ELECTRIC CIRCUITS Chapter Twenty: Electric Circuits 20.1 Charge 20.2 Electric Circuits 20.3 Current and Voltage 20.4 Resistance and Ohms Law Chapter 20.1 Learning Goals Define static electricity and discuss its causes. Explain what it means when an object is electrically charged. Discuss the relationship between like and unlike charges. Investigation 20C Key Question: What is static electricity? Electric Charge 20.1 Electric charge Electric charge, like mass, is also fundamental property of matter. Inside atoms found in matter, attraction between positive and negative charges holds the atoms together. 20.1 Charge Virtually all the matter around you has electric charge because atoms are made of electrons and protons (and neutrons). Because ordinary matter has zero net (total) charge, most matter acts as if there is no electric charge at all. 20.1 Electric and magnetic forces Whether two charges attract or repel depends on whether they have the same or opposite sign. A positive charge attracts a negative charge and vice versa. Two similar charges repel each other. 20.1 Electrical forces The unit of charge is the coulomb (C). The name was chosen in honor of Charles Augustin de Coulomb ( ), the French physicist who performed the first accurate measurements of the force between charges. 20.1 Electrical forces Electric forces are incredibly strong. A millimeter cube of carbon the size of a pencil point contains about 77 coulombs of positive and negative charge. 20.1 Electrical forces Lightning is caused by a giant buildup of static charge. The cloud, air, and ground can act like a giant circuit. All the accumulated negative charges flow from the cloud to the ground, heating the air along the path (to as much as 20,000C) so that it glows like a bright streak of light. Chapter Twenty: Electric Circuits 20.1 Charge 20.2 Electric Circuits 20.3 Current and Voltage 20.4 Resistance and Ohms Law Chapter 20.2 Learning Goals Define electricity. Describe the components of an electric circuit. Explain the difference between a closed circuit and an open circuit. 20.2 Electric current Electric current is caused by moving electric charge. Electric current comes from the motion of electrons. current 20.2 Electric current Electric current is similar in some ways to a current of water. Like electric current, water current can carry energy and do work. A waterwheel turns when a current of water exerts a force on it. 20.2 Electric Circuits An electric circuit is a complete path through which electric current travels. A good example of a circuit is the one found in an electric toaster. 20.2 Electric Circuits Wires in electric circuits are similar in some ways to pipes and hoses that carry water. 20.2 Electric Circuits When drawing a circuit diagram, symbols are used to represent each part of the circuit. 20.2 Electric Circuits Electrical symbols are quicker and easier to draw than realistic pictures of the components. 20.2 Resistors A resistor is an electrical device that uses the energy carried by electric current in a specific way. Any electrical device that uses energy can be shown with a resistor symbol. 20.2 Current in a circuit Current only flows when there is a complete and unbroken path, or a closed circuit. Flipping a switch to the off position creates an open circuit by making a break in the wire. Chapter Twenty: Electric Circuits 20.1 Charge 20.2 Electric Circuits 20.3 Current and Voltage 20.4 Resistance and Ohms Law Chapter 20.3 Learning Goals Explain how current flows in an electric circuit. Define voltage and describe how it is measured. Discuss the function of a battery in an electric circuit. Investigation 20A Key Question: How do you measure voltage and current in electric circuits? Electricity 20.3 Current and voltage Electric current is measured in units called amperes, or amps (A) for short. One amp is a flow of a certain quantity of electricity in one second. The amount of electric current entering a circuit always equals the amount exiting the circuit. 20.3 Voltage Voltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy. Voltage is measured in volts (V). A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second. 20.3 Voltage A difference in voltage provides the energy that causes current to flow. 20.3 Voltage A useful meter is a multimeter, which can measure voltage or current, and sometimes resistance. To measure voltage, the meters probes are touched to two places in a circuit or across a battery. 20.3 Batteries A battery uses stored chemical energy to create the voltage difference. Three 1.5-volt batteries can be stacked to make a total voltage of 4.5 volts in a flashlight. 20.3 Batteries A pump is like a battery because it brings water from a position of low energy to high energy. 20.3 Measuring current I f you want to measure current you must force the current to pass through the meter. Multimeters can measure two types of current: alternating current (AC) and direct current (DC). 20.3 Measuring current Circuit breakers and fuses are two kinds of devices that protect circuits from too much current by making a break that stops the current. Chapter Twenty: Electric Circuits 20.1 Charge 20.2 Electric Circuits 20.3 Current and Voltage 20.4 Resistance and Ohms Law Chapter 20.4 Learning Goals Use Ohms law to relate current, voltage and resistance. Apply Ohms law to solve problems. Classify materials as conductors, insulators, and semiconductors. Investigation 20B Key Question: What is the relationship between current and voltage in a circuit? Resistance and Ohms Law 20.4 Resistance Resistance is the measure of how strongly an object resists current flowing through it. The relationship between electric current and resistance can be compared with water flowing from the open end of a bottle. 20.4 Resistance The total amount of resistance in a circuit determines the amount of current in the circuit for a given voltage. 20.4 Resistance Electrical resistance is measured in units called ohms. This unit is abbreviated with the Greek letter omega ( ). 20.4 Ohms Law The current in a circuit depends on voltage and resistance. Ohms law relates current, voltage, and resistance with one formula. If you know two of the three quantities, you can use Ohms law to find the third. Solving Problems A toaster oven has a resistance of 12 ohms and is plugged into a 120- volt outlet. How much current does it draw? 1.Looking for: current in amps 2.Given R = 12 ; V = 120 V 3.Relationships: I = V R 4.Solution I = 120 V 12 Solving Problems = 10 A 20.4 Resistance of common objects Every electrical device is designed with a resistor that causes the right amount of current to flow when the device is connected to voltage. 20.4 Resistance of common objects The resistance of many electrical devices varies with temperature and current. A light bulbs resistance increases when there is more current because the bulb gets hotter when more current passes through it. You are Wired! Your nervous system uses specialized cells called neurons to transfer electrical signals from one part of your body to another. A neuron has three basic parts: the cell body; a long, thin portion called the axon; and fingerlike projections called dendrites.