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Electricity, Circuits, Voltage and Batteries 17.1 Objectives • Define electric current. • Define voltage source. • Identify the electric symbols for basic circuit elements. • Distinguish between parallel and series arrangements of batteries. • Use a multimeter to test continuity. • Identify and construct open, closed, and short circuits. • Construct electric circuits with batteries connected in series and in parallel. • Calculate the combined voltage of batteries connected in series and in parallel. Physics terms • electric current • voltage • ampere (A) • volt (V) • electric circuit • battery • open circuit • voltmeter • closed circuit • short circuit What is electricity? Electricity is the flow of electric charges, typically through wires, conductors, and electrical devices. Sometimes we can see the effects of electricity in nature. What is electric current? Electric current is the flow of charged particles, usually through wires and circuits. Current is measured in amperes (A), also called amps. Animated illustration, page 472 How do we measure electric current? Electric current is measured using an ammeter or, more commonly, a digital multimeter. What is an electric circuit? An electric circuit is a conducting path through which electric current can flow. The symbol for electric current is I. In this circuit, I = 0.3 amps. Charge is conserved Electric current I must be the same at all points in a simple, single-loop circuit. I Charge can never be created or destroyed. All the charge that flows through one point in the circuit must flow through the next point also. I Open and closed circuits An open circuit does not have a complete path so no current flows. A closed circuit has a complete path, allowing current to flow. Common circuit elements Knife blade switches A knife blade switch uses a mechanical lever to open/close a circuit connection. What is a short circuit? Assessment 1. Which statement below is a correct definition of electric current? Electric current is . . . A. the energy of moving electrical charges. B. the flow of electric charges through a wire or circuit. C. the number of electrons in a circuit element. D. the attraction between charged particles in a wire. Assessment 2. Label each of these electrical symbols with the name of the electrical component it represents: battery; resistor; lamp; switch; or wire. Assessment 4. Identify the following circuits: A reliable source of current Prior to 1800, no one knew how to create a steady supply of electric current. Alessandro Volta was the first to succeed. • He discovered that placing two different metals in an acid or salt solution produced an electric current between the terminals. • He connected multiple voltaic piles together to create a battery of them. Adolphe Ganot, Elementary Treatise on Physics, 1893 What is a battery? A battery is a device that converts stored chemical energy into electrical energy. Batteries come in many different kinds – even lemons! Reading a battery Batteries come in many sizes and voltages: 1.5, 6, 9, 12 V, for example. Voltage is the electric potential difference between the two terminals of a battery. Voltage is measured in volts (V), named after Volta, the inventor of the battery. How is voltage measured? Voltage is measured using a voltmeter or a digital multimeter. Volts and amps Current and voltage are the two most important physical quantities for understanding electric circuits. • Current measures the flow of charge through a wire. • Voltage measures the change in electrical potential energy per unit of charge. A useful analogy Charges in a wire are analogous to water in a pipe: • Electric current is analogous to the flow of the water. • The battery is analogous to the water pump. • Voltage is analogous to the change in gravitational Ep. What is a series connection? In a series connection, the batteries are connected one after the other. The positive terminal of one battery is connected to the negative terminal of the next battery. What is a parallel connection? In a parallel connection, the same ends of both batteries are connected together. Batteries in Series or Parallel To calculate the voltage across batteries connected in series or in parallel: • When batteries are connected in series, the voltages add together. • When identical batteries are connected in parallel, the total voltage remains the same. Why does it work this way? When these batteries are connected in series, the voltages add together. Electric charge (1 coulomb) passing through a 1.5 volt battery gains 1.5 joules of Ep. Passing through a second 1.5 volt battery gains another 1.5 joules of Ep. It’s like climbing a stairway to the second floor, then another stairway to the third floor. Why does it work that way? When these batteries are connected in parallel, the total voltage remains the same. Electric charge passes through one battery OR the other. It ONLY gains 1.5 joules of Ep because each charge ONLY passes through one battery. It’s like climbing either of two stairways from the first to second floor. Assessment 1. Which statement below provides a correct practical definition of a voltage source? A voltage source . . . A. creates an electric potential difference between two points in a circuit, such as the two ends of a battery. B. provides the electrical potential energy needed for a circuit to operate. C. is analogous to the pump in a system of circulating water. D. all of the above. Assessment 2. Identify which circuit has batteries connected in series and which has batteries in parallel. Assessment 3. Calculate the voltage across the batteries for each circuit. Assessment 4. What does the “1.5 V” label on the battery mean? A. The voltage of the positive terminal is 1.5 V. B. The voltage of the negative terminal is 0 V. C. The voltage between the positive and negative terminals is 1.5 V. D. All of the above