s resistance What causes an electric current? increases because electrons collide more often. In order for charge to flow in a conducting A superconductor is a material that has wire, the wire must be connected in a almost zero resistance when it is cooled to complete loop that includes a source of low temperatures. electrical energy.
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Voltage Voltage Potential Difference A pump lifts water to Water falls spontaneously from a higher to a lower the top of the height. Likewise, electric charges flow from a fountain, increasing higher to a lower potential energy. the gravitational • Potential difference is the difference in electrical potential energy of potential energy between two places in an electric the water. A voltage field. source increases • Potential difference is measured in joules per the electrical coulomb, or volts. Because it is measured in volts, potential energy of potential difference is also called voltage. electric charges.
3 20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Voltage Ohm’s Law Voltage Sources A source of voltage does work to increase the potential energy of electric charges. Three How are voltage, current, and resistance common voltage sources are batteries, solar cells, related? and generators. Increasing the voltage increases the current . A battery is a device that converts chemical Keeping the same voltage and increasing energy to electrical energy. the resistance decreases the current.
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Ohm’s Law Ohm’s Law A multimeter can be According to Ohm’s law, the voltage (V) in a used to measure circuit equals the product of the current (I) current, voltage, or and the resistance (R). resistance. Here the voltage of a 9-volt battery is measured.
When the current is in amperes, and the resistance is in ohms, the voltage is in volts.
20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law Ohm’s Law You Try What is the voltage if the resistance is 3 ohms and the current is 3 amps? 4. A 9-volt battery drives an electric current through V = I × R = 3 amps × 3 ohms = 9 volts a circuit with 4-ohm resistance. What is the electric current running through the circuit? a. 0.44 A b. 2.25 A c. 5 A d. 36 A
4 20.2 Electric Current and Ohm’s Law 20.2 Electric Current and Ohm’s Law
You Try Result 4. A 9-volt battery drives an electric current through EXIT a circuit with 4-ohm resistance. What is the electric current running through the circuit? 1. Which of the following materials is a good conductor of electric current? a. 0.44 A V = 9V R = 4Ω a. wood b. 2.25 A b.glass c. 5 A V c. air d. 36 A V = I × R I = R d. iron ANS: B 9V I = = 2.25A 4Ω
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